Gerhard Sessler

Elko:

Today is April 23, 2010, and we’re at Baltimore at the ASA meeting. We’re here today to talk to Gerhard Sessler to take his oral history. I think, Gerhard, you read the first paragraph. You know what the intent, the purpose of this is. So, I think we’ll follow the format that was suggested. Is that okay?

Sessler:

Yes, sure, AIP format.

Elko:

Yeah, the AIP oral history. So, I’m supposed to know when you were born. I know your age, but I haven’t calculated back what that is. Why don’t you say something about that, where you were born and something about your family and your parents and such stuff?

Sessler:

Well, I don't really remember much about these early events in my life! [Laughs] It was on February 15, 1931, when I was born in a small town in southwestern Germany. Rosenfeld is the name of the town. My parents were living there. My father was a veterinarian. He practiced in that town. He had a university education in Munich, and his first practice was in this little city. We soon moved to the vicinity of Stuttgart, to another small city there. Then in 1936, when I was five years old, we moved to Waiblingen, which is also close to Stuttgart, and this is the place where I lived until I graduated from high school.

I finished high school in 1949. During the war, my father was drafted and he was a veterinarian officer there in France, and then in Russia. Fortunately, at the end of the war, he was transferred to Italy, so he was not a POW in Russia, which would have been unfavorable. So, in 1945 he was captured by the Americans, and he soon thereafter was released. He took on other responsibilities such as the inspection of meat products. That was in his responsibilities as a veterinarian.

Elko:

Was this back in Waiblingen?

Sessler:

This was in Waiblingen, in this city. Yes.

Elko:

He came back to Waiblingen after the war?

Sessler:

He came back to Waiblingen. So, I grew up there. I went to high school. Mathematics and physics always fascinated me. We had a very good teacher there.

Elko:

A good math teacher?

Sessler:

Yes. He also taught some fields of physics like theory of relativity. I still remember that course.

Elko:

In high school in 1949!

Sessler:

Yeah, I was but 17 or 18 years old. Yes.

Elko:

That’s amazing.

Sessler:

It was just fascinating, the way he presented it. We discussed it and all the things — the twin paradox, for instance — that for a young person who is interested in science is just fascinating.

Elko:

Yeah, absolutely. So, that really sparked your interest.

Sessler:

So, this is, in the end, why I decided to study physics. I was kind of undecided, even when I started my studies. Thus, parallel to physics I studied law for two semesters.

Elko:

Was that the influence of your…

Sessler:

But then I decided one topic was enough, you know. [Laughs]

Elko:

What about your parents? Were your parents interested in you becoming a scientist?

Sessler:

Well, my father would have been more interested in me studying something connected with medicine — animal medicine, for example, or human medicine — because he was in that field. He said, “Why don’t you do that? You can take over my practice and become a veterinarian.”

Elko:

His practice, was it just him or did he have other…?

Sessler:

No, it was just him. In those days, they didn’t have these medical centers, you know.

Elko:

So, he would go to the farm or he would go wherever…

Sessler:

He would go to the farm. He would see after the horses and cows and dogs, cats.

Elko:

Did you accompany him a couple times and figured out you didn’t want to do that?

Sessler:

Yes, exactly. So, my interest in physics and natural sciences was so fierce.

Elko:

It’s amazing. You mention that a good high school teacher had a huge impact on you. I think that’s probably a recurrent theme through a lot of people that I’ve talked to.

Sessler:

Yes I think that’s the age where you have to be infected with these ideas, you know? So, that just carries for your entire life.

Elko:

Okay, good. So, you already answered almost all the questions in this youth part here. So, you graduated from high school. How did you select what university you were going to go to, and how did you get in touch with those folks?

Sessler:

First, I selected a university that was pretty close, Freiburg, also in southwestern Germany. It was a beautiful city in the Black Forest. From the mountains, you could see the Swiss Alps, which I always liked. We did a lot of hiking there.

Elko:

Was that an important part of going to Freiburg?

Sessler:

That was a fun part of it.

Elko:

You went there to study physics then, I guess.

Sessler:

Yes, I did study physics, but also law in Freiburg.

Elko:

Okay, the two years of law.

Sessler:

I stayed in Freiburg only two semesters, and during these two semesters, I studied both law and physics. I made some exams in law and also in physics, of course. Physics means of course physics, mathematics, and chemistry. It was this combination, but with the emphasis on physics. So, we had also a few good professors there. One was Staudinger. He was a Nobel Prize-winner in chemistry.

Elko:

Really?

Sessler:

Yes, he gave very good lectures, which I enjoyed. But I still decided that I wanted to major in physics and not in chemistry.

Elko:

Oh, interesting. Did he want you as a student when you were taking his course?

Sessler:

Well, I was too young. He didn’t even know me.

Elko:

You were undergrad.

Sessler:

He was a big professor standing there, 500 students in the auditorium, and he lectured there. And he was a Nobel Prize-winner, so…

Elko:

Yeah, so he would never talk to an undergraduate.

Sessler:

No, he would never.

Elko:

He probably wouldn’t even talk to his PhD students, right? [Laughing]

Sessler:

Yes, right. So, from there I went to Munich, which was my next…

Elko:

You decided, “I’m not going to do chemistry. If I was going to do chemistry, I’d stay here. I’m not going to do law. I want to do physics, so I’m going to go to Munich.”

Sessler:

I wanted to study physics, yes. So, I went to Munich. There I concentrated on physics, mathematics, and chemistry. There were a few good teachers there, like Gerlach, and a few famous people like Sommerfeld.

Elko:

Really? He was there?

Sessler:

He was there, and he was still alive. He unfortunately soon died because he had a car accident, I think, and he died from that. But he was already in his eighties at that time. But he still lectured occasionally.

Elko:

You took lectures from Sommerfeld?

Sessler:

Yes. He was very famous, of course, very good. He missed a Nobel Prize, and many people said that the biggest blunder of the Nobel Prize committee was that they didn’t give a prize to him.

Elko:

Yeah! That is amazing.

Sessler:

In physics he was so famous.

Elko:

Yeah. Absolutely.

Sessler:

So, I made my bachelor’s degree, the “Vordiplom” in Germany, in Munich.

Elko:

The Diplom. So, that’s like five years basically for Diplom?

Sessler:

No, a little less for the Vordiplom, about three years. Then, after I had my Vordiplom, I went to Göttingen. Göttingen was of course still the Mecca of physics in those days. There were several Nobel Prize-winners there: Hahn, Heisenberg, two of them, and some other famous people like Pohl. I saw these people mostly in the colloquium.

Elko:

You knew that Göttingen was like holy grail. It was the best university for physics, and you wanted to do your PhD.

Sessler:

Yes. Now these guys, Hahn and Heisenberg, didn’t lecture anymore. They gave occasionally a talk, but lecturing was not… They were of course involved in research. Hahn was already retired, but he was there. If there was a colloquium, for instance, he was always there and asked some questions.

Elko:

That must have been interesting.

Sessler:

Heisenberg was often there. So, it was fascinating. My immediate teacher later on was Erwin Meyer, who is very famous in acoustics.

Elko:

Yeah, very famous.

Sessler:

He was among the younger professors — actually the most famous one in Göttingen.

Elko:

The most famous young physics professor?

Sessler:

He was… Young means in his forties or fifties.

Elko:

Yeah. He was energetic. He’s just starting his career.

Sessler:

Meyer was energetic. He was well-known internationally for his scientific contributions, more than the other young physics professors who were active in Göttingen in those days.

Elko:

And the focus he had was acoustics, right?

Sessler:

The focus was acoustics.

Elko:

That’s interesting.

Sessler:

So, this is why I got involved with acoustics.

Elko:

Did you happen to like the lectures he gave? What got you into becoming his student?

Sessler:

Meyer had very good lectures. He also had experiments in his lectures, which made his lectures much more lively. He was very fond of analogies between acoustics and electrical engineering and physics, so this was interesting, the way he looked at these subjects. I enjoyed his lectures.

Elko:

You’re one of his younger students, right? Still around. I mean there were some others. He had a lot of famous students… like Manfred Schroeder.

Sessler:

Manfred was one of his first students.

Elko:

Konrad Tamm, right, was a very famous student of his too.

Sessler:

Tamm, yes. Tamm was famous in physics. His field was relaxation phenomena. He later on went to Heidelberg. Kurtze was one of his students. Kuttruff, Eisenmenger, Mechel were all students of Meyer. Many of these later on assumed professorships at various universities in Germany.

Elko:

So, Meyer was like the grandfather, like the Johnny Appleseed of…

Sessler:

So, he was a professor who had about half a dozen, or even more, well-known students. He was the father whose sons were offered professorships later on at various German universities.

Elko:

Right. Wow. So, he was an inspiring person as well.

Sessler:

Yeah, he was inspiring. And he traveled often to the States.

Elko:

What was he doing in the States?

Sessler:

Well, Meyer visited many laboratories and universities there: Bell Labs, the National Bureau of Standards, Harvard University, and Bendix Labs, for instance. He gave lectures and talks in these places and at numerous meetings of the ASA.

Elko:

Is that right?

Sessler:

Well, Manfred Schroeder went to Bell Labs.

Elko:

Oh, Manfred! Yeah, of course.

Sessler:

So, Meyer had a connection there. Even before Manfred, Win Kock was a director at Bell Labs, and Meyer knew him very well.

Elko:

Oh really?

Sessler:

Yes.

Elko:

Because he would come to Germany or…?

Sessler:

So, that was actually the connection which got Manfred to Bell Labs, the Win Kock connection.

Elko:

Win Kock to Meyer.

Sessler:

Yes, Win Kock to Meyer.

Elko:

Oh, interesting. I never knew that.

Sessler:

So, Win Kock hired Manfred and Manfred hired me. [Laughter]

Elko:

Wow! I didn’t know that. That’s a very interesting piece of [inaudible].

Sessler:

Yes, because I didn’t know Manfred from Göttingen then.

Elko:

He was already over in the U.S., right?

Sessler:

He was out of the institute before I came.

Elko:

Okay. So, it’s asking how did you support yourself as a grad student back then?

Sessler:

Well, it was my father who supported me.

Elko:

He did, all through graduate school?

Sessler:

I worked occasionally a little bit, but my father was well situated.

Elko:

But you didn’t have to pay tuition, right, at that time?

Sessler:

There was some tuition, but it was very low, not comparable to tuition at American universities.

Elko:

It wasn’t a hardship. It wasn’t hard for your father. Basically, he had to pay for your apartment and your food, right?

Sessler:

Yes, it was 200 marks for tuition, corresponding to 50 U.S. dollars at that time, per semester, and about the same per month for living expenses, which was nothing.

Elko:

So, when you went to Göttingen, did you find it to be a really exciting place to be?

Sessler:

It was an exciting place to be, yes.

Elko:

Yeah, I can believe that.

Sessler:

Particularly the colloquium, which I mentioned before, was very exciting because they invited the leading physicists from all over the world.

Elko:

Absolutely. Wow. What an opportunity.

Sessler:

Göttingen, of course, was great in the 1920s and early 1930s before the Nazis came in and threw all these people out.

Elko:

They all got thrown out.

Sessler:

But there were many Nobel Prize-winners in the 1920s in Göttingen, you know. It was worldwide, I think, the leading place, or one of the leading places. But even in the 1950s, it was still good because there were some of the people there whom I mentioned.

Elko:

When did Meyer finally retire?

Sessler:

Meyer retired in 1969, at the age of about 70.

Elko:

Oh, late. Much later.

Sessler:

And Manfred Schroeder followed him.

Elko:

Right, he followed him and took his chair as a professor, is that right? So, when his slot vacated, Manfred filled that slot?

Sessler:

Yes, right.

Elko:

So, he had many more students that came on after… There were many… Or did he stop taking students after a while?

Sessler:

Yes, there were many later students. Volker Mellert, for instance, was one of these who went to Oldenburg. Birger Kollmeier.

Elko:

Well, Kollmeier wasn’t a student of Meyer, though, was he?

Sessler:

I think he was a student of Manfred already.

Elko:

Yeah, yeah. I think Manfred…

Sessler:

Yes, he’s younger.

Elko:

Good. So, I think we covered your graduate training, unless there’s anything else you want to say. If something pops into your head, just feel free to say it.

Sessler:

Well, perhaps I should mention my Diplom thesis, master’s thesis. This was about sound propagation in rarefied gases, very-low-pressure gases, pressures of something like 10^-2 torr. That means about 10^-5 atmospheres. So, these are conditions which you find in the higher atmosphere. Sound still propagates there, but as you go to lower and lower pressures, the mean free path of the molecules increases. At those pressures which I mentioned, the mean free path is about as large as the distance between the transducers, you know. If you measure, for instance, sound attenuation, you use two transducers at a short distance (usually a few centimeters apart or something like this) because the attenuation is very high. The mean free path now was longer than the distance between the two transducers, the sender and the receiver. Completely new physics, how it’s there, you know. I did some theoretical work on this in my thesis and also experimental work.

Elko:

Is it quantum?

Sessler:

Meyer always called it a ping pong game because molecules are flying from the sender to the receiver. So, this was interesting. We wrote a paper which was published in the Zeitschrift für Physik, and it was quoted many, many times later on. It was a paper by Meyer and myself. He was the first author, although he didn’t do the work, you know. [Laughter]

Elko:

This is Germany.

Sessler:

But he had the idea, of course. He was interested in the progress of the work.

Elko:

You had to develop your own transducers at that point as well, right?

Sessler:

Yes. We used new transducers, and of course that was now the beginning of the electret work, so to speak. The emphasis was on the physics of the sound transmission, and we just used these transducers, but we noticed some of the effects, you know. These were condenser transducers biased with an external bias, but after some time they charged up. They became electrets. We just saw it.

Elko:

You observed something happening.

Sessler:

We observed it. We saw it as a side effect. It had been observed before. There’s this famous paper by Kuhl, Schodder, and Schröder — a different Schröder, by the way, not Manfred — and they described these effects. This paper came out in 1954, and it was based on another paper by Sell that was already published before. This paper described capacitive transducers in which a solid dielectric was introduced in the air gap. They were called Sell transducers. We used these transducers in the late 1950s. My Diplom examination was in 1957 and the doctoral examination in 1959. Both, in the master’s thesis (the Diplom thesis) and in the doctor’s thesis I used these transducers.

Elko:

Were they like tensioned membranes?

Sessler:

Tensioned membranes, yes.

Elko:

Just like a classic condenser mic.

Sessler:

Right.

Elko:

But with a polymer?

Sessler:

But with a solid dielectric in between. That was the difference.

Elko:

Yeah, that’s different. Yeah, yeah.

Sessler:

Yes. The solid dielectric charged up because a DC bias of 200 volts was applied.

Elko:

Was there a gap or did it just lay flat on a rough surface or something?

Sessler:

Some of these were used with a gap; some of them were used with just contact between the membrane and the back plate. There were some irregular air layers, so the membrane could be deflected.

Elko:

Yeah, yeah. So, the cut-off frequency, that had a pretty wide bandwidth.

Sessler:

Yes, well, the resonance frequency, there was a very damped resonance at about 100 kHz or something like this. So, we used them at the higher frequencies.

Elko:

What did you bias it at? What voltage?

Sessler:

We still biased them at that time.

Elko:

You did like 200 volts or something?

Sessler:

200 volts, yes.

Elko:

So, you used batteries, I guess.

Sessler:

We used batteries, yes.

Elko:

Oh cool. A lot of batteries in series, I guess, built it that way.

Sessler:

Yes. Some similar work was done by Moe Greenspan in this country, and Meyer of course also knew Greenspan. That was actually the reason why he suggested to me to do this work.

Elko:

The Sell transducer? Oh, you mean to look at the…

Sessler:

The Sell transducer and the sound propagation in rarefied low-pressure gases, because Meyer had seen it at the National Bureau of Standards, where Greenspan worked. We actually extended the parameter frequency over pressure. This corresponds to the ratio mean free path of the gas molecules to wavelength and this ratio was extended beyond to what Greenspan had done and investigated, because it turned out that Greenspan used piezoelectric transducers. The condenser transducers which we used were actually more useful in this range because they could be used at much lower gas pressure, so we could extend the range, and that was nice. Because we wanted to explore sound propagation for the case where the mean free path was comparable or larger than the wavelength.

Elko:

After that, did Greenspan switch to condenser Sell transducers?

Sessler:

Well, no, he went to other topics. He did this work in the early 1950s and then he went on to other things.

Elko:

So, after you graduated, you came directly to Bell Labs?

Sessler:

Yes, directly after my doctoral examination. Meyer established the contact with Manfred at Bell Labs. We didn’t even have an interview, you know. I mean Meyer recommended me.

Elko:

Oh, that he was good enough, Manfred knew.

Sessler:

And Manfred accepted me, so I went to Bell Labs in late 1959.

Elko:

Wow, just all first straight out of school.

Sessler:

Yes, yes.

Elko:

Meyer said, “Hey, would you like to work in the United States?” or “You would like to work with Bell Labs?”

Sessler:

Yes. I was always interested in coming to the United States. I even applied for a Fulbright scholarship while I studied, but there were so many applications. Everybody in Germany wanted to go to the United States, so there were thousands of applications and only a few people could go, so I didn’t succeed. But then after finishing my doctoral work, there was this possibility, and I went.

Elko:

At that point, Bell Labs was just trying to hire the best people they could find. They didn’t care what your background was as long as it was good, right?

Sessler:

Yes.

Elko:

You came recommended from a famous professor or something. They didn’t really have something in mind for you. They said, “We’re going to hire this guy…”

Sessler:

Yes. Well, Manfred had some ideas, of course, what could be done. He was a department head at that time.

Elko:

Of Acoustics Research?

Sessler:

In this department. The name was Acoustics Research, and various fields of acoustics were covered by him at that time. He did of course work on architectural acoustics and speech and auditory research, and also in electroacoustics.

Elko:

So, was he interested in the work you were doing for your PhD? He didn’t want you to continue that? He had some other ideas? What did he want you to do?

Sessler:

Well, he was now more interested in the transducer aspects. My PhD work and my Diplom work was on sound propagation in various media, but now the emphasis was on the transducers. This is then why we started with the electret work.

Elko:

So, the electret work was actually thought about from the perspective of “okay, we need to…”

Sessler:

Kind of a continuation of your work. We tried it at the…

Elko:

So, there was still the idea there that, “Look, let’s work on something that pertains to communication systems,” right?

Sessler:

Yes.

Elko:

It wasn’t like, “Let’s just win a Nobel Prize.” Is that right?

Sessler:

Yes.

Elko:

So, there were certain directions. There was a “Hey, let’s see if we can solve something on this problem.” Were you actually…?

Sessler:

Yeah, but this steering was very remote. I mean, I came to Manfred and said, “Well, wouldn’t it be nice to investigate these transducers a little more so that they can be used in communications?” But I was still interested in propagation problems, sound propagation, so I suggested, “Well, why not try sound propagation in plasma, in ionized media?” So, I started a project on sound propagation in plasma, and instead of sound, I soon found out that there are ion waves and electron waves. So, I got interested in the propagation of ion waves, particularly electron waves were at very high frequencies. So, I didn’t have the equipment there to do this work, but for acoustic ion waves, as they were called, much of the equipment was there. Some of the things were constructed in the workshop there. I bought a large magnet because these ion waves are modified if you put them in a magnetic field. So, I had this huge magnet there which was actually an excuse to get an air conditioner in my room. [Laughs] Bell Labs was not air conditioned in the early 1960s. They started air conditioning in the mid-1960s.

Elko:

But the air conditioner wasn’t for the magnet, or was it? It was an electromagnet?

Sessler:

It was a huge electromagnet, which used a lot of power, generated a lot of heat. I then made an application for an air conditioning system, which was needed for personal comfort, and it was approved. [Laughter]

Elko:

Only vice presidents and Gerhard got their air conditioners. So, your lab was popular in the summer, I think, right?

Sessler:

Yeah. You could do all kinds of work at Bell Labs in those days. Some work was done on gravitational waves, later on by other people which certainly had nothing to do with communications. But it still was tolerated as long as it was scientifically of interest. Bell Labs was very tolerant about these things in the 1960s and 1970s [although gravitational waves were detected only in 2016, several decades after the first work was done there].

Elko:

Yeah. That changed. That changed.

Sessler:

That changed in the 1970s and 1980s as you know.

Elko:

Yeah, it did, and the ’90s very much so. If you were a star, or you had to establish yourself, or you had a Nobel Prize, they weren’t going to bother you. But yeah, there was definitely much more interest in trying to focus… I mean, when I came on board, it was definitely… They had things that they wanted to do, but, okay. So, I think we actually covered… It’s funny. We’re just chatting here, and we’re actually doing a very good job of…

Sessler:

Yes. Another thing that came up apart from our electroacoustics work… Of course, as far as the electroacoustics work is concerned, I should mention Jim West. I started collaborating with him right when I came to Bell Labs, and we did this work on the electret microphone, and also electret earphone, of course, together over the years. It started in late 1959 or 1960, and the first major publication was in 1962.

Elko:

In ’62 on the electret itself.

Sessler:

On the electret microphone.

Elko:

Yes. But to backtrack just a little bit, I think Jim was trying to do something with earphones, right?

Sessler:

He was also working in electroacoustics, of course. Before I came, he joined Bell Labs a couple of years earlier, and he was working with earphones. He was probably also inspired by this paper by Kuhl, Schodder, and Schröder, which inspired us in Göttingen. It was a very parallel development. I worked there to use these transducers for sound propagation measurements, and he experimented with them for headphones.

Elko:

He was observing, I think, some of the things that you said, where you saw as a function of time the sensitivity would drop, and I think Jim had mentioned to me that…

Sessler:

Kuhl, Schodder, Schröder saw these things.

Elko:

Yeah, and I think there was some idea of flipping the bias or something.

Sessler:

Flipping the bias, yes.

Elko:

Yeah, or taking it off or whatever. Then that would…?

Sessler:

The sensitivity went down and down and down, and then you flip the bias and the sensitivity was right up there again. It was an effect. So, when I came there, we combined forces, and that was very good. It was very inspiring to have somebody to discuss these things with.

Elko:

Yeah, what was going on and why these things were failing.

Sessler:

What was going on, yeah.

Elko:

Oh, yeah. It says here your department head was Manfred. What was his style as a department head? Was he pretty laissez-faire, hands-off?

Sessler:

His style was that he gave us a lot of freedom. He said, “Okay, do what you find interesting, as long as something comes out” — some publications or patents, then that’s good. At that time, that was possible. He got us involved, of course, also in some things that were closer to him like room acoustics, architectural acoustics. He had this project on Philharmonic Hall which started in 1962, or was it in 1964?

Elko:

I don't know the time.

Sessler:

It was started in those years, and it immediately turned out that the acoustics were deficient. Manfred was one of the four or five consultants that were asked in, and he then wanted to start a measuring project there. So, we had to develop measuring methods, and these were actually the first measuring methods fully dependent on digital signal processing. We generated our measuring signals, tone bursts, on the digital computer. We used for the tone bursts a temporal Hamming window, to reduce spectral side lobes, which would have been difficult to do with analog equipment. After we had these tone bursts, we went into the hall, played them on loudspeakers, and recorded the response of the hall.

Elko:

Digitally. It was all digital.

Sessler:

Digitally. Well, it was recorded. Of course, we received it through microphones. It was analog signal, but then it was digitized and evaluated digitally.

Elko:

So, where was this hardware, the A to D, D to A, found? Did Bell Labs make it themselves?

Sessler:

Oh yes. All of these. Well, not the A to D converters, but all the other hardware was made there.

Elko:

That’s interesting. So, this is Manfred’s interest in digital computers using…?

Sessler:

Yes.

Elko:

It’s the beginning of signal processing, really, the first — yeah, that’s amazing.

Sessler:

Yes, of course it was amazing. It was nice. We had these room responses to our tone bursts, and then everything was done on the digital computer. It was very different. In those days, you could not… It was usually still done with analog equipment.

Elko:

How would that work? Did you record the bursts on analog tape?

Sessler:

Yes.

Elko:

Because the computer was like huge. You couldn’t lug that.

Sessler:

No, you could not bring a computer into the hall.

Elko:

It wasn’t like a laptop to bring to the hall.

Sessler:

That part was still analog, but then we had the signal on tape, and then we could evaluate it on the computer at Bell Labs.

Elko:

And somebody else would program some program, something.

Sessler:

Yes, right. There were some programmers there.

Elko:

Wow. Amazing. Did you ever program the computer?

Sessler:

No. I didn’t do that.

Elko:

Oh, you were going to say… So, Manfred — one of the things that I remember hearing, and maybe you can tell if this is true or not — the reason that Manfred was invited was that an AT&T board member was also a board member of Philharmonic Hall, the New York Philharmonic.

Sessler:

Yes.

Elko:

And therefore, they said, “Hey, maybe somebody from Bell Labs could help us.” Is that true?

Sessler:

Yes, that’s true.

Elko:

Okay, so Manfred, you, Jim, and Bishnu Atal?

Sessler:

Yes, I think AT&T was approached by Philharmonic Hall, I think, or by Lincoln Center. Of course, AT&T knew that there was Manfred Schroeder at Bell Labs, so they contacted him and then he was in charge of this project.

Elko:

So, what did you find out on your project for that?

Sessler:

Well, we found all kinds of things, for instance that the room was deficient in low frequencies because of these reflectors on the ceiling that were too small to reflect low frequencies. Disturbing echoes came from the back walls and lateral reflections arrived too late.

Elko:

They were absorbing too much low frequency?

Sessler:

They absorbed… well, the low frequencies went through the reflectors and were lost behind the reflectors in the space between the reflectors and the ceiling. So, they later on decided to do away with these reflectors.

Elko:

They called them clouds, too.

Sessler:

The clouds, yes.

Elko:

So, they just pulled them out completely, just removed them?

Sessler:

In the first modification of the hall, they closed the gaps between the clouds. After this did not result in satisfactory acoustics, they rebuilt the entire interior of the hall without using clouds.

Elko:

Did that improve the acoustics?

Sessler:

Not really. Low frequencies were still weak on the main floor. Thus, the distribution of acoustics in the room was poor. There were some seats that were subjectively very good. It was the seat A15 on the second balcony, I think, which I still remember. It was a very famous seat. Its acoustics was good because of the unhindered sound propagation from the stage to this seat. The low frequencies were relatively strong and there was good lateral reflection from the side walls. It was discussed I think in the New York Times that there were very good seats.

Elko:

The critics liked that.

Sessler:

The music critics detected many deficiencies. We wanted to find out why these seats were good, so we evaluated, for instance, the reflections from the side walls. There were very good side wall reflections on these seats. So, we found out that side wall reflections are more important than reflections from the ceiling, and this means of course that a concert hall that is high and not broad is better because the ceiling reflections arrive later. The first reflections are from the side walls. Another thing which we detected and described first was the seat dip effect. If sound propagates over theater seats, there’s a dip in the frequency response of the received sound at about 200 Hz or something like this.

Elko:

It was like a quarter wave or something?

Sessler:

This was detected simultaneously by people from BBN and by us. Schultz from BBN detected it, and we described it at the same time. There are two papers in the Journal of the Acoustical Society.

Elko:

So, how did they combat that? Reduce the regularity of the seats or heights or spacings?

Sessler:

Well, there’s very little you can do about this because you need seats, obviously. [Laughs] With the seats, you cannot do very much. You can introduce, however, same raking of the seats. That means inclination from the horizontal with increasing distance from the stage. Obviously, you cannot make them with different height, for instance, to broaden the missing frequency band.

Elko:

Was it a depth thing, that the height of the chair…?

Sessler:

It was the depth, yes. The height of the seats is involved. But it’s a difficult problem, a diffraction problem.

Elko:

Okay, that’s interesting. So, I wonder if your observations of the lateral reflections becoming important led to — I know Manfred did this — interaural cross correlation stuff, with a bunch of his students later at Göttingen when he went back to Göttingen. He was very much enamored of this idea of trying to understand why there’s a preference for lateral reflections. That was the beginning of that, actually.

Sessler:

Yes, that’s right.

Elko:

That’s what’s preferred. People like that. That’s interesting. I think there are some general science questions they wanted to know. I guess if you’ve served on review boards or grant boards or committees that fund research.

Sessler:

Well, let’s discuss a little more the electret microphone work, because we have not said very much about this. [Laughs]

Elko:

Oh, yeah. [Laughing] We’re going to skip that one. No, go ahead.

Sessler:

Okay. Well, we have discussed the history of it, so I’ll just recall how we got to work on this. The first electret microphones which we made used Mylar films. Or Mylar is the electret material, and of course Mylar…

Elko:

How did you find Mylar?

Sessler:

Well, it is a mechanically stable polymer and the material is readily available. No, it was a little more than this. The transducers which we used before, which I mentioned which were still biased without an applied bias, they usually used Mylar films.

Elko:

They did?

Sessler:

Yes, because Mylar is a very stiff material that does not flow under pressure. It is available in very thin films down to about 2 μm thickness. So, all these were advantages of the Mylar. For this reason, Mylar was used in the externally biased transducers, and we found that there’s this charging effect. So, we said, “Okay, let’s try to cultivate this. Let’s see whether we can deliberately charge the Mylar and bring it up to a charge that is permanent and high enough to bias the microphones.” So, this is how we started. Then, we soon found that Mylar was okay for some time, but that it lost its charge over time. The Mylar microphones lasted perhaps for six months or a year or something like this, but then their sensitivity went down. So, in 1962 when we described the first electret microphone in the Journal of the Acoustical Society — the first thin film polymer electret microphone, I should say, because there were these wax electret microphones before in the Second World War, and they were good only for half a year or so. So, when we described this first thin-polymer-film electret microphone in JASA in 1962, it was still a Mylar electret. But we knew already at that time — and it’s actually described in the paper that Mylar is not an optimal material for this and we were looking for better electret materials. I checked some of the literature, you know, on volume resistivity of polymers, and there was the fluorocarbon polymer Teflon as a superior polymer in this respect.

Elko:

Which was a new material back in those days probably.

Sessler:

Teflon was fairly new at that time, about 20 years old.

Elko:

DuPont, right?

Sessler:

By DuPont. We then used the Teflon. Other people got aware of the electret microphone business and started, also worked there — for instance, Northern Electric. They also looked for better materials, and they found polycarbonate. So, for some years there was a debate which was the better material. Polycarbonate was better mechanically, and the Teflon was better electrically. So, eventually the Teflon was the material that was being used, particularly since the electret was then cemented onto the rigid back plate to stabilize the mechanical properties by preventing its flowing under mechanical stress.

Elko:

With the Sell transducer you were using at Göttingen, was that Mylar?

Sessler:

That was Mylar, yes.

Elko:

That was Mylar?

Sessler:

Well, it was the German Mylar, the Hostaphan material, which is also a PET material, also referred to as a polyester. But here in the States, we used Mylar.

Elko:

When you started this, you observed basically the sensitivity dropping with time. Did anybody just take the batteries off and see that you got good output again just by taking the batteries off?

Sessler:

Kuhl, Schodder, Schröder did this.

Elko:

They did?

Sessler:

Well, they took the bias off.

Elko:

Yeah, that was quite an “aha!” moment, right? Like, “Wait a second. We don’t have a bias on this thing and it’s working.”

Sessler:

They could have invented the electret microphone about six or eight years earlier, but they didn’t think of it like this. For them it was an unwanted effect.

Elko:

They were focusing on something different?

Sessler:

They didn’t think of charging this deliberately.

Elko:

They didn’t think of what was going on. They didn’t understand what was — or did they understand that it was happening?

Sessler:

Well, they understood that the material was charging up. They understood that, you know. But they never used the word electret, for instance, for it, and they certainly did not cultivate this effect. They did not say, “Okay, let’s use the effect…”

Elko:

Yeah, that’s amazing!

Sessler:

“…to bias the microphone.” Often you stand on the shoulders of the people who worked, who were there before, and we stood on the shoulders of Kuhl, Schodder, and Schröder.

Elko:

And you wonder why they didn’t know… Well, they never thought they could really stabilize the effect sufficiently.

Sessler:

Then it was a small step there.

Elko:

That’s actually a big step, to be honest with you.

Sessler:

It’s the way you see it.

Elko:

You have to think differently, right? You have to think, “Wait a second. I could use… That’s a great… That’s something!” Like you said, you started to look at, could you use this effect, and then that got you on the path of thinking about, “Can we put charge there and keep it there,” right?

Sessler:

Yes right. Well, this is the same, I think, with all inventions. Think of the telephone. Bell was standing on the shoulders of other people. I don't mention their names. [Laughter]

Elko:

We know it’s not Philipp Reis, okay? [Laughter] No, actually he did. Joseph Henry, I think, played a very big role in helping Bell, right?

Sessler:

Yes, yes.

Elko:

Because I think Bell himself was not… He was not a scientist. He was not an engineer. He was just kind of an inventor. Maybe Edison… I don't know. But he had a little better understanding, I think.

Sessler:

But I think electromagnetic transducers were around before Bell.

Elko:

Yeah, they were.

Sessler:

And Bell’s transducer was an electromagnetic transducer.

Elko:

Yeah, that’s right. That’s right.

Sessler:

But he used it in a clever way to make a telephone out of this.

Elko:

Yeah, that’s right.

Sessler:

I think the same is true even for the transistor. There was work before the transistor that was very close to what Shockley, Bardeen, and Brattain did. But you make one more step and then all of a sudden you have something which is viable, which can be used, and which industry picks up and develops, and which then is made in millions or billions.

Elko:

Manfred could never have imagined bringing you in and saying, “Maybe you should look at something with transducers that may have some impact on the telephone” — how much impact you and Jim would have with that invention.

Sessler:

Well, nobody knew at that time.

Elko:

Nobody knew! Did you know? Did you know?

Sessler:

At about the same time… Well, once in a while we questioned whether our electret was the optimal solution. There was a transistor microphone developed at that time. What the man did was he just modulated the pn-junction of a transistor with a needle. The needle was connected to a diaphragm, and you talked into the diaphragm. The needle was somehow sitting on the pn-junction and modulated that junction, and you had a modulation of the current of the transistor. It was a very sensitive microphone because you modulated the transistor mechanically directly, but it was not very stable. I remember this fellow brought his microphone over to the anechoic chamber.

Elko:

Was this Tikanoff [?]? What’s his name?

Sessler:

It was Sikorsky. He brought it over, and by the time he put it on in the free space room, the sensitivity was down by 20 or 40 dB. Just at the end you have to be very careful, you know. But it was quite… And people said, “Well, look. This is the microphone of the future,” the FET microphone. Some people said, “Well, you with your electret, forget it. He has the microphone of the future.”

Elko:

Oh. Everybody was transistor-happy.

Sessler:

So, for a few years, it was interesting, but then it disappeared somehow.

Elko:

Never to come back again. Never to be seen again.

Sessler:

Never could be made reliable enough to be something that is commercially of interest.

Elko:

When you actually came up and you understood what was happening, you came up with some nice equations. You wrote that seminal paper. Did you realize at that point that that was a fundamental… going to be a huge shift in technology?

Sessler:

No, of course we didn’t realize this.

Elko:

You didn’t know that.

Sessler:

As a matter of fact, we were very impatient. We were disappointed to a certain degree because it took six years before industry picked this up. Our first paper was in 1962. The first commercial implementation was by Sony in 1968, and I learned about this. I was at an ICA meeting — International Commission of Acoustics meeting — in Japan. I gave a paper there about the electret microphone. After the talk somebody came up to me and said, “Do you know that Sony has been producing this for a couple of months?” I said, “No! No, I didn’t know that.”

Elko:

Has been producing it!

Sessler:

So, they had just started production.

Elko:

Somebody from Sony or somebody else?

Sessler:

I think it was somebody else who told me. So, it took six years, which was an eternity in those days, to me. So, we said, “Well, we are not really sure whether this is the invention that you file patents on it,” of course.

Elko:

So, why didn’t Western Electric work on it? Because of the Sikorsky mic? They thought that was the future, so they weren't going to work on the electret, so they just pushed that aside?

Sessler:

Well, there were several effects. One was the NIH factor, Not Invented Here, which was applied by our Development Department. The Development Department of course still tried to improve the carbon microphone. They had some electrodynamic microphones, and they didn’t believe in the electret microphone.

Elko:

Are these the folks from Indianapolis?

Sessler:

Yeah, folks in Indianapolis and also in Holmdel. They said, “Well, the electret is much too expensive.” That was one of the arguments. “Look,” they said, “we can make a carbon microphone for 30 cents now, and the electret will cost $5.” Now of course we know that even in 2010 the electret cost only 10 cents. [Laughter] And the dollar was worth much less in those days, you know.

Elko:

Amazing.

Sessler:

So, they didn’t believe that this could be made at a cost that would be competitive.

Elko:

No one saw that. It’s like a stretched piece of plastic in a FET. That’s not that expensive.

Sessler:

Nobody saw that in those days.

Elko:

Well, the piece of carbon was probably pretty simple, but they knew that carbon was limited, though, right?

Sessler:

Yes, yes.

Elko:

There are classic cases of, you have to bang your receiver on the table to get it to work again.

Sessler:

Well, they were looking for some other microphones like the dynamic microphones, but these were much too clumsy and much too complicated for a telephone. Of course, one of the problems in the early days of the electret microphone was the stability. We described this Mylar electret microphone, and it had a lifetime of half a year or one year, so that’s not good enough. Of course, every invention at the beginning is not what it will be after five or ten years. You just have to put in some work and then look for better solutions, improve on the work that is done.

Elko:

So, I’m getting demoralized here, because I used to think that the older engineers were wiser, but maybe they weren't so. They were the same as today’s engineers! [Laughing] All the axes to grind, politics, and their own biases which cloud their view of what really can… you know. So, it usually takes, I guess, for a technology like this, a visionary to do something. So, you think Sony played that role? Did they realize that?

Sessler:

They somehow picked this up, and they said, “That must be…”

Elko:

Do you know why they picked up the electret?

Sessler:

“…an inexpensive, good solution.” They developed the microphone. Initially, of course, it was also more costly. It didn’t cost pennies in those days. It cost dollars.

Elko:

Do you know why? Why did Sony want that microphone? Did it have something to do with the vibration in their device? Do you know why they wanted… What was attractive to them in the electret?

Sessler:

Well, we never found out, but it must have been some of these characteristics. We of course described the vibration sensitivity of electret microphones a couple of years after we invented this type of microphone.

Elko:

Because I think these microphones that Sony made were in mechanical tape-recording systems, right?

Sessler:

Yes, yes.

Elko:

So, they may have had motor noise issues and stuff when they used other mics. They may have needed something that was smaller, much lower vibration sensitivity.

Sessler:

That was certainly one of the major reasons why they looked for other microphones. You could not build a dynamic microphone into a tape recorder since its vibration sensitivity is proportional to the mass per unit area of the membrane including the voice coil. Thus, such a microphone picked up too much vibration.

Elko:

Too much of the vibration that was in the tape recorder, yeah. Just for posterity’s sake, we’re using electret microphones to record this interview. [Laughter]

Sessler:

Good!

Elko:

And it’s on a Sony, no less! [Laughter] But it doesn't have any moving parts, but still. That’s funny. Did you want to say anything more about the electret?

Sessler:

I think we said most of the important things. Well, later on we got involved…

Elko:

So, what took… Eventually, Western Electric started making electret microphones. What finally convinced them?

Sessler:

Well, it was perhaps the fact that the electret microphone was made now by Sony. They said, “Well, something must be good about it.” I made some microphones. These were experimental microphones, the ones Western Electric made. Actually, they never made a microphone that was produced and used within the Bell system. I think they bought their microphones, the electret, later on, when they switched to electret microphones…

Elko:

No, I think that’s not completely true there. The EL-3 was put in some business telephones.

Sessler:

Was it put in some business telephones?

Elko:

That was after you left, though. EL-3 did make it into products.

Sessler:

I see.

Elko:

But again, a goofy thing about the EL-3 that just carries that… They put 18 dB of gain in thinking that the noise immunity, since you only had, I don't know, was it 4, 8 mV/Pa. So, they were worried about such a low signal traveling on an unshielded cable, so they put in… But you know what happened is they blew it and the amplifier put in more noise than… [Laughter]

Sessler:

I see.

Elko:

And it turned out that it didn’t matter anyhow. So, it was at a cost, and then eventually EL-3 got pushed out and they used Primo, Panasonic, or other things. Oh, I know. I was going to ask you one other thing. What was I going to ask you? Oh, I forgot now. Oh, yeah. So, at Bell Labs, clearly the electret wasn’t the only thing you did there, right? You worked on other projects there as well.

Sessler:

Yes. Well, as I mentioned before, we worked on room acoustical projects, architectural acoustics. That means Philharmonic Hall. Later on, after Manfred left, Jim Flanagan was our director, and he was very interested in these room acoustic projects. We went to some halls, took measurements — some other halls, you know, like the Academy of Music in Philadelphia or the Symphony Hall in Newark, New Jersey.

Elko:

You were looking at…

Sessler:

So, we went on and did such measurements. That work extended over quite some period, into the early 1970s. Then the work on plasma waves continued also until about 1972 or 1973.

Elko:

So, all during this time you continued to work on this…

Sessler:

Well, we also worked on directional electret microphones. That was the next step. We started with that in the late 1960s and worked into the 1970s. We came up with second-order gradient microphones, toroidal microphones, for instance, or bi-directional microphones, unidirectional microphones based on the second-order gradient principle. We also made some dimensional microphones. That was essentially the work on microphones.

Elko:

So, what made you decide to… Eventually, you got to a point where, I guess, Germany called you back. You got an offer. Someone picked up the phone and said, “Gerhard, would you please come back to Germany?” Or what happened?

Sessler:

I got actually two offers. One was to Delft University in the Netherlands, and the other one was to Darmstadt University in Germany. I negotiated with both places and then decided to go to Darmstadt. Well, there were several reasons to go to a university, of course. One was that, in an industrial corporation, you have to be young. I think you should be a young person. An older person has more experience, and that’s valued more at a university.

Elko:

Because of the experience.

Sessler:

Because of the experience. This is one of the things… I thought about the future, and I thought, “Well, do I still want to be at Bell Labs at 65?” And I said, “Well, I would rather be at a university at that time. But if I want to go to a university, I have to change now.”

Elko:

How old were you when you made that decision?

Sessler:

I was 44 or 43.

Elko:

Oh, early then.

Sessler:

Yes. But at European universities, they usually don’t hire people who are 65. This is different in the States, you know. Jim Flanagan went to Rutgers when he was 65 and Jim West went to Johns Hopkins when he was 70. This could never happen at a German university. You have to join when you are in your forties at the latest, so I had to make up my mind.

Elko:

This is like a woman, huh? She would have to have a baby before 35, otherwise she’s not going to have one. So, that was a realization on your part. That’s actually very good. You were thinking ahead.

Sessler:

I was thinking ahead.

Elko:

Did you have something in mind where you wanted to go? Did you put the word out, “I’m looking for a university position,” or did you just wait for them to come to you? Were you proactive about it?

Sessler:

I was, in a sense, proactive. I of course always maintained my contacts. Originally when I came to the States, I wanted to be in the States for one to one-and-a-half years, as a postdoc.

Elko:

That was the plan.

Sessler:

That was my original plan. I always wanted to go back to Germany. But of course it was great at Bell Labs.

Elko:

Too much fun!

Sessler:

I enjoyed it so much, too much fun. Instead of one to one-and-a-half years, I stayed for 15 years, almost 16 years.

Elko:

Wow. That’s great. You were a supervisor, right?

Sessler:

I was a supervisor at the end, yes.

Elko:

Electroacoustics there.

Sessler:

Yes, supervising a small group.

Elko:

So, you negotiated between Delft and Darmstadt and then ended up in Darmstadt.

Sessler:

Yes.

Elko:

And that place I think already had developed some acoustics. There were some folks there…

Sessler:

Kuttruff was there prior to my time in Darmstadt. He was my predecessor, and he built and left of course quite a few facilities which I could use.

Elko:

So, it was attractive for you.

Sessler:

There was a huge anechoic chamber, a huge reverberation chamber there. There was some equipment there, and this was one of the advantages of this lab.

Elko:

Was there anybody else in audio or acoustics there when you went there?

Sessler:

Not in acoustics at that time, since Kuttruff had left already a few years before I went there.

Elko:

They were looking…

Sessler:

Well, Werner Endres was in Darmstadt, working on speech research. But he was not at the university; he was at the Research Institute of the German Telecommunications System.

Elko:

And he had an appointment where he could teach or something? How did that work?

Sessler:

He taught in Darmstadt also, at our university, mostly on the recognition, synthesis, and transmission of speech. He specifically worked on systems for speech synthesis, work that was of importance for his institute. He had the position of an Honorary Professor at our university, which means he was employed by a different organization but he was expected to do some teaching and also some research work at our university. Thus, I had some collaborations with him.

Elko:

So, when you were thinking about where you would go, were you worried?

Sessler:

That was a factor, of course.

Elko:

Was it? That you had somebody there that had some background in acoustics…?

Sessler:

Yes, who was in the same field of acoustics. Somebody with whom I could have discussions on fields of joint interest, such as electro-acoustics.

Elko:

Yeah. And you were connected… Being one of Meyer’s prized students, you were connected into the whole German academic system, so it would be easier maybe for you to find openings for your students, because people would be looking for you, as it were, and telling you, “hey,” leads to look here, look there.

Sessler:

Yes, well, I had an offer from UCLA, although that was actually quite a bit earlier. I left Bell Labs in 1975 for Darmstadt. But the offer from UCLA came perhaps in 1970. But at that time, I felt it was too early to go to a university.

Elko:

If they had offered you four years later, you might have gone to L.A.?

Sessler:

I might have considered it, yes.

Elko:

So, when you went back to the university, obviously you got some students. So, what were the kind of projects or areas you were trying to do work in?

Sessler:

Of course, I continued with projects in electroacoustic, materials research, and room acoustics. That was still interesting. And we looked, for example, at some other electroacoustic transducer materials like PVDF, the piezoelectric polymer. One of my first students, Reinhard Lerch, wrote his nice dissertation about PVDF transducers. The same with Reimund Gerhard, who was also one of my early students. He looked at some other materials, and he wrote a comprehensive dissertation about that topic. But we also continued with electret research. Heinz von Seggern was actually my first student. He started in 1976 and finished his dissertation on the properties of FEP Teflon electrets in 1979. He did very interesting work with thermally stimulated currents, and he found, for instance, the difference between surface traps and volume traps in this material. So, that was another field which we still pursued. We looked at electret microphones, tried to do some improvements on these. Rudolf Zahn, another of my early students, looked at this. Thomas Andert studied some problems of room acoustic diffusion. He specifically considered the acoustics of our reverberation chamber. So, these were some of the topics we pursued during the first years of my time in Darmstadt. Later on, of course, we got into some other transducer work, acoustic arrays. Jens Meyer was one of the students who pursued this. He worked on beamforming arrays and finished his dissertation in 2001.

Elko:

Carsten Sydow?

Sessler:

Yes, right. He was earlier, in 1996, and worked on self-steering microphone arrays.

Elko:

He did some nice work, too, I think I remember.

Sessler:

Yes.

Elko:

You got some really very good students that worked for you, that worked with you. And varied, too.

Sessler:

Well, after the first years in Darmstadt, there was a big project that we developed in the early 1980s on silicon microphones, or, as they are called now, MEMS (Micro Electro Mechanical Systems) microphones, which we have not mentioned so far. Many of my students worked in this field. Dietmar Hohm started this activity and he described and constructed the first MEMS capacitive microphone worldwide in 1983.

Elko:

So, what was the interest in the silicon mic? I mean that’s…

Sessler:

Well, there were no silicon mics before we started. There were other silicon devices.

Elko:

Well, technically Sikorsky’s was a silicon mic, right? [Laughs]

Sessler:

But it was not an all-silicon mic. He used a transistor and he put a diaphragm on this, which was not silicon, the stylus which was not silicon. So, we came up with the first all-silicon mics in Darmstadt in 1982 or ’83.

Elko:

So, what was the first student that started working on that? Was that Hohm?

Sessler:

It was Dietmar Hohm, yes. I read in the literature about silicon sensors like pressure sensors, accelerometers. There were publications on devices of this kind, and I asked myself, “Well, why not a silicon microphone?” So, I asked Dietmar Hohm to get involved in this, and we came up with the first paper in 1983 describing a single-chip electret-type silicon microphone. It was the first all-silicon microphone suggested worldwide.

Elko:

A single chip? This was not the dual… the flip one?

Sessler:

It was a dual-chip microphone, a membrane chip and a backplate chip bonded together. As we heard this morning in this paper by Yasuno, he has now a single-chip electret silicon transducer. So, it took quite some time to come up with this because in between they got away from the electret biasing. They used external biasing. Of course, you need only 10 volts, so biasing is not that much of a problem. But still, if you can do away with it, it’s better.

Elko:

The microphones that we’ve worked with use less than 9 volts. They use like 6 volts, 4 volts.

Sessler:

So, the first silicon microphone which we made was a silicon condenser microphone. At the same time, there was in the literature a description of a silicon piezoelectric microphone. I think both papers were in 1983, the same year.

Elko:

And who… That was not from your group; that was another group.

Sessler:

Yes, that was from another group. The piezoelectric microphone was from another group.

Elko:

Do you remember who?

Sessler:

Royer from Honeywell. But eventually, of course, the silicon condenser microphone was the one that was used and produced, starting in about 2002.

Elko:

Yeah, and now as we know, this is now becoming a huge success… They’re selling in huge quantities.

Sessler:

Yes, they are making a few hundred million — actually, a few billion annually now.

Elko:

Hundreds of millions per year and growing rapidly. Well, that’s amazing. So, what’s the next transducer we need to start looking at, Gerhard? [Laughter]

Sessler:

It’s a piezoelectric electret transducer, which I described in the meeting this morning, using cellular electrets that show a piezoelectric effect different from the classical piezoelectric activity. Different in the sense that the piezoelectricity is not due to a microscopic crystalline lattice, but due to a dipolar polarization of macroscopic, millimeter-size cavities in the cellular material. We suggested in 2004 to call it the “piezoelectret” microphone.

Elko:

You think so?

Sessler:

Well, there is a question whether it’s being made commercially now [in 2010]. So, we had the electret microphone in 1962. It took about six years until it was produced commercially, in 1968. We had the silicon microphone. It took about 20 years, from 1983 until 2002. Now, with the piezoelectret microphone, I hope it will not again take 20 years. This morning we heard from Yasuno that by hearsay there are already some commercial production.

Elko:

Yeah, I’m sorry I missed your talk.

Sessler:

We don’t know the company.

Elko:

No? What company is it?

Sessler:

Yasuno didn’t tell us the company. It’s still secret. It’s confidential. It’s a Japanese company that makes it. You have to ask Yasuno. He knows it, but he won’t tell us the company that makes it.

Elko:

Interesting. Okay.

Sessler:

So, we hope, of course, having suggested this first in a publication together with Joachim Hillenbrand, and then in the dissertation of Reiner Kressmann, this microphone with the cellular piezoelectret is sufficiently advanced…

Elko:

Oh, so Yasuno presented this in today’s talk…

Sessler:

No. Yasuno presented this morning in his talk a silicon electret microphone.

Elko:

But he talked more privately about the piezoelectret one? I missed his talk.

Sessler:

He told me last night at the social hour that somebody is producing the piezoelectret microphone. I asked him again this morning, but he said he cannot disclose the name of the company. So, we’ll see. If it comes about, we’ll be happy about it because we described it first.

Elko:

Absolutely. I’ll be interested to keep an eye on it. They’re not shipping it, I guess.

Sessler:

So, about every 20 years, something new comes up. The electret microphone in 1962, the silicon microphone in 1983, and the piezoelectret (cellular) — well, first described in 2004.

Elko:

Why do you think the piezoelectret will, say, outperform the MEMS? What’s the advantage of that?

Sessler:

Well, the advantage is that it’s very simple. It doesn't need an air gap. The air gap is built into the film. So, a microphone which consists just of a film, period, that’s it. Of course, you need some housing. You need some contacts. You need some shielding. But the microphone as such is just a piece of film.

Elko:

But you need an impedance converter as well.

Sessler:

No, not for air.

Elko:

No, no, I mean from the electrical side. Its output impedance is still pretty high, so you still need an…

Sessler:

Well, the same with the electret microphone. It’s no different from the electret and the MEMS microphones in that respect.

Elko:

So, the idea will be this will be very cheap then, I guess. Is that right?

Sessler:

Yes.

Elko:

What other properties? The signal-to-noise ratio? Is that good? Is it controlled?

Sessler:

We measured the A-weighted noise level and it’s 26 dB.

Elko:

It’s a big one, though. How big a piece was that?

Sessler:

It’s about a centimeter in diameter. Of course, if you make it smaller, noise will go up, but the noise is still amplifier noise.

Elko:

Is it?

Sessler:

Yes. The best amplifiers we could get had a noise level of about 3 microvolts A-weighted.

Elko:

Yeah, that’s probably about right. There were some other general questions. At your university, I guess you played some advisory roles there as well, probably in the administration and whatnot. Or you weren't interested in that? You were interested in science and your students, right? You didn’t play a big role there?

Sessler:

Yes, well, I did some administrative work. At the university, of course I was dean of the department twice, and I was on some committees, of course. You have to do that at a normal university.

Elko:

Yeah, but it wasn’t something you aspired to, right?

Sessler:

No, no, I didn’t. I just did it because we had to assume such responsibilities occasionally.

Elko:

You were just doing it as a good citizen.

Sessler:

Right, yeah. Of course, outside the university I had also some responsibilities. I was chairman of the first German acoustics association, the DAGA, for six years, which was from 1984…

Elko:

From the beginning of DAGA?

Sessler:

No, the first chairman was Kurtze, and then I followed him in 1984 until 1990. Then of course, the German Acoustics Society, the DEGA, was founded. I was one of the founding members and the first vice president, because, at that time, I was still president of DAGA, so I could not be both. [Laughter] So, I was vice president of the DEGA. Then, later on, DEGA assumed all the responsibilities because it was now the German Acoustics Society.

Elko:

So, now has that morphed into a broader acoustics society like the European… Is that the EAA?

Sessler:

Well, the DEGA is…

Elko:

It’s still German.

Sessler:

…one of the members of the EAA, one of the member societies.

Elko:

Okay. So, you still have DAGA meetings.

Sessler:

We have DAGA meetings. The meetings are still called DAGA meetings.

Elko:

And they’re always held in Germany, or they’re held anywhere in Europe, or…?

Sessler:

Occasionally somewhere else in Europe. We had meetings in Vienna, in Paris, in Rotterdam last year.

Elko:

But the EAA, does it have its own big meeting as well then?

Sessler:

It has an annual meeting, yes.

Elko:

Is it large? Is it like the size of the ASA?

Sessler:

It’s not that large. It’s about as large as the DAGA meetings. DAGA meetings nowadays have about 1,200 people in attendance.

Elko:

Really?

Sessler:

Yes, they are very large meetings.

Elko:

That’s huge! That’s bigger than ASA, or at least the same size.

Sessler:

Well, it’s about the same size. But it’s only once per year. The ASA is twice per year.

Elko:

Do you want to say anything else about, for instance, things you helped develop, like professionally in the societies or in university? I mean chairman of DAGA for six years is pretty impressive. Was there a six-year limit? Was there a term limit?

Sessler:

Well, I was elected for three years and then reelected again for another three years, and six years was the limit, yes.

Elko:

And you received some nice medals along the way, I think, right? Maybe you want to say something about… Maybe people from the ASA will know that you’re the winner of an Interdisciplinary Silver Medal in engineering acoustics and in physical acoustics, and, later on, the winner of the ASA Gold Medal.

Sessler:

Well, one of the most important medals I’ll receive next week. That’s the Benjamin Franklin Medal in Electrical Engineering in Philadelphia. I will receive this together with Jim West for our work on the electret microphone, so that’s a very big honor. Jim and I were also elected into the National Inventors Hall of Fame in 1999. That was also one of the highlights, of course, in my life.

Elko:

Yeah, that’s great!

Sessler:

And I received a few other medals like the Helmholtz Medal of the DEGA, the German Acoustic Society, which is their highest medal, their highest award, and also the George R. Stibitz Award from AT&T, for instance, which is a patent medal.

Elko:

It’s funny that they would give you a medal for a patent that they kind of bungled, right? [Laughs]

Sessler:

Yeah. Well, we eventually got some patents, you know. They were not worth as much as they could have been.

Elko:

Yeah. I think I remember hearing Jim say this once, and maybe you said it, too. It was like if he could have a penny for every electret that was sold, he would be a wealthy man!

Sessler:

Two billion, you know. [Laughter] Two billion pennies per year.

Elko:

Pretty wealthy. Yeah, that’s a lot of pennies. Exactly. So, I think also you were inducted into the New Jersey Inventor Hall of Fame as well, right?

Sessler:

Yes, right. I got some other awards, like the Rhein Prize in Germany, which is the highest prize in Germany for electrical engineering.

Elko:

Was that recent? When was that?

Sessler:

It was three years ago in 2007.

Elko:

Okay. Yeah, I do remember that. Good. So, if you’re okay with that, maybe we could talk a little bit about your family. I think there are some things about your family, your wife, your children, and your grandchildren, things like that. There are some suggestions about writing, about books or articles that you were fond of, and then some history of your lab… Well, you already talked about your students, but your lab. So, maybe you want to talk a little bit about your background and from your family…

Sessler:

Yes. Well, of course I talked about my parents already. That’s already covered. My family: my wife and I have been married for 49 years now, I think. We got married in 1961. Her name is Renate, and we have three children aged between 35, I think, and 43. Two daughters and one son. My son is also an electrical engineer. He works for the European Space Agency, ESA, on the further development of their system of ground stations. My two daughters, the older one is an orthodontist. She has a practice in the same city I live in. The younger one is a biologist and pharmacist, and she works on some regulatory aspects concerning the products her company sells. Thus, both daughters are professional people. Well, we live in Darmstadt and in the neighboring city of Wiesbaden. I guess that’s the things which I wanted to say about my family.

Elko:

But they’ve been very supportive of you, right?

Sessler:

Yes, they have been very supportive.

Elko:

And your grandchildren as well.

Sessler:

Yes, I do have five [as of 2021, now seven] grandchildren. Particularly Renate, my wife, always supported me very much, which is of course very important. If you want to achieve something, you have to work for endless hours. [Laughter]

Elko:

So, you met Renate in Göttingen?

Sessler:

I met her in Göttingen, yes, while I was still a student.

Elko:

She was a student?

Sessler:

Yes. When I went to the States in 1959, she also came to the States the following year. She went to New Orleans, to stay with her sister’s family and also to work in this city. But we already had decided at that time that we would get married, and so we got married in Summit, New Jersey.

Elko:

Oh, I didn’t know that.

Sessler:

Well, the marriage was in New York City.

Elko:

Oh really? Where in New York?

Sessler:

In Riverside Church.

Elko:

Really? Was it a big ceremony? Did you have a lot of people come from…

Sessler:

It was a nice ceremony. We had some people from Bell Labs there.

Elko:

That’s cool! I didn’t know that.

Sessler:

It was soon after I came to this country, a little more than a year, a year-and-a-half after that. We did not know so many people whom we could invite, but it was a nice party.

Elko:

Wow, Riverside is a big church, too.

Sessler:

Yes, well, we didn’t fill it. The ceremony was in the chapel. There’s a chapel right next to it. It actually belongs to the church. And we invited a few friends to the marriage ceremony, from Bell Labs, from Germany, the States, and Europe. From Bell Labs, Manfred Schroeder was invited. He was actually my best man. Also, Ed David, our director who later became the science advisor of President Nixon. Also present was an old friend from Austria, Herwig Kogelnik, who is a well-known laser specialist and the later director of Photonics Research at the Labs. Also, Bishnu Atal, an authority in speech research, and a few more people from the Labs, all with their wives.

Elko:

So, I think there were a few additional things that I suggested to discuss.

Sessler:

Well, perhaps I should mention a few of the colleagues at Bell Labs. Manfred Schroeder, whom I mentioned several times already, was our first department head and later on our director of the Lab, and he was a very close friend of mine. He went back to Göttingen in 1969 to succeed Erwin Meyer as director of the Physics Institute of the university. He unfortunately died a couple of months ago. Jim Flanagan, who will be in Philadelphia at the meeting tomorrow and give one of the talks there, pretty much succeeded Manfred after Manfred left in 1969.

Elko:

At the awards ceremony tomorrow, he’s giving a talk for you?

Sessler:

Yes, he is.

Elko:

At your symposium, right?

Sessler:

At the symposium, yes.

Elko:

It’s a very nice talk, by the way. I’ve read it.

Sessler:

Bishnu Atal was one of the colleagues at Bell Labs with whom we had close relations. We collaborated on the Philharmonic Hall project, room acoustics project, as mentioned already. Then there were many other people at the Labs whom I remember like Ben Logan, for instance, Doug Carroll. These are people who helped me with many projects. Ben Logan assisted us with the mathematics, of course. Doug Carrroll with the multi-dimensional scaling, which we used for some of our evaluations in room acoustics. Then, Bernhard Gross I should mention, who was not with Bell Labs but who was a consultant. We hired him as a consultant, and he was at Bell Labs many, many times over the years, from about 1972 to the early 1980s, when I was already in Darmstadt, but still visiting annually, also as a consultant. Later on, I invited him also to Darmstadt. We co-authored many papers together. We had projects on electrets, on charging with electron beams, and on charge retention in the thin films. These were the main topics of our work with him. So, he died about five years ago at a very high age. He was already retired when he visited us at Bell Labs and in Darmstadt in those days. We decided that he could still contribute quite a lot with his knowledge in physics and with his experience, and this is what actually happened. So, the papers which we published with him, some of these are really still very relevant and cited quite a lot.

Elko:

Yeah. I think Jim speaks really highly of that. He played a very nice role with you and had some very good insights and helped you understand some electret story, some of the physics of that. … I guess they’re asking about if any historians helped document your work. Are there records of your work, I guess maybe in your office?

Sessler:

There were a few notebooks at Bell Labs. Of course, I left these at the Labs. But we never had very extensive entries into these notebooks. Once in a while you’d put something in, you know, but we didn’t use them that much. Of course, the major documents are the publications we have, because we believed more in publications.

Elko:

Yeah, which is the best way, to publish it.

Sessler:

Yes, we published.

Elko:

So everyone can have access.

Sessler:

Of course, there are patents around. But I think the major sources are the publications. The patents are often more confusing than clarifying.

Elko:

Okay. Is there anything else we should discuss?

Sessler:

Oh, about documentation: perhaps I should mention in Germany we had an annual report of our institute, which is in German of course, but many of the things are documented there.

Elko:

Yeah, what the group did while you were there, while you were leading the group.

Sessler:

Yes, what the group did, what we did in the institute.

Elko:

So, is it a couple of pages or is it a paragraph?

Sessler:

Well, no. These are annual reports which are about 50 pages each year, 50 or 100 pages, where all the talks we gave are documented, all the papers and reports we published, all the student-lectures we presented, all the outside talks we gave, and all the visitors we had. Last, not least, we described in these annual reports in detail all the projects we were working on during the year.

Elko:

Where are those stored?

Sessler:

These reports were sent to people who were interested in them. We sent out perhaps 200 copies each year to other institutes and we kept copies at the Institute.

Elko:

In the library?

Sessler:

In our Institute library, yes.

Elko:

I think they’re asking that in case a historian is interested in digging some more, right?

Sessler:

Yes, they could find these. They would have to contact the institute, and they would be available.

Elko:

But like you said, the publications speak for themselves. If you want to see what day-to-day, some real background and how things went, maybe they could document it that way.

Sessler:

Yes.

Elko:

Is there anything else you would like to say, anything that we left out? Did we miss something? We can continue the conversation as well, later. We can always add to this. But is there anything…?

Sessler:

I think we have covered most relevant aspects. I can’t think right now of anything else that would be important in this context here.

Elko:

So, it says here also to ask if you can suggest somebody else to interview for this project.

Sessler:

Well, many of the people I know like Jim Flanagan or Manfred Schroeder had these, of course. Also, Jim West? [If you check Google you find that he had many interviews.] Who is in charge of the oral history project?

Elko:

At this point, it belongs to Jay Maynard. I’ll ask Jay. My apologies for taking so long. I think I signed on to do this how many years ago now, Gerhard? Maybe three years ago or so? My apologies for taking…

Sessler:

Three years [and I delayed it for several years thereafter by not correcting the transcript].

Elko:

I know. We’ve had opportunities in the past. It’s just that we didn’t have the time to sit down and do this. So, I appreciate it. If there’s anything that you want to discuss, I’ll be in Germany in the not-too-distant future. I can bring my recorders. We can add to it if you think of something. Clearly, I understand they’re going to give you a transcript of this, so you’ll be able to correct it and fix things that aren’t right or omit things or add things, I think, there as well.

Sessler:

I’m sure many of the names have to be corrected because they’re probably not known to the person who does the transcript.

Elko:

Yeah, exactly, how to spell their names. I don't know how long that takes. I really don’t know. But somebody has to transcribe this discussion, and then you’ll get a chance to look it over. Like I said, if at any opportunity you feel that there’s something we glossed over and you thought, “Oh, I should have said that,” just let me know and we’ll pick up where we left off.

Sessler:

Thank you very much, Gary, for taking this interview. I enjoyed your questioning and I think we have covered many of the most important aspects of my life. Thank you cordially again.

Elko:

All right. Bye bye.

[End of recording]