D.C. Mun. Regs. tit. 20, r. 20-2905
Unless impracticable, earphones shall be used occasionally while noise measurements are being taken as a periodic check to ensure that the meter is responding properly to the noise source being monitored.
In the case of steady (including intermittent, slowly fluctuating, or quasi-steady) noise, a measurement of the ambient noise level shall be required.
If the ambient noise level of steady noise is within ten (10) dB(A) of the source sound level, the corrections specified in the table of § 2905.7 of this section shall be applied to determine the noise level due solely to the source being investigated, and the source shall be interpolated as necessary.
If the ambient level of steady noise is less than three (3) dB(A) from the noise source, no correction may be made and measurements cannot indicate true source noise level.
Before citing a violation of a noise limit which may not be exceeded regardless of duration, a two and five tenths (2.5) dB(A) grace shall be allowed to account for all possible meter inaccuracies.
Several measurements showing a violation shall be taken with at least twenty (20) seconds between each measurement.
The following table shall be used for making corrections to measured ambient sound pressure levels:
Difference (in decibels) between sound pressure level measured with sound source operating and ambient sound pressure level along dB(A) | Correction (in decibels) to be subtracted from sound pressure level measured with sound source operating to obtain sound pressure level due to sound source along d B(A) |
Less than 4 | Measurement can not be made |
4 | 2.2 |
5 | 1.7 |
6 | 1.3 |
7 | 1.0 |
8 | 0.8 |
9 | 0.6 |
10 | 0.4 |
more than 10 | .0 |
D.C. Mun. Regs. tit. 20, r. 20-2905