15
NOTE: When making out-of-doors measurements, it is usually preferable to point
the microphone upward (to avoid interference from reflected high frequencies)
and as far from the body as is convenient.
Grease and dirt on the windscreen will affect sound level measurements. To
check for dirt or grease on the windscreen, measure the level of continuous noise
indoors with or without the windscreen in place. To clean the windscreen, wash
with a mild detergent. Thoroughly rinse and dry before placing over microphone.
3.7 Meter Fluctuations
If the meter indication fluctuates more than 3 dB, set the meter response switch of
the Instrument to the SLOW position unless otherwise specified in the applicable
specification. Then if the fluctuations are less than 6 dB, record a value of 3 dB
less than maximum indication. Infrequent, high peaks customarily are disre-
garded. (When making noise survey measurements record the highest peak.)
NOTE: As pointed out in Table 3-1, applicable standards require SLOW meter
response when making noise surveys.
3.8 Selecting A Weighting Characteristic
The weighting characteristic must be selected according to individual application
requirements. Noise codes and sound test procedures frequently specify the one
to be used. For example: “A” weighting often is employed for measurement of
speech interference by office and plant noise. Federal regulations such as the
Walsh-Healy Act require use of the “A” response for all tests. When a standard
test procedure is not involved, data can be taken with each of the three character-
istics and thereby provide information on overall frequency content.
NOTE: If the weighting is not specified, common practice is to assume “A” weight-
ing (ANSI S1.4-1983). Be sure to indicate the weighting used on all recorded
data.
3.9 Impact Noise
Impact (impulse) noise refers to noise impulses of less than 1 second duration,
occurring once or at intervals greater than 1 second, such as gunshots or drop-
hammers. Impulse noise measurements are made with an impulse noise meter
such as the Simpson 899.
Impulse noise measurements can also be made using an oscilloscope and the
886-2. The oscilloscope is connected to the RMS output jack and calibrated,
using a steady sound source and correlating the scope deflection with the indica-
tion on the sound level meter.
A rough check of impulse noise having a duration greater than 5 milliseconds
can be made with a sound level meter. It is set on “C” weighting, “FAST” meter
response, and range switch in the 120 or 130 position. An indication of 125 dB is
approximately equivalent to an actual impulse noise of 140 dB (limit established
by the Walsh-Healy Act).
NOTE: Impulse noises having durations longer than 50 milliseconds and peak
levels of 140 dB will give readings higher than 125 dB. The Simpson 899 Impulse
Sound Level Meter may be used to take measurements such as these.
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