Time period. The time taken by an oscillating body to complete one oscillation is called its time period. It is denoted by T. Its SI unit is second (s).
Frequency. The number of oscillations or vibrations completed by an oscillating body in one second is called its frequency. It is denoted by v (Greek letter nu).
SI unit of frequency=per second (s^-1) = cycles per second (cps) = hertz (Hz).
Relation between time period and frequency:
Let T=time period of an oscillating body. Then number of oscillations completed in T second = 1
Number of oscillations completed in 1 second =\(\frac { 1 }{ T } \)
But number of oscillations completed in 1 second = frequency (v)
∴ \(v=\frac { 1 }{ T } \)
Hence frequency is equal to the reciprocal of time period.

The sound waves having frequencies greater than 20,000 Hz are called ultrasonics.
Applications of Ultrasonic Waves:
1. In the treatment of muscular pains.
2. In the detection of defects or flaws in metal blocks.

1. Loudness is a physiological response of the ear to the intensity of sound. It distinguishes between a loud sound and a low sound.
2. Loudness depends on two factors:
(i) Intensity of sound which is directly proportional to the square of amplitude of the sound wave.
(ii) Sensitivity of the ear.

Frequency of the sound wave, v = 220 Hz
Speed of the sound wave, v = 440 m s⁻¹
For a sound wave,
Speed = Wavelength × Frequency
\(\begin{aligned} &v=\lambda \times \nu \end{aligned}\) 
\(\therefore \lambda=\frac{v}{\nu}=\frac{440}{220}=2 m\) 
Hence, the wavelength of the sound wave is 2 m.

When the school bell vibrates, it forces the adjacent particles in the air to vibrate. This disturbance gives rise to a wave and when the bell moves forward, it pushes the air in front of it. This creates a region of high pressures known as compression. When the bell moves backward, it creates a region of low pressure known as rarefaction. As the bell continues to move forward and backward, it produces a series of compressions and rarefactions. This makes the sound of a bell propagate through air.

When a wave propagates along the surface of a medium, it generally has both transverse and longitudinal components. When such a wave travels through the medium, its particles move either along a circle or an ellipse. Examples of such waves are:
(i) Seismic waves travelling along the surface of the earth during an earthquake or explosion.
(ii) Ocean waves in deep water.
(iii) Wave pulses produced by beetle's motion along the sand's surface.

Mechanical waves. The waves which require a material medium for their propagation are called mechanical waves. They are also called elastic waves because their propagation depends on the elastic properties of the medium.
Examples of mechanical waves:
(i) Sound waves in air.
(ii) Waves over water surface.
(iii) Waves produced during earthquake. These are known as seismic waves.

First we calculate time t taken by the stone to reach the water level in the well by using the relation, \(s=ut+\frac { 1 }{ 2 } gt^{ 2 }\)
But   \(s=44.1\quad m,\quad u=0,g=9.8\quad m/s^{ 2 }\)
∴  \(44.1=0+\frac { 1 }{ 2 } \times 9.8\times t^{ 2 }\)
or  \(t^{ 2 }=\frac { 44.1\times 2 }{ 9.8 } =9\)
or t = 3 s
Time taken by sound to reach the mouth of the well 't' = 3.13 - 3 = 0.13 
\(Speed\quad of\quad sound=\frac { Distance }{ Time } =\frac { 44.1\quad m }{ 0.13\quad s } =339.23\quad m/s.\)

The audible range of an average human ear lies between 20 Hz to 20,000 Hz. Humans cannot hear sounds having frequency less than 20 Hz and greater than 20,000 Hz.

The speed of sound (344 m/s) is less than the speed of light (3 x 10⁸ m/s). The sound of thunder takes more time to reach the Earth as compared to light. Hence, a flash is seen before we hear a rumble of thunder.

Sound needs a medium to propagate. Since the moon is devoid of any atmosphere, you cannot hear any sound on the moon.