Larson Davis SoundExpert LxT, Руководство пользователя

Страница: 295 / 307

C
-
18 Glossary and Formulas LxT Manual
A microphone measures sound 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.
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.
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 acoustic near field,
the effect of the type of sound source is significant. Because
wavelength varies with frequency, 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
geometric 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 geometric near field, the sound of a weaker, but closer,
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.