Larson Davis System 824, Technical Reference Manual

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possible. Use a tripod whenever possible.
2. Measure the influence of other sources by measuring the
background sound level without the source of interest. You
may have to correct for the background.
Near Field
There are two types of near fields: the acoustic near field
and the geometric near field .
Acoustic Near Field : The distance from a source of sound is
less than an acoustic wavelength. In the near field, the effect
of the type of sound source is significant. Since the
wavelength varies with frequency (See the definition of
Wavelength), the distance will vary with frequency. The
most common example of a near field is driving an
automobile with an open window. As you move your ear to
the plane of the window, the sound pressure level builds up
rapidly (wind noise) since most of the pressure changes are
to move the air and very little of it compresses the air to
create sound. Persons not far way, can hardly hear what you
hear. The acoustic near field is characterized by pressures
that do not create sound that can be measured in the far field.
Therefore measurements made here are not useful in
predicting the sound levels far way or the sound power of
the source.
Geometric Near Field : The distance from a source of sound
is less than the largest dimension of the sound source. In the
near field, effect of source geometry is significant. Sound
sources often have a variety of specific sources within them,
such as exhaust and intake noise. When in the near field, the
sound of a weaker, but close, source can be louder than that
of a more distant, but stronger, source. Therefore
measurements made here can be used to separate the various
sources of sound, but are not useful in predicting the sound
levels and sound spectrum far from the source.
Noise
Typically it is unwanted sound. This word adds the response
of humans to the physical phenomenon of sound. The
descriptor should be used only when negative effects on
people are known to occur. Unfortunately, this word is used
also to describe sounds with no tonal content (random):
Ambient: The all encompassing sound at a given location
caused by all sources of sound. It is generally random, but
need not be.