Basic Documentation

Chapter 2–Physics of Sound
The sound pressure level is the most widely used parameter in the field of acoustical
engineering since it is the closest thing to what we experience in terms of loudness or
softness of a sound. In the previous analogy, the thickness of the paint coating on the
spherical surface represented the effect that a certain quantity of paint could have on the
surface of the sphere. Therefore ,sound pressure expresses the effect that the sound power
energy has when impinging upon a unit area of the receiver.
Sound Measurement Parameters
To use sound generation and attenuation data in the design of HVAC systems, it is
necessary to understand the measurement parameters with which sound power, sound
pressure, and other factors involved are quantified. These key factors are each listed below
with an explanation of their units of rating or measurement.
Sound Power Level
As discussed above, sound power expresses the overall sound energy of the sound source
and sound power level is represented in terms of watts. However, as was previously
indicated, the range between the highest and lowest sound power levels is too large to
conveniently use actual watt values. In addition, the actual watt values that apply to the
typical sound power levels encountered in HVAC are so small (that is, 10
-3
to 10
-11
watts) that
too many zeros would be needed after the decimal point to express specific watt values.
A more convenient scale is to represent sound power level so the decibel (dB) unit is used.
With decibels, sound power low level begins at 0 decibels, which is just enough power for the
human ear to begin hearing something that is right next to the ear. At the upper end of the
scale is the sound power level of a jetliner taking off, which could be 160 dB. (An explanation
of decibels and how specific physical measurements are expressed in terms of decibels
follows.)
Be sure not to confuse sound power level with sound pressure level (even though both are
expressed in terms of decibels). Remember:
• Sound power level expresses the power or energy of the sound source.
• Sound pressure level expresses the loudness or effect of the sound at the receiver.
When we hear a soft whisper, we are experiencing a sound power level of approximately
0.000000001 (10
-9
) watts. When we converse in a normal voice, we are experiencing a
sound power at about 0.00001 (10
-5
) watts. If we had the unfortunate experience to be just
below a jetliner taking off, we could later tell everyone that we experienced a sound power
level of around 10,000 (10
4
) watts. (However, it may take us a few days before we could
again hear their reply.)
As previously discussed, wattage based direct sound measurements are non-linear and vary
over a very large range, thus it becomes clumsy to stay with watts as the basic unit of
measurement.
8 Siemens Building Technologies, Inc.