A source of sound pulses, a measuring tape, a sound receiver, and a stop watch.
Procedure:
(i) Measure the distance "d" between the source of sound pulse and the reflecting surface using the measuring tape.
(ii) The receiver is also placed adjacent to the source. A sound pulse is emitted by the source.
(iii) The stop watch is used to note the time interval between the instant at which the sound pulse is sent and the instant at which the echo is received by the receiver. Note the time interval as "t".
(iv) Repeat the experiment for three or four times. The average time taken for the given number of pulses is calculated.
Calculation of speed of sound:
 (i) The sound pulse emitted by the source travels a total distance of 2d while travelling from the source to the wall and then back to the receiver.
(ii) The time taken for this has been observed to be "t", Hence, the speed of sound wave is given by
Speed of sound \(=\frac { Distance\ travelled }{ Time\ taken } \)
\(\\ =\frac { 2d }{ t } \)

(a) Audible waves:
(i) Sound wave with frequency ranging between 20 Hz and 20,000 Hz.
(ii) These are generated by vibrating bodies such as vocal cords, stretched strings etc.
(b) Infrasonic waves:
(i) Sound wave with frequency below 20 Hz that cannot be heard by human ear.
(ii) E.g.: waves produced during earth quake, ocean waves, sound produced by whales, etc.
(c) Ultrasonic waves:
(i) Sound wave with frequency greater than 20 kHz.
(ii) Human ear cannot detect these waves, but certain creatures like mosquito, dogs, bats, dolphins can detect these waves.
(iii) E.g.: waves produced by bats.

The velocity of sound waves is maximum in solids because they are more elastic in nature than liquids and gases.
Velocity = \(\frac{Distance}{Time Taken}\)
If the distance traveled by one wave is taken as one wavelength (A) and, the taken for this propagation is one time period (T), then, velocity can be written as
\(v=\frac { \lambda }{ T } \quad ....(1)\)
Therefore, velocity can be defined as the distance traveled per second by a sound wave. Since, Frequency (n) = 1/T, equation (1) can be written as
v = nλ

(i) Some animals communicate with each other over long distances and also locate objects by sending sound signals and receiving the echo as reflected from targets.
(ii) The principle of echo is used in obstetric ultrasonography, which is used to create real-time visual images of the developing embryo or fetus in the mother's uterus. This is a safe testing tool, as it does not use any harmful radiations.
(iii) Echo is used to determine the velocity of sound waves in any medium.