Hey all. I know this will be a fairly simple question for anyone who knows anything from physics to psycholinguistics but I don't have sufficient background in this to appropriately answer my own concern. Does two sources of the same sound decibel increase the sound itself? Is there an interaction between two sources of sound of the same decibel? Simple put, I want to know whether two sources of 5 decibels would be louder than one source of 5 decibels? Thanks!
We don't discuss acoustics much so thanks for the question. There isn't enough information to answer the question but I'll give you the short answer. Depending on the phase of the two sound waves, they will add and/or subtract over the volume they occupy. In ideal circumstances (always in phase) they will add and double the sound pressure level. In worst case they will cancel (like noise cancelling headphones). Decibels is a relative measure of gain or attenuation, relative to the reference. Sound pressure level is measured at a reference of 0 dB = 20 µPa. If the sound pressure doubles then we say the gain is 10 log (40/20) = 3.01 dB, or else we say the SPL is 3 dB, and it's understood that this means 3 dB[sub]20 µPa[/sub] which means "3 db referenced to 20 µPa". Unfortunately, there is no exact answer to your question. Depending on the nature of the signals, and the position of the ear or transducer picking up the sound, the SPL will vary. A lot of questions in science are like this: "It depends on what you mean". But I suppose the best answer is "+3 dB". So your expected maximum sound pressure level will be 8 dB.
Aqueous Id's answer is basically correct - I'll just add that for low frequencies, (wave lengths great compared to separation of your ears), what he said is true even with a mix of frequencies; but with mix of frequencies also including many with wave lengths less than ear separation some of those shorter waves can be mutually re-enforcing and others mutually destructive. One condition in one ear and the other in the other ear, but your brain is clever and still get good approximation of where a point sound source is as it compares the arrival times of the phases (peaks for example) to do that not the amplitude. Furthermore you don't notice the amplitude modulate as function of frequency as brain basally add the signal from both ears. - much like you don't know which eye is reading this, if both have good focus. As you grow old, typically your sensitivity for high frequency components falls off faster than for more base notes. So old people like me don't notice anything strange happening even if the try - concentrate on the sound.
