Basic question about sound.

Posted by Gary Gibbs [IP: 128.155.16.195] on May 24, 1999 at 10:58:24
Using Mozilla/4.5 [en] (Win98; I):

In Reply to: Basic question about sound. posted by Chris Jensen on May 21, 1999 at 16:58:38:

I agree with Chris on most of his points, and am glad to see him include the appropriate math. The only point where I disagree is as follows. When you have two coherent (driven by the same signal) sound sources creating sound in an acoustic environment (e.g. a room) at acoustic levels that are less than 140 dB (thus wave equation is still valid) than the total response at a point is definitely the sum of the pressures (created by each speaker) at that point. Thus the SPL is found by
SPL = 20*log10(Ptotal/Pref) where Pref is 20E-6 Pa.
If the two speakers are collocated then the total pressure everywhere is twice the pressure due to one speaker and the SPL goes up 6 dB. This collocation is a function of the wavelength and thus is only (practically) valid at lower frequency. At higher frequencies the two speakers produce pressures at a point which at some frequencies add constructively and other frequencies add destructively. Thus the total output of the two speakers over the high frequency band tends to look like to incoherent sources which produce SPL's that are the sum of the powers:
SPL = 20*log10(I/Io) As stated by Chris.
Thus two speakers spaced as close as possible to one another and driven by the same signal tend to produce a 6 dB increase in SPL at low frequency and a 3 dB increase at high frequency.

All of this discussion has been based on the entire system being linear. As discussed by other posters, higher efficiency speakers and the addition of two speakers together tend to show reduced speaker cone displacement for a given SPL thus the sound is much less distorted than the equivalent SPL sound level output from a single low efficiency speaker.

Gary

Follow Ups:

• Basic question about sound. - Chris Jensen 15:56:33 05/25/99 (0)

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