Ultrasonic Sound

The term "ultrasonic" applied to sound refers to anything above the frequencies of audible sound, and nominally includes anything over 20,000 Hz. Frequencies used for medical diagnostic ultrasound scans extend to 10 MHz and beyond.

Sounds in the range 20-100kHz are commonly used for communication and navigation by bats, dolphins, and some other species. Much higher frequencies, in the range 1-20 MHz, are used for medical ultrasound. Such sounds are produced by ultrasonic transducers. A wide variety of medical diagnostic applications use both the echo time and the Doppler shift of the reflected sounds to measure the distance to internal organs and structures and the speed of movement of those structures. Typical is the echocardiogram, in which a moving image of the heart's action is produced in video form with false colors to indicate the speed and direction of blood flow and heart valve movements. Ultrasound imaging near the surface of the body is capable of resolutions less than a millimeter. The resolution decreases with the depth of penetration since lower frequencies must be used (the attenuation of the waves in tissue goes up with increasing frequency.) The use of longer wavelengths implies lower resolution since the maximum resolution of any imaging process is proportional to the wavelength of the imaging wave.

Bats and Ultrasound

Bats use ultrasonic sound for navigation. Their ability to catch flying insects while flying full speed in pitch darkness is astounding. Their sophisticated echolocation permits them to distinguish between a moth (supper) and a falling leaf.

About 800 species of bats grouped into 17 families. The ultrasonic signals utilized by these bats fall into three main categories. 1. short clicks, 2. Frequency-swept pulses, and 3. constant frequency pulses. There are two suborders, Megachiroptera and Microchiroptera. Megas use short clicks, Micros use the other two. Tongue clicks produce click pairs separated by about 30ms, with 140-430 ms between pairs. (Sales and Pye, Ultrasonic Communication by Animals). 10-60 kHz in frequency swept clicks. One kind of bat, the verspertilionidae, have frequency swept pulses 78 kHz to 39 kHz in 2.3 ms. Emits pulses 8 to 15 times a second, but can increase to 150-200/s when there is a tricky maneuver to be made.

Cetacean Sound

Orcas produce a wide variety of clicks, whistles and pulsed calls. They vary in frequency from 1 to 25 kHz. Individual pods of whales have their own distinctive dialect of calls, similar to songbirds. Some such calls are known to be stable over a period of 10 years. Humpback whales produce a variety of moans, snores, and groans that are repeated to form what we might call songs. The frequency of these songs range from about 40 Hz to 5 kHz. Singing whales are usually solitary males who exhibit it in a shallow smooth-bottomed area where sound propagates well. They are interpreted as territorial and mating calls. Whales are also known to produce some very intense low frequency sounds which they may use to stun or disorient small fish for prey. Bottlenose dolphins produce sounds in the range 7 to 15 kHz which are continuously variable in pitch. In addition, they produce short burst from 20 to 170 kHz, presumably for better echolocation.

A dolphin's clicks come from small knobs near its blowhole. There are no vocal cords.

Ultrasonic Transducers

Ultrasonic sound can be produced by transducers which operate either by the piezoelectric effect or the magnetostrictive effect. The magnetostrictive transducers can be used to produce high intensity ultrasonic sound in the 20-40 kHz range for ultrasonic cleaning and other mechanical applications.

Ultrasonic medical imaging typically uses much higher ultrasound frequencies in the range 1-20 MHz. Such ultrasound is produced by applying the output of an electronic oscillator to a thin wafer of piezoelectric material such as lead zirconate titanate. The higher frequencies imply shorter wavelengths and therefore higher resolution for the imaging process. The application of the basic ideas of imaging (e.g., the Rayleigh criterion) suggests that the resolution of any imaging process is limited by diffraction to a dimension similar to the wavelength of the wave used for the imaging process.

Lead Zirconate Titanate (PZT)

The piezoelectric material which has had the widest application as an ultrasonic transducer for medical diagnostic applications is lead zirconate titanate, commonly referred to as PZT, a shortened form of the chemical symbols PbZrTi.