Adding Decibels Summing the contributions from multiple sound sources to obtain the total SPL is not done simply by adding the decibel levels because SPL is a logarithmic quantity. Imagine a fan produces a moderate SPL of 65 dBA at 6 feet. If a second identical fan were turned on the resulting SPL would not be 130 dBA. This would be equivalent to a commercial jetliner taking off at close range. The correct method of adding the SPL from each source is to sum the acoustic power produced by each source. This implies that each time the number of sources having equal SPL is doubled, the SPL will increase by 3 dBA. Therefore, in the example with two fans, the correct total SPL would be 68 dBA. More examples with multiple sources producing equal SPL are shown in 2.4. 65 dBA + 65 dBA ยน 130 dBA WRONG (A2.2) 65 dBA + 65 dBA = 68 dBA RIGHT (A2.3)
1 8 Source Sources Sources Sources 65 dBA 68 dBA 71 dBA 74 dBA 2 4
Figure A2.4. Total SPL from Multiple Sources with Equal SPL Output
51 of 58 Spendiarian & Willis Acoustics & Noise Control LLC
12/15/2019
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