Appendix B
B: Glossary of Terms
C-A
The C-A measurement is an Lavg that enhances the low- frequency components of the sound signal.
It is the result of subtracting an A-weighted Lavg from a simultaneously collected C-weighted Lavg
and is sometimes used as the NRR (noise reduction rating) factor to determine if hearing protection
devices are adequate for usage in the field.
Relative Response (dB)
Frequency (Hz)
Figure B-1: C-A weighting response curve
Combining Noise sources
Because sound levels are measured in decibels they cannot be simply added together. A sound level
of 90 dB plus 90 dB does not yield 180 dB but rather 93 dB. The procedure for mathematically
determining a combined level of sound from independent measurements made for two noise sources
is given below.
1. Make independent measurements for both noise sources, where each exists without the
presence of the other.
2. Determine the decibel difference between these two measurements.
3. Find that difference on the x-axis of the figure below.
4. Find the point on the curve directly above this difference.
5. Read the corresponding value on the y-axis for that point (the Amount to Add).
6. Add this value to the larger of the two measurements.
Example: Independent measurements for the two noise sources yielded levels of 90 dB and 91 dB.
For a difference of 1 dB, the correction factor taken from the curve below is 2.5 dB. Therefore, the
combined level when these two sound sources are present in the measurement area at the same time
93.5 dB.
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