Originally Posted by tsyoo
Thanks for replay. I don't think I fully understand why, with my limited knowledge
So in other words, it is better to have multiple subs than have one powerful sub in terms of total output?
Originally Posted by goneten
If two subwoofers are co-located then the gains increase. The maximum output increase after co-locating two identical subwoofers will be 6 dB's due to coupling.
The idea that two speakers will pick up additional *gain* if co-located is based on the fact that two speakers co-located will become far more directional because they make up one larger speaker.
The counterpoint is that the law of conservation of energy always holds. Two speakers receiving the same amount of energy will always radiate the same total amount of energy or perhaps less if there is cancellation. Never more.
However, if two co-located speakers are more directional than one, the intensity of sound in the preferred direction will increase. The intensity of sound in the non-preferred directions will experience a corresponding decrease. The law of conservation of energy is thus obeyed.
Subwoofers are a special case because they are generally non-directional. The drivers are so much smaller than a wavelength at the highest frequency they handle that they remain essentially non-directional. So, if you put two of them into a room, their joint directionality will remain negligable. There will be no preferred direction. There will be no non-preferred direction.
An obvious exception would be a subwoofer that is directional such as a fully horn-loaded subwoofer. Cardioid subwoofers might be another exception.
If two subwoofers are spaced apart then coupling between them starts to weaken and output increases are not as significant. Usually 3-4 dB's across the band. Until, of course, frequencies match the 1/4 wavelength distance that the subwoofers are spaced apart, you should see full maximum coupling and so should expect a 6 dB output gain.
The reason for this is because you get positive reinforcement when the wavelengths interact at 1/4 wavelength distance between two sources. The levels, frequency and phase also needs to be set identically to reap the full benefit of coupling.
As soon as you start getting wavelength-related effects, response of course becomes very non-flat.
I'm reminded that speakers are basically antennas, and this is becoming more apparent as we start working with arrays of them.
We think of antennas as having "gain" but of course they can't have any overall gain if they are passive. What antennas do commonly have is directionality, and they obtain that property we call gain by the same means as loudspeakers - by means of directionality. If you measure the power radiated by a so-called high gain antenna in all possible directions, there is no over-all gain. Instead we see the power being concentrated in one direction at the expense of all the rest.