Monday, 28 April 2014

Nature of Sound

Nature of Sound: Sound is a longitudinal wave motion. It has compressions and rarefraction in medium of travel. Air acts as medium for sound propagation. Sound follow simple Harmonic motion.

When sound wave strikes the ear drum, they are changed into Audio signals. These signals are carried by nerves to the Brain, where they are sensed as sound. Sound has three dimensional motion in air.
When source of sound i.e., tuning fork is gently hit by wooden hammer, a pure tone is generated. This tone remains for some time and gradually decreases.

Due to vibrations of tuning fork, compressions and rarefractions are produced in air. When they reach the Human ear, they cause sensation of sound. Observe that the direction of propagation is longitudinal. There are compressions and shown with closed vertical lines and rare fractions have been shown with vertical lines which are apart. The changes in pressure produced by compressions are shown by curve of continuous line, whereas pressure changes produced by rarefraction are shown in figure.

A wave has Amplitude, frequency, wave length, phase and velocity. Let us know about them.
Amplitude: It is the intensity of compression and Rarefraction produced in the medium. For a pure tone the intensity can be represented by a sine wave.

A cycle has compression (+ve side) and rare fraction (-ve side).

Frequency: The number of repeated compressions and rare fractions in one second is termed as Frequency. Unit of frequency (f) is cycles/second or Hertz (Hz).

Time Period: The time taken to complete one cycle of sound wave.
Time Period, T = 1/f Hz Seconds.

Wave Length: Wave Length is the distance travelled by the sound wave in one cycle. It is denoted by λ.      λ = V/f
V = Velocity of sound in meters / second.
f = Frequency in Hertz
λ= Wave length in meters

Velocity: Distance travelled by the sound wave in one second. The velocity of sound in space is 344 m/s at 20 degree Celsius. At 0 degree Celsius it is 332 m/s. It is clear that sound travels slowly when compared with light.

The relation between Velocity and Temperature can be given by,
V2 = V1 √(T2/T1)
V1 = Velocity at T1 Degree Kelvin
V2 = Velocity at T2 Degree Kelvin

So, Velocity of sound is affected by Temperature, but not by pressure.
Also, velocity of sound depends on density of Atmosphere.
V = √(rp/d)
V = Velocity in m/s
p = Pressure of gas in dynes / cm2
d = Density in gm/cm3
r = A constant of medium of propagation for air. It is 1.41.

So, we can observe from the above, velocity of sound is inversely proportional to density of medium.
Velocity of Sound is slow in gases.
Velocity of Sound is faster in liquids.
Velocity of Sound is fastest in Solids.

Phase: It gives instantaneous motion relative to some reference. Phase can be expressed in angle or wave length or time period.The phase angle shown above cvan be mentioned as 90 degree or T/4 or λ/4.

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