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Library: The Physics of Sound

Books

Author: merja
Date:Jul 15 2003


KEY TERMS:
*acoustics *oscillation *reflection of waves *sonar *frequency *wavelength
*hertz (Hz) *ultrasoni *Doppler effect *intensity *amplitude *decibel
*acoustic interference  *resonance *forced vibration
     
           *      *     *
     
The science of sound and how it is perceived is called *acoustics*, from the
Greek word for hearing. Both sound and light occur in the form of waves. To
start, then, we should know something about waves and how they act.
      
            *      *      *
THE NATURE OF WAVES. If you drop a stone in a quiet pond, you will see a set
of waves spreading outward in ever-widening circles from the point where the
stone entered the water. The size of each circular ripple grows at a constant
rate. However, dropping a stone into water would only produce a few crests and
hollows. To create a continuous wave motion you would have to introduce a
regular series of repetitive disturbances, for example, by inserting some
vibrating object like a tuning fork into the water.
     
In any wave motion, no single particle of material carrying the waves ever
moves very far from its normal place. If you were to watch a floating hip of
wood in the water where you dropped the stone, you would see that it bobs up
and down as each wave strikes it but does not move outsward with each wave.
     Different kinds of waves move particles in different ways. For example,
particles moved by a compression wave, the kind that produces sound,
*oscillate* (move back and forth about a center) along the line in which the
waves are moving. For this reason they are called longitudinal waves. In other
kinds of waves, the disturbed particles move perpendicular to the line of
advance of the waves; these are called transverse waves. Still other waves are
combinations of longitudinal and transverse motions; for example, the floating
woodchip moves slightly forward and upward as the crest of a wave meets it,
then back and downward as the next trough comes by.
      When waves traveling on water strike a wide obstacle such as a floating
board, you can see a new set of ripples starting back from the obstacle. The
waves are said to be reflected from it. Sound waves and light waves can also
be reflected, as you will see. You can observe the reflection of both water
and light waves with a simple experiment.
     
Exploration 9.1 You can investigate the reflection of waves from flat and
curved surfaces by using a large, flat pan with some water in it. Drop in some
more water. The ripples you produce are easily seen if lighted by an unshaded
lamp placed some distance above the pan. By adding a flat piece f wood or
metal for the waves to hit, you can see how the waves will be reflected back.
     
As you watched the direct and reflected water waves, you may have noticed that
the two sets of ripples can pass right through each other without having any
mutual effect. This is true of any kind of wave, including sound waves. We
will talk more about the effect of this phenomenon, which produces what are
called stationary wave patterns, when we talk about sound.
     
SOUND WAVES
Suppose that, instead of tossing a stone into a pond, you were to explode a
firecracker outdoors. You can demonstrte what would happen with an experiment
of your own.. ,


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