ACOUSTIC WAVES

The sensation of sound appears due to mechanical oscillation of eardrum. This oscillation is excited by acoustic wave propagating from the source of sound to the ear. Any vibrating object generates the acoustic wave, but we can hear only the sound with the frequency in the range 20 Hz- 20 kHz. The sound with the higher frequencies (ultrasound) and lower frequencies (infrasound) can be detected by special devices. Let us consider the process of the sound generation by loudspeaker. The alternating current being applied to the loudspeaker causes the mechanical oscillation of a coil connected with cone of the loudspeaker. As a result of this oscillation some regions of the air turn out to be compressed, while the other regions are rarefied. The regions with excessive pressure are propagating in the space in the form of longitudinal waves. When such a wave reaches the ear, it forces the eardrum to be oscillated and we hear the sound

.

If the size of sound source is much less as compared to the wavelength, then the spherical wave is generated and the source of sound is considered as the point source. Otherwise, the flat acoustic wave will propagate. The velocity of the acoustic wave depends upon the properties of media / gas where it propagates.

The formula for the velocity v of the acoustic wave was given by French scientist P.Laplace (1749-1827):

where

g is the adiabatic factor, R is the universal gas constant, T is the temperature, m is molecular weight of gas. This formula was derived in the assumption that the sound propagation is the adiabatic process. According to this formula the velocity of sound in air at T=273 K equals 330 m/s that is in a good correspondence with experimental results.