Mention one practical use of echoes.

The rear view mirror of a motorbike starts vibrating violently at a particular speed of the motorbike.
$(a)$ Why does this happen?
$(b)$ What is the name of the phenomenon taking place?
$(c)$ What could be done to stop the violent vibration?

What do you understand by the term ultrasonic vibrations?

State three factors on which the speed of sound in a medium depends.

State the relationship between frequency and time period of a wave. The wavelength of vibrations produced on the surface of water is $2 \, cm$. If the wave velocity is $16 \, m s^{-1}$, find its frequency and time period.

Give reasons for the following:
$(a)$ The reverberation time of a hall used for speeches should be very short.
$(b)$ $A$ vibrating body produces sound. However,no sound is heard when a simple pendulum oscillates in air.
$(c)$ Sounds of same loudness and pitch but produced by different musical instruments like a violin and flute are distinguishable.