Sound is a Wave Motion:
There are two possible modes of transference of energy from the source to the ear. One is that the source of sound produces waves in the air and they strike our ears. The second is that from the sounding source, some particles are shot off which reach our ears. In the second method, the sounding body would gradually lose weight, which has never been detected. Consequently, the natural conclusion is that sound travels in the form of waves.
The view that sound travels in the form of waves is further supported by the following facts.
- Sound requires a medium.
- Sound travels with definite speed.
- Sound waves are reflected.
- Sound waves are refracted.
- Sound waves show interference.
- Sound waves are diffracted.
Sound waves do not show polarization effects, hence it is concluded that sound waves are longitudinal waves. During the propagation of sound waves, the molecules of the medium vibrate along the direction of propagation and cause pressure at a point to fluctuate about the average pressure with the frequency of the wave. Infact sound waves are the result of a pressure disturbance in an elastic medium.
The average human ear is sensitive to detect pressure variations in the frequency range from 20 Hz to 20 kHz, which roughly defines the range of the audible spectrum. The pressure waves having frequencies in between 20 Hz and 20 kHz are, therefore, called sonic waves. Although human ears are not capable of detecting pressure waves of frequencies below 20 Hz and above 20 kHz, these waves can be detected with the help of instruments. Pressure waves having frequencies below 20 Hz are called infrasonic waves and waves above 20 kHz are called ultrasonic waves.
The specialized study of sound waves is called acoustics. Acoustics is a large field of study and includes such topics as noise, musical scale, acoustics of buildings, pitch and loudness, Doppler effect, etc.
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