7.1.2 Faint Noises and Correcting Operating Noise with Respect to Faint
Faint noises are defined as peripheral sounds existing while the unit is not running, which are
picked up when measuring operating noise. If these faint noises are 10 dB or more than the
noise produced by the unit, the measured value can be taken as the operating noise of the unit.
But, the difference must be corrected if less than 10 dB, because of the effect these noises have
on the actual measured value. Also, when the sound meter remains unchanged even while the
unit is stopped, we can determine the operational noise to be at least 10 dB less than the faint
noises, but we cannot pinpoint the operating noise exactly.
For example, if the faint noises are some 65 dB and the noise produced by the unit in operation
is 70 dB, the indicated difference comes to 5 dB. Using Table 3, we recommend you correct the
operating noise by about 2 dB to 68 dB.
Difference between when noise
is produced and when not
7.1.3 Calculating Operating Noise
When two or more units are running at the same time, the amount of operating noise they
produce rises. The total amount of noise produced can be obtained ahead of time with Chart 4.
Sample calculation 1
49 = 1, the corrective value is 2.5, therefore 50 + 2.5 = 52.5 dB.
Sample calculation 2
When sounds of 40 dB, 38 dB, 37 dB and 40 dB are placed in order of magnitude, we obtain the
40 dB, 40 dB, 38 dB, 37 dB
To start, the difference between 40 dB and 40 dB is 0, therefore we take a corrective value of
3dB and obtain 40 + 3 = 43 dB. The compounded sound of 43 dB and 38 dB has a 5.0 dB
difference, thus a corrective value of 1.2 dB, which gives us 44.2 dB from 43 + 1.2. In the same
manner, the corrective value for 44.2 dB and 37 dB is approximately 0.7 dB, or in other words,
44.2 + 0.7 = 44.9 dB.
are given as compounded sounds of 50 and 49 dB respectively. Since L
Table 3 Correcting the effect of faint noises
Sum L of L
L = L
+ corrective value
The corrective value relative to
(L - L
) is added to L
Refer to sample calculation (L
= 50 –