Whenever there is a relative motion between a source and a listener, the frequency of sound heard by the listener is different from the original frequency of sound emitted by the source. This is known as "Doppler effect".

The velocity with which the particles of the medium vibrate to transfer energy in the form of wave is called particle velocity.

Wave length (A) is the distance between two consecutive compressions or two consecutive rarefactions.

Architectural acoustician is a person who designs concert halls.

The minimum distance required to hear an echo is 1/20^th part of the magnitude of the velocity of sound in air.
i.e.  \(\frac{1}{20} \times 344 = 16.7m\)

If the particles of the medium vibrate along the direction of propagation of the wave, then the waves are said to be longitudinal.

(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.

The speed of light is so phenomenal that it will render its contribution to the time taken negligible so
Distance = Velocity x time
= 330 x 9.8
Distance = 3234 m

(i) During rainy days, the moisture content is more in the atmosphere and speed of sound increases with increase in humidity.
(ii) Hence the sound travels faster on rainy days.

Factors affecting velocity of sound:
(i) The following factors affect the velocity of sound waves.
Effect of density:
(i) The velocity of sound in a gas is inversely proportional to the square root of the density of the gas.
(ii) Hence, the velocity decreases as the density of the gas increases.
\(V \propto \frac{1}{\sqrt{d}}\)
Effect of temperature:
(i) The velocity of sound in a gas is directly proportional to the square root of its temperature.
(ii) The velocity of sound in a gas increases with the increase in temperature. \(v \propto \sqrt{\mathrm{T}}.\)
(iii) Velocity at temperature T is given by the following equation:
\(\mathrm{v}_{\mathrm{T}}=\left(\mathrm{v}_{0}+0.61 \mathrm{~T}\right) \mathrm{m} \mathrm{s}^{-1}\)
(i) Here, v_o is the velocity of sound in the gas at \(0^{\circ} \mathrm{C}\).
(ii) For air, \(\mathrm{v}_{\mathrm{o}}=331 \mathrm{~m} \mathrm{~s}^{-1}\).
(iii) Hence, the velocity of sound changes by 0.61 ms^-1 when the temperature changes by one degree celsius.
Effect of relative humidity:
(i) When humidity increases, the speed of sound increases.
(ii) That is why you can hear sound from long distances clearly during rainy seasons.

(c) Assertion is true but reason is false.

b. If both the assertion and the reason are true but the reason is not the correct explanation of the assertion.