Sound Waves

•    Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving.
•    If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it.
•    The motion of the particles is parallel (and anti-parallel) to the direction of the energy transport. This is what characterizes sound waves in air as longitudinal waves.
•    The mechanical vibrations which can be said as sound are able to travel through all forms of matter i.e. solids, liquids and gases. The matter which allows the sound to travel through it is called the medium.
•    Sound cannot travel through a vacuum.

Equipment for dealing with sound

Some of the equipment for generating sound: musical instrument, sound box, hearing phones, sonar systems, sound reproduction, and broadcasting equipment. Many of these use electro-acoustic transducers for input: microphones.

Sound Wave Properties

All waves have certain properties. The three most important ones for audio work are shown here:

Wavelength: The distance between any point on a wave and the equivalent point on the next phase. Literally, the length of the wave.

Amplitude: The strength or power of a wave signal. The "height" of a wave when viewed as a graph.

Higher amplitudes are interpreted as a higher volume, hence the name "amplifier" for a device that increases amplitude.

Frequency: The number of times the wavelength occurs in one second. Measured in kilohertz (Khz), or cycles per second. The faster the sound source vibrates, the higher the frequency.

Higher frequencies are interpreted as a higher pitch. For example, when you sing in a high-pitched voice you are forcing your vocal chords to vibrate quickly.

Speed of Sound

•    Speed of sound is different in different mediums. In a medium the speed of sound basically depends upon elasticity and density of medium.
•    Speed of sound is maximum in solids and minimum in gases.
•    When sound enters from one medium to another medium, its speed and wavelength changes but frequency remains unchanged. In a medium, the speed of sound is independent of frequency.

Sound in different material mediums

Speed of Sound
60 m/s
Air at 40oC
355 m/s
Air at 20oC
343 m/s
1210 m/s
3240 m/s
4540 m/s
4600 m/s
6320 m/s
Carbon dioxide
260 m/s
Steam at 1000C
405 m/s
1213 m/s
1269 m/s
1450 m/s
Water at 20oC
1482 m/s
Sea Water
1533 m/s
3560 m/s
5130 m/s
6000 m/s

Effect of Temperature on Speed of Sound- The speed of sound increases with the increase in temperature of the medium through which it is traveling. The speed of sound in air increases by 0.61 m/s when the temperature raises by 1 degree Celsius.

Effect of Humidity on Speed of Sound- The speed of sound is less in dry air and more in humid air, this owes to density; the humid air is denser than dry air.

Echo- The sound waves received after being reflected from a high tower or mountain or any other obstacle is called echo.

Characteristics of Echo

•    To hear echo the minimum distance between the observer and reflector should be 17 m or more precisely 16.6 m.
•    Persistence of ear i.e. 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 very large amplitude. This phenomenon is called Echo.

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 called interference of sound.

If two waves meet at a point in same phase, intensity of sound is maximum at that point. Such type of interference is called constructive interference. Similarly, when 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.

Diffraction of Sound- Wavelength of sound is of the order of 1m. If an obstacle of that range comes in the path of sound, sound deviates at the edge of obstacle and propagate forward. This phenomenon is called diffraction of sound.

When the distance between the source and observer decreases the apparent frequency increases and vice versa.
Mach Number- The Mach Number is defined as the ratio of speed of sound source to the speed of sound in the same medium under the conditions of temperature and pressure.

If Mach Number is > 1, body is called supersonic.
If Mach Number is > 5, body is called hypersonic.
If Mach Number is < 1, the body is said to be moving with subsonic speed.

Shock Waves-  When the speed of a source exceeds the speed of sound (v > c) the wave fronts lag behind the source in a cone-shaped region with the source at the vertex. The edge of the cone forms a supersonic wave front with an unusually large amplitude called a "shock wave". When a shock wave reaches an observer a "sonic boom" is heard.

Unlike ordinary sound waves, the speed of a shock wave varies with its amplitude. The speed of a shock wave is always greater than the speed of sound in the fluid and decreases as the amplitude of the wave decreases. When the shock wave speed equals the normal speed, the shock wave dies and is reduced to an ordinary sound wave.

Bow Waves- The Bow Wave is a progressive disturbance propagated through a fluid such as water or air as the result of displacement by the foremost point of an object moving through it at a speed greater than the speed of a wave moving across the water. Viewed from above, the crest of the bow wave of a moving ship is V-shaped; the angle of the V is determined by the relative speeds of the ship and of the propagation of waves in the water.In three-dimensional space-for example, when describing the wave produced by a plane flying at supersonic speed-the bow wave is conical in shape. In this case an observer on the ground experiences a sonic boom when the bow wave passes. See also wave motion.

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