echo

Repetition of a sound wave, or of a radar or sonar signal, by reflection from a hard surface such as a wall or building. By accurately measuring the time taken for an echo to return to the transmitter, and by knowing the speed of a radar signal (the speed of light) or a sonar signal (the speed of sound in water), it is possible to calculate the range of the object causing the echo (echolocation).

The speed of sound can be measured using an echo. The time taken for the sound to be reflected back over a known distance is recorded. The speed of sound can then be calculated using the formula: speed = distance/time.

For an echo to occur, the wavelength of the incident sound must be smaller than the size of the object. Sound obeys the same rules of reflection as for light. If the wavelength of the sound is the same as or greater than the size of the object, then diffraction takes place.