14
3.6
Windscreen
When making measurements where wind is present, a low frequency interfer-
ence can be introduced by air passing across the microphone. Try to make mea-
surements where wind is not present; if this is not practical a windscreen should
be employed. When using this screen, indicated wind noise will be attenuated
approximately 20 dB. The loss of system sensitivity occurring with use of the
windscreen is shown in Figure 3-3.
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.
3.6.1
Loss of System Sensitivity Due to Windscreen
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 disregarded.
(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
Background Noise
Ideally, any sound source being measured should be isolated from effects due to
extraneous or background noise. If the background noise is 10 dB or more below
the sound source level desired, the effect from the background noise can be
considered negligible. In instances where the ratio is less than 10 dB, an approxi-
+5
+4
+3
+2
+1
0
–1
–2
–3
–4
–5
200
1000
500
2000
5000
10,000
FRE
QU
ENC
Y
RESPONSE
ERR
OR
DU
E
TO
WIN
D
SCREEN
(dB)
FRE
QU
ENC
Y
(Hz)
Figure 3-3. Loss of System Sensitivity Due to Windscreen
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