Kind of a misleading statement... The drivers could be completely dis-coupled (as you state it) and loose zero net output if there are no phase differentials... It is simply about phase issues between drivers, that is rarely an issue if the box is somewhat symmetrical... In a properly build enclosure, there is never any output gained by common airspace, the only exception being minimum vent dimensions... not completely I made that statement after years of testing with a termlab , I'v even built removeable baffles so I could test the difference with the subs together and apart , the improvement is not always dramatic but I always get better results with the subs almost kissing each other in the same cavity the old school rule of thumb is to be with in a voice coil diameter of each other but I try and keep them about an inch apart Mutual Coupling of Loudspeakers Mutual coupling of two or more loudspeakers occurs then they are reproducing th same signal with their acoustic centers close together and pointing in about the same direction at wavelengths smaller than the distance between the two centers. The combined sound signal is propagating forward like one single waveform, the two or more smaller drivers behave like one larger one. The centers of two loudspeakers must be close enough together between 1/4 wavelength and 1/2 wavelength. The higher the wavelength the shorter the necessary distance between the two centers to build up mutual coupling. The actual effect is always direction depending because of cancellations off-axis. Actually (theoretically) the coupling exactly on-axis occurs always independent of the spacing. At smaller wavelengths cancellation occurs earlier off-axis. At longer wavelengths the angle off-axis without strong cancellations is larger. The mutual coupling should be seen a an effect of the average sound level on-axis and off-axis. The acoustic centers' distance and the wavelength of the signal actually determine the angle over which mutual coupling occurs. no mutual coupling with acoustic centers more than 1/2 wavelength separated mutual coupling with acoustic centers less than 1/2 wavelength separated With two loudspeakers next to each other at a distance of more than the wavelength of the reproduced frequency the average measured volume on-axis and off-axis adds up for +3dB (double power). With two loudspeakers close enough to get mutual coupling the volume adds up for +6dB. Because of the physical size of loudspeaker chassis mutual coupling always occurs only at frequencies below about 500 Hz. So would this still work when using 4 subs or more a inch apart
