Basic Documentation

Sound Wave Parameters
Unless there is a barrier, the sound waves continue to travel outward in all directions in a
spherical manner, until they either are absorbed by an object or their energy level is
dissipated by the surrounding air.
With regard to understanding the science of sound and its effects, it is necessary to have an
understanding of two fundamental terms: sound power and sound pressure. These terms are
not interchangeable and it is important to have a clear understanding of each term.
The intensity of the sound at the source is expressed in terms of sound power and
establishes the energy level of the sound. Sound power is the parameter that indicates the
total energy or power output of the sound. It is universally expressed in terms of watts. The
sound power spectrum that we are familiar with ranges from a high point of 10,000 (10
4
)
watts of sound power for a jetliner takeoff or gun fire, to a low of 0.000000009 (10
-9
) watts for
a soft whisper.
Ultimately, sound waves, which in our context, are really compressed air waves, will
ultimately impinge on a receiver and at that point their effect is expressed in terms of the
sound pressure.
For our purposes, the most common receiver will normally be the eardrum of a person who
hears the sound. Another common example of a sound receiver is a microphone that is part
of a sound amplification system. Sound power itself, however, does not really establish
whether a sound will be interpreted as loud or soft by the receiver. That is entirely dependent
upon the amount of energy loss or attenuation of the sound waves that occurs prior to
impinging upon the receiver as sound pressure.
Attenuation, which is simply a decrease in the sound power before it gets to the listener,
occurs primarily due to two factors: distance and physical barriers. When sound is generated
in an open or unconfined space, as in Figure 1, the primary attenuation factor is the distance
between the sound source and the receiver.
When a sound source generates sound, the sound power energy is radiated outward in all
directions, as shown in Figure 1, and the sound power energy is dissipated over a rapidly
increasing area. This can be likened to an ever expanding sphere surrounding the source of
the sound. An analogy of the effect of sound power radiation would be like having a fixed
quantity of paint and the task of achieving a uniform thickness paint coating on the surface of
the sphere. In this analogy the paint quantity represents the available sound power level,
while the resulting thickness of the paint coating on the surface of the sphere represents the
sound pressure level.
As the radius of a sphere increases, the surface area also increases and the thickness of the
surface coating must be decreased. If a sphere having a radius of 1 foot (surface area 12.56
square feet) is expanded until the radius is doubled (becomes 2 feet), the surface area would
have increased to 50.24 square feet or four times the original surface area. This of course
means that the paint coating on the surface could then only be 1/4 of the previous thickness.
Likewise, each time the distance between a sound source and the receiver is doubled, the
effect at the receiver that is the sound pressure level is reduced by a factor of 4.
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