BACKGROUND: Physical oceanographer Jeff Nyustuen is giving scientists and managers a way to sift through and identify the sounds present in various marine ecosystems. Passive Aquatic Listeners (PALS) are devices that sink ten to thousands of meters below the water surface and are set to listen for a few seconds every few minutes. PALs can identify sounds coming from such things as ships, whales, volcanic eruptions, rainfall and breaking waves. The result is a record of all the noise and its intensity in the ocean environment, which can help biologists sort out what levels of noise go unnoticed, or can cause harm to marine mammals, for example.
HOW IT WORKS: PALs don't try to record every single sound in the ocean. That would take too much memory. Instead, Nyusten is developing software that allows the PALs to sift through the racket, identify and sort sound sources by frequencies as they are received.
ABOUT SOUND: Sound waves are pressure waves: the result of a vibrating object that creates a disturbance in the surrounding air. For instance, when the telephone rings, the ringer vibrates very quickly, sending energy radiating outward through the air. These vibrations disturb the molecules that make up the air. The air molecules push closer together as the object moves one way ý an effect known as compression -- and then create a space between themselves and the vibrating object as it moves the other way, called rarefaction. The motion disturbs the neighboring molecules in turn, creating an outward ripple effect, much like a stone cast in a quiet pond will cause waves to ripple outward from the spot where the stone hit.
WHAT'S YOUR FREQUENCY? All sound waves have wavelength and frequency. The distance between compressions determines the wavelength. Objects that vibrate very quickly create short wavelengths because there is very little space between the compressions, creating a high-pitched sound. Objects that vibrate very slowly create long wavelengths because the compressions are spaced further apart. This creates a low-pitched sound. Frequency measures how many crests, or compressions, occur within one second; the measurement of this speed of vibration is called a Hertz, and 1 Hertz is equivalent to 1 vibration per second. Pitch simply means those frequencies within the range of human hearing (from about 20 Hertz to 20,000 Hertz). The faster the rate of vibration, the higher the pitch; the slower the rate of vibration, the lower the pitch.
SOUND SENSE: Bats emit a series of ultrasonic pulses that bounce off objects in its environment. How long it takes for the sound to be reflected back to the bat indicates how close (or far) a given object might be, enabling the bat to orient itself as it flies, and to detect food. Modern sonar technology is based on the same principle. The more feedback the bat receives, in terms of incoming reflections, the more accurately it can pinpoint a given object's location That's why the rate of the ultrasonic calls increases as the bat nears its prey, climaxing into a "feeding buzz" as the bat locks in on its target and prepares to strike. In contrast, whales appear to use sounds (or "songs") to communicate, emitting a complex sequence of low moans, high squeals and clicking noises that can last as long as 30 minutes. The songs appear to be related to mating cycles.
STOP THAT RACKET: Noise cancellation tries to block the unwanted sound at its source, rather than merely trying to prevent it from entering our ears. If we add two waves together, and the peaks of one line up with the valleys of the other, they will cancel each other out. Digital signal processors (DSPs) are microelectronic devices that determine which sound wave is required to cancel the unwanted sound wave (noise). It then creates that sound and amplifies it through speakers or headphones. The end result is near silence. Most cell phones, CD players, and hearing aids now contain one or more DSP devices.