PHYSICS OF SOUND
Definition: Sound - a disturbance which propagates through an
Elastic material at a speed which is characteristic of that material.
Sound is usually caused by radiation from a solid vibrating
Surface or fluid disturbances.
Types of waves
Example: find the wavelength for a wave of Frequency (1.33)
Hz through a rope of 425 cm in length in 10 sec.
42.5/1.33= 31.955m
• The speed of sound is a property of the
material: it depends upon
elasticity and density of the material
• c ≈ 340m/s in air at room temp
• c ≈1400m/s in water
• C ≈ 5000m/s in steel
The human ear
1. The ear changes sound pressure waves from the
outside world into nerve impulses sent to the brain.
2. The sound is amplified through the middle portion
of the ear and passed from of air into a liquid.
3. The hollow channels of the inner ear are filled with
liquid, and contain microscopic "hairs" that project
out into the fluid. The hair cells are receptors that
release a chemical signal when
stimulated.
Wave propagation Wave propagation is any of the ways in
which waves travel. With respect to the direction of the
oscillation relative to the propagation direction, we can
distinguish between longitudinal wave and transverse waves.
Closed space source
Isotropic source
Shock Waves
Shock wave, is a type of propagating disturbance that moves faster
than the local speed of sound in the medium. Like an ordinary wave, a
shock wave carries energy and can propagate through a medium.
A sonic boom is the sound associated with the shock waves created
whenever an object travelling through the air travels faster than the
speed of sound. Sonic booms generate enormous amounts of sound
energy, sounding similar to an explosion or a thunderclap to the
human ear.
If a plane were traveling at the speed of sound, what would the wave crests looks
like?
They would be bunched up in front of the aircraft and an observer (to the right)
would Measure λ=0
Intensity of Sound
We define the intensity I of a wave, or the power per unit area,
to be the rate at which the energy being transported by the
wave transfers through a unit area A perpendicular to the
direction of travel of the wave:
The average power pav emitted by the source must be distributed
uniformly over this spherical surface of area
intensity at a distance r from the source is:
4πr2. Hence, the wave
Example:
An audio speaker at a concert emits sound uniformly in all directions at a rate of
100 W. calculate the sound intensity experienced by a listener at a distance of 8
m from the speaker.
I=100/4*3.14*8^2=0.124W/m2