What is sound pressure level?
Sound pressure level (SPL) is a logarithmic measure of acoustic pressure relative to a reference of 20 micropascals, expressed in decibels (dB). Because decibels are logarithmic, you cannot simply add the levels of two noise sources together. This calculator handles the correct logarithmic combination of up to three sources and then projects the result to any distance.
How to use this calculator
Enter the SPL of each source measured at the same reference distance, then set the reference distance d_1 and the target distance d_2 where you want to know the level. Leave the second and third source fields blank to work with fewer sources. The tool reports the combined level at the reference point, the attenuation caused by distance, and the final SPL at the target.
The formulas explained
Multiple incoherent sources combine on an energy basis:
where L_i is the level of each source in dB. For a point source radiating into free space, the level decreases as distance increases:
Here d_1 is the reference distance and d_2 the target distance. Doubling the distance reduces the level by about 6 dB.
Worked example
Suppose two machines each produce 80 dB at 1 m. Combining them gives:
A single 90 dB source measured at 1 m, evaluated at 2 m, becomes:
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