Larson Davis SoundAdvisor 831C Reference Manual page 212

Near Field
SoundAdvisor Model 831C
is applied on one-half the diaphragm and an equal positive pressure is applied to
the other half, the net force is zero and essentially no motion of the diaphragm
occurs. This occurs at high frequencies and for specific orientations of the
microphone.
Rules:
1. Do not use a microphone at frequencies higher than specified by the manufac-
turer; to increase the frequency response choose smaller microphones.
2. Choose a microphone for free field or random incidence to minimize the influ-
ence of orientation.
A microphone influences the sound being measured. The microphone measures
very small forces, low level sound can run about one-billionth of a PSI! Every
measurement instrument changes the thing being measured, and for very small
forces that effect can be significant. When sound impinges directly on a
microphone the incident wave must be reflected since it cannot pass through the
microphone. This results in the extra force required to reflect the sound and a
microphone output that is higher than would exist if the microphone were not
there. This is more important at high frequencies and when the microphone is
facing the sound source.
Rules:
1. Do not use a microphone at frequencies higher than specified by the manufac-
turer; to increase the frequency response choose smaller microphones.
2. Choose a microphone for free field or random incidence to minimize the influ-
ence of orientation.
A microphone measures what is there from any direction: Most measurements
are intended to measure the sound level of a specific source, but most
microphones are not directional so they measure whatever is there, regardless of
source.
Rules:
1. When making hand-held measurements, keep your body at right angles to the
direction of the sound you are interested in and hold the meter as far from your
body as 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.
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.
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