Spartan 821/721 Manual
Glossary
C-11
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.
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 away 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. The word is used also to describe sounds with no tonal
content (random).
Noise Dose (Dose)
Noise Dose, or “Dose”, 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.
The allowed time of exposure is determined by the Criterion Duration and by the sound
level (the higher the level, the shorter the allowed time). The sound levels must be
measured with A-frequency weighting and “Slow” Time Weighting.
Dose 100
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