c. Standards for sound level meters specify performance characteristics in order
that all conforming instruments will yield consistent readings under identical
circumstances. The more important characteristics specified are frequency response,
signal averaging, and tolerance.
d. Three weighting networks (A, B, and C) are provided on standard sound level
meters. The purpose of these is to give a number that is an approximate evaluation of
the total loudness level. The (A) scale emphasizes those frequencies, which the human
ear responds to. The (B) scale is an intermediate scale. The (C) scale has a flat
response, which indicates the actual sound pressure level.
e. The A-weighting network is the most useful one on the sound level meter. It
indicates the A-weighted sound level, often abbreviated dB(A) from which most human
responses can be predicted quite adequately (see para 3-2m).
f. Action of the indicating meter may be selected as "fast" or "slow." Steady
state sounds are measured with the more sluggish "slow" response to reduce meter
g. The speed of meter response affects the readings obtained for transient
sounds. For example, the level of a whistle toot lasting 1/5 second should be indicated
no more than 2 dB low on the "fast" scale. On the "slow" scale, the level of a toot
lasting 1/5 second would read 3 to 5 dB low. The "slow" meter response must be used
for steady-state noise surveys.
h. American standard sound level meters are furnished in three types offering
varying degrees of precision. Designated Types 0, 1, and 2 in order of increasing
tolerances, the Type 2 generally measures within 2 or 3 dB of true levels, which is
satisfactory for most purposes.
i. By combining a device known as an octave band filter with a sound level
meter, it is possible to determine the pattern of the distribution of sound pressure at
different points or areas along the scale of audible frequencies (see Figure 3-11). Using
this equipment, it is possible to make a spectrum analysis of complex sounds or noises.
3-19. USE OF THE SOUND LEVEL METERS (SLM)
a. Since the hazardous criteria for steady state noise is 85 dB(A) s, the SLM
should be set for a range of 80 to 90 dB on the weighted scale.
b. Starting at a point a distance from the noise source, the technician moves
toward the noise, holding the SLM at a 70o angle to the noise source. When the SLM
registers 85 dB (A), he stops recording the spot on a diagram of the area. He then
repeats the procedure 3 to 5 mores times from different points, thereby making it
possible to establish a noise contour.