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58 Appendix B
Glossary of Terms
053-644, Rev E Page 58 eg4 & eg3
Combining Noise sources
Because sound levels are measured in decibels they cannot be simply added together. A sound level of
90dB plus 90dB does not yield 180dB but rather 93dB. 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.
Crest Factor
Crest Factor is the ratio of the instantaneous peak value of a wave to its RMS value. This is
a performance specification of a meter’s ability to process signals that have peaks that are
substantially higher than their RMS averages.
Criterion Level (CL)
Criterion level is the average SPL that will result in a 100% dose over the Criterion time,
usually 8 hours. The Criterion Level is typically set by a regulating agency, such as OSHA,
and is not usually applicable for community noise monitoring. Examples: OSHA mandates
the Criterion Level (maximum allowable accumulated noise exposure) to be 90 dB for 8
hours. For an 8-hour sample, an average level (LAVG) of 90 dB will result in 100% dose.
For the OSHA HEARING CONSERVATION AMENDMENT, the “action level” is 85 dB for 8
hours. This would result in a 50% dose reading. Note that the Criterion Level has not
changed. (If the Criterion Level were changed to 85 dB then an 8-hour average of 85 dB
would result in 100% dose.)