##### Physics
General Knowledge

# Characteristics of Sound waves

## Characteristics of Sound waves

Sound waves have the following three Characteristics :
1. Intensity
2. Pitch
3. Quality

## 1. Intensity

Intensity of sound at any point in space is defined as amount of energy passing normally per unit area held around that point per unit time. SI Unit of Intensity is watt/m2.

»   Intensity of sound at a point inversely proportional to the square of the distance of point from the source.
»   Intensity of sound at a point directly proportional to square of amplitude of vibration, square of frequency and density of the medium.
Due to intensity, a sound appears loud or faint to the ear. Actually, the sensation sound perceived in ear is measured by another term called loudness which depends on intensity of sound and sensitiveness of the ear. Unit of loudness is bel. A practical unit of loudness is decibel (dB) which of equal to 1 / 10th of bel. Another unit of loudness is phon.

## 2. Pitch

Pitch is that characteristic of sound which distinguishes a sharp (or shrill) sound from a grave (dull or flat) sound. Pitch depends upon frequency. Higher the frequency, higher will be the pitch and shriller will be the sound. Lower the frequency, lower will be the pitch and grave will be the sound.

## 3. Quality

Quality is that characteristic of sound which enables us to distinguish between sounds produced by two sources having the same intensity and pitch. The quality depends upon number, frequency and relative intensities of overtones.

## echo

The sound waves received after being reflected from a high tower or mountains is called echo. »   To hear echo, the minimum distance between the observer and reflector should be 17m (16.6 m).
»   Persistence of ear (effect of sound on ear) is 1 /10 sec.
»   Due to refraction, sound is heard at longer distances in nights than in day.

## Resonance

If the frequency of imposed periodic force is equal to the natural frequency of a body, the body oscillates with a very large amplitude. This phenomenon is called resonance.

## Interference of sound

The modification or redistribution of energy at a point due to superposition of two (or, more) sound waves of same frequency is ca e interference of sound.
If two waves meet at a point in same phase, intensity of sound is maximum, Such type of interference is called constructive interference. similarly two waves meet at a point in opposite phase, intensity of sound at that point is minimum. Such type of interference is called destructive interference.

## Stationary wave

When two progressive waves of same type (i.e. both longitudinal or both transverse) having the same amplitude an same time period / frequency /wavelength travelling with same speed along the same straight line in opposite directions, superimpose, a new set of waves are formed which are called stationary of standing waves.

## Diffraction of sound

Wavelength of sound is of the order of 1 m. If an obstacle of that range appears in the path of sound, sound deviates at the edge of obstacle and propagates forward. This phenomenon is called diffraction of sound.

## Doppler's Effect

If there is a relative motion between source of sound and observer, the apparent frequency of sound heard by the observer is different from the actual frequency of sound emitted by the source. This phenomenon is called Doppler's effect.
When the distance between the source and observer decreases, the apparent frequency increases and vice-versa.

## Mach Number

It is defined as the ratio of speed of source of sound to the speed of sound in the same medium under the same condition of temperature and pressure.
»   If Mach number > 1, body is called supersonic.
»   If mach number > 5, body is called hypersonic.
»   If mach number < 1, the body (source) is said to be moving with subsonic speed.

## Shock waves

A body moving with supersonic speed in air leaves behind it a conical region of disturbance which spreads continuously. Such a disturbance is called shock wave. This wave carries huge energy and may even make cracks in window panes or even damage a building.

## Bow Waves

When a motor boat in a sea travels faster than sound, then waves just like shock-waves are produced on the surface of water. These waves are called bow waves.