Sound is a mechanical three dimensional and longitudinal wave that is created by a vibrating source such as a
guitar string, the human vocal cords, the prongs of a tuning fork or the diaphragm of a loudspeaker. Being a
FREQUENCY AND PITCH OF SOUND WAVES
We can describe sound waves either in terms of excess pressure (equation 1.1) or in terms of the longitudinal
displacement suffered by the particles of the medium w.r.t. mean position.
SPEED OF SOUND WAVES
1 Velocity of sound waves in a linear solid medium is given by
INTENSITY OF SOUND WAVES :
Like any other progressive wave, sound waves also carry energy from one point of space to the other. This energy
can be used to do work, for example, forcing the eardrums to vibrate or in the extreme case of a sonic boom
created by a supersonic jet, can even cause glass panes of windows to crack.
The amount of energy carried per unit time by a wave is called its power and power per unit area held perpendicular
to the direction of energy flow is called intensity.
The ability of human to perceive intensity at different frequency is different. The perception of intensity is maximum
at 1000 Hz and perception of intensity decreases as the frequency decreases or increases from 1000 Hz.
INTERFERENCE OF SOUND WAVES
Reflection of sound waves from a rigid boundary (e.g. closed end of an organ pipe) is
analogous to reflection of a string wave from rigid boundary; reflection accompanied by
an inversion i.e. an abrupt phase change of ?. This is consistent with the requirement of
displacement amplitude to remain zero at the rigid end, since a medium particle at the
rigid end can not vibrate
LONGITUDINAL STANDING WAVESTwo longitudinal waves of same frequency and amplitude travelling in opposite directions
interfere to produce a standing wave.
If the two interfering waves are given by
Standing waves can be set up in air-columns trapped inside cylindrical
tubes if frequency of the tuning fork sounding the air column matches
one of the natural frequency of air columns. In such a case the sound of
the tuning fork becomes markedly louder,
Closed organ pipe
When two sound waves of same amplitude and different frequency superimpose, then intensity at any point
in space varies periodically with time. This effect is called beats
When there is relative motion between the source of a sound/light wave & an observer along the line joining them,
the actual frequency observed is different from the frequency of the source. This phenomenon is called Doppler’s
Effect. If the observer and source are moving towards each other, the observed frequency is greater than the
frequency of the source. If the observer and source move away from each other, the observed frequency is less
than the frequency of source