Because the decibel scale is a logarithmic scale, if we have two different sound sources combining together we can't simply add the sound pressure levels - what we have to do is to add the intensities. That means we have to convert our decibel values into intensities (with units of W/m2).
How do sound intensities add?
A decibel corresponds to an increase of intensity or energy in a ratio of approximately 5/4, two decibels to a ratio of (5/4)2, three to a ratio of (5/4)3= 2,... and so on. Thus doubling the intensity of a sound, as when two similar voices sing the same note, corresponds to a change of 3 decibels.
How do you find the combined sound intensity?
Sound Intensity and Decibels
The equation used to calculate this intensity, I, is:I=PA I = P A Where P is the power going through the area, A. The SI unit for intensity is watts per meter squared or,Wm2 W m 2 . This is the general intensity formula, but lets look at it from a sound perspective.
