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57 Appendix B
Glossary of Terms
053-644, Rev E Page 57 eg4 & eg3
Background Noise, effects of
Background noise can cause considerable error in measurement when its level is high relative to the
level of a noise source of interest. If you can make measurements both with and without the
contribution of a noise source of interest, you can mathematically subtract the background noise from
the combined measurement, yielding the level of the noise source of interest alone. The accuracy of
this determination improves as the difference between the two measurements increases.
To remove background noise
1. Obtain a measurement that includes the contribution of the noise source of interest (the
combined measurement).
2. Make a second measurement under exactly the same conditions as in Step 1 but without the
presence of the noise source of interest (the background noise measurement).
3. Determine the decibel difference between these two measurements.
4. Find that difference on the x-axis of the figure below.
5. Find the point on the curve directly above this difference.
6. Read the corresponding value on the y-axis for that point (the correction value).
7. Subtract the correction value from the combined measurement.
Example:
Measurements with and without the noise source of interest are 91 dB and 90 dB,
respectively. For a difference of 1 dB, the background noise correction value is 7 dB. Therefore the
level for the noise source of interest without the noisy background is 84 dB.
APPENDIX
B