Sound Location

the determination of the bearing and position of an object from the sound field created by the object (passive location) or from the reflection by the object of sound generated by special devices (active location, echolocation).

Pulsed and continuous sound sources are used in echolocation. In pulsed echolocation the distance to the object is determined according to the delay of the reflected signal. In continuous echolocation a frequency-modulated signal may be used to determine the distance to the object according to the difference in frequencies between the transmitted and reflected signals. Passive location of noise-generating objects is done with highly directional sound receivers operating in a narrow frequency band or with the correlation method of reception (during operation with broad-band sources).

Sound location is widely used in the frequency range from infrasound to ultrasound, with propagation in air, earth, water, and metals. The infrasound frequencies (from fractions of a hertz to dozens of hertz) are used in locating earthquakes, in finding petroleum and gas deposits by seismic prospecting, and in long-range detection of ships in distress at sea (using explosive sound sources). The sonic and ultrasonic frequencies (hundreds of hertz to dozens of kilohertz) are used in operating sonar, directional hydrophones, and fathometers. Ultrasonic frequencies (hundreds of kilohertz and megahertz) are used in ultrasonic flaw detection.

Sound location by humans and animals. All living beings are capable of finding the bearing of a sound source; this capability is a result of the binaural effect. In the course of evolution some animals become capable of echolocation. Among such animals are bats, porpoises, and whales and some kinds of birds, such as the oilbird. Bats emit sound pulses of several milliseconds’ duration that contain high-frequency oscillations (10–150 kHz). Several species of bats (for example, horseshoe bats) emit pulses of almost pure tones; others emit broad-band pulses. The most common species emit frequency-modulated signals. For all species of bats the pulse repetition rate depends on the distance to the target, increasing from 10–20 Hz when far from the target to 250 Hz when close to the target. Dolphins emit squeaking sounds of a few milliseconds’ duration; here, too, the pulse repetition rate depends on the distance to the target (1-2 Hz to hundreds of hertz). Animals using sound location are capable of detecting weak useful signals against a background of interfering reflections and a multitude of similar signals emitted by other individuals.

To some degree, humans are also able to detect obstacles from a sound echo. Blind persons have been found to sense the approach to an obstacle by the reflected sound of steps or of a tapping cane.