16.11 - Summary
Sound waves are longitudinal mechanical waves. Instead of the peaks and troughs
of transverse waves, sound waves are composed of condensations and rarefactions
of the medium through which they travel. Particles in a sound wave move in simple
harmonic motion along the direction that the wave travels.
The intensity of a sound is the sound power passing perpendicularly through a unit
area. For a sound that spreads radially, such as from a fireworks explosion in
midair, the sound intensity is inversely proportional to the square of the distance
from the source.
Do not confuse sound intensity with the sound level. The sound level takes into
account the logarithmic perception of sound by the human ear.
The Doppler effect is the change in frequency of a wave due to the relative motion
of the source and/or the listener. A common example is the change in frequency
heard as the siren on a fire engine races by you. As the fire engine moves toward
you, the sound waves “pile up” and their wavelength decreases. Since the speed of
sound remains the same, this results in you hearing a higher frequency.
Speed of sound in airSound intensityI = P/A
Sound spreading radiallySound levelDoppler effect, source moves toward
listenerDoppler effect, source moves away
from listenerMach angle