Medical ultrasound machines are versatile devices that rely on high-frequency sound waves to produce pictures of organs, blood vessels, and fetuses. An ultrasound probe includes a sound source, which generates ultrasonic pulses roughly a thousand times higher in pitch than the highest pitch humans can hear, and a microphone to detect echoes of the sound pulses. The pulses travel through the body and are reflected by the boundaries between different types of material. In the nervous system, sound waves can travel through the solid matter and bounce off spinal fluid. In a fetal ultrasound, the pulses travel through the amniotic fluid and bounce off the developing infant's skin. A computer constructs an image based on the time it takes for the pulses to return to the probe.
In principle, the echolocation of bats and dolphins is much like an ultrasound imaging system, with squeaks and chirps taking the place of ultrasound pulses, and the animals' ears acting as echo detectors. Bats and dolphins effectively see the world around them with sound rather than light, just as an ultrasound machine helps us to see inside the body.
Sound is made of waves, similar to the waves on the surface of a body of water. The detail in an ultrasound image is determined by the length of the waves that the system uses. The waves that correspond to human speech, for example, range in length from about half an inch to more than fifty feet. The high-frequency chirps of a bat have wavelengths just fractions of an inch long, comparable to the sizes of the insects they catch.
Typical ultrasound waves are hundreds of times shorter than the wavelengths humans hear -- a few thousandths of an inch long -- and can produce images of structures as small as blood vessels. That's why short-wavelength ultrasound pulses can easily reproduce the detail of a developing child's face and fingers.