Mixing subs (Reflex)

Maze wrote: I'm looking for some advice... I have 4x double 18" reflex subs that I'm pretty happy with. However I have been playing with the idea of adding two more to make it 6 in total.

You want the exact same box and drivers for maximum gains, everything else results in something less than what it could be.

The question is how much less?

If the speakers match and sum coherently,increasing the cabinet count from 4 to 6 will result in a maximum additional 3.5dB level. Even with identical cabinets, due to the fact that the drive units can't occupy the same physical space the summation will already be less than optimal.

If different cabinets are used, there is a possibility that the frequency amplitude response might be different form the originals. Thus the summation will be different across the frequency range. If the new cabinets put out more acoustic power with the same electrical power input as your existing cabinets the combined output may be more than 3.5dB, but on the flip side it could be less.

As earplug alluded to, different cabinets also might have different phase characteristics. Doing random simulations, in winisd, with different drive units and cabinet designs, the phase response never exceeded 40 degrees. Summing two identical sources, with a 40 degree phase shift results in pretty good summation. This can be simulated in a DAW using a couple of sine waves.

So, whilst I agree with madboffin, and earplug, that similarish cabinets will work together, there are some practical considerations. If your existing cabinets operate down to 30Hz, and the new cabinets operate down to 40Hz, running them with the same high pass on the dsp may cause over excursion of the drive units in the new cabinets. Even with the same transfer function (low signal frequency response) different drivers can excursion limit at different amplitudes. This can be mitigated by running the new speakers at a lower level (with their own amplifier), with their own dsp settings, or raising the high pass filter on the existing dsp. Unfortunately doing any of the above will lead to less additional output than you may have anticipated. As I mentioned above, on the flip side it could be better. Without details of the drive units and cabinet design, there is no way of predicting.

If you were building your own cabinets, all the above could be calculated in advance, but as I assume that you are buying commercially made cabinets, that is not possible. Self powered cabinets would get round the problem.

The final consideration is the physical size of different cabinets. Will they integrate in to your existing system.

Renting some cabinets and testing, should answer some of these questions.

Edited by Maze - 02 January 2026 at 1:00pm