Larson Davis System 824, Справочное руководство

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D-14 824 Reference Manual 11/3/00
ested in and hold the meter as far from your body as possi-
ble. 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 wave-
length varies with frequency (See the definition of Wave-
length), 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. Per-
sons not far way, can hardly hear what you hear. The acous-
tic near field is characterized by pressures that do not create
sound that can be measured in the far field. Therefore mea-
surements 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 measure-
ments 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.