Larson Davis SoundAdvisor 831C Reference Manual page 208

Near Field
Noise
Noise Dose (D)
SoundAdvisor Model 831C
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.
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. There-
fore 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.
Typically it is unwanted sound. This word adds the response of humans to the physical phenome-
non 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 gen-
erally random, but need not be.
Background: The all encompassing sound at a given location caused by all sources of sound, but
the source to be measured. It is essentially the sound that interferes with a measurement.
Pink: It is a random sound that maintains constant energy per octave. Pink light is similar to pink
noise in that it has a higher level at the lower frequencies (red end of the spectrum).
White: It is a random sound that contains equal energy at each frequency. In this respect, it is sim-
ilar to white light.
It is the percentage of time a person is exposed to noise that is potentially damaging to hearing.
Zero represents no exposure and 100 or more represents complete exposure. It is calculated by
dividing the actual time of exposure by the allowed time of exposure. The allowed time of expo-
sure is determined by the Criterion Duration and by the sound level (the higher the level, the
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