I'm really surprised to see that the "Double Bass Array" (DBA) is not very popular outside Germany. I searched for key words, but found only one thread in the AudioCircle forum.
Maybe someone is interested in this concept, since it is by far the smartest way to get a linear frequency response and completely get rid of room modes.
The DBA was described first in a paper from Klein+Hummel
which unfortunately exists only in german. I hope my english is good enough to give a small overview here.
A DBA consists of two identical arrays of subwoofers. One on the front and one one on the rear side.
The subwoofers have to be mounted on special positions on the wall. For example if you have 4 drivers in one array (that means overall 8) ranged in a square, their correct place is at 1/4 and 3/4 of the wall's width and height. Like this (my room in an early stage):
The side walls work like mirrors and have the same effect like more equidistant bass sources. This completely eliminates the room modes between the side walls and between floor and ceiling.
With this order the front array produces a plane wave which propagetes through the room. When it arrives on the rear wall the second subwoofer array creates the same signal but with inverted polarity. So both waves compensate each other and no reflection on the rear wall occur. The bass is completely free of modes!
Of course it only works, if the rear array is delayed by the time the sound needs to travel from the front array to the rear (delay = speed of sound / room length). Such a delay can be achieved by using cheap DSP equalizers like the Behringer Ultracurve 2496.
The level of the rear array usually has to be a bit lower than the level of the front array, since there is always a bit loss in real rooms when the wave propagates. But with measuring equipment the best setting is easy to find.
The subwoofers itself should have a low depth so reflection from the mounting wall get minimized. Ideally the drivers are build directly into the wall.
Of course a DBA also works with more or less drivers per wall. It is only important that the distance between 2 drivers is twice as long as the distance between the driver closest to a side wall and the side wall itself.
Both dimensions can be considered completely independent of each other.
For example if you want to use only 2 drivers per array, they have to be mounted on 1/4 and 3/4 of the room width and on the middle between floor and ceiling.
Denser driver grids conclude in a higher frequency where a plane wave will still be formed. With common room dimensions 4 drivers per array are enough to ensure a plane wave up to the LFE cut-off frequency.
The big advantage of this concept is to be completely free of room modes and to get exact the frequency response of the simulation. The maximum sound pressure of the DBA is defined as the number of subwoofers in one (!) array times the maximum sound pressure of one single subwoofer.
Another advantage is that the bass is fine on a large area and not limited to one seat. This makes it suitable for large home cinemas. Also the subwoofers can be integrated perfectly because of their small depth. A simple curtain is enough to hide them completely (my front looks like this
now). I have seen other solutions which integrates them into self-built shelves in the living room.
And with a DBA playing "Sokoban" at home is over!
The only disadvantage I know of is the adaption to the front speakers, since their sound pressure usually lowers with 6 dB when doubling the distance and DBA's sound pressure stays constant in the whole room. So if you adjust a linear frequency response for the first row of seats the second will get slightly more bass (or in fact lower sound pressure from the fronts).
The costs of a DBA strongly depend on the components you want to use. My solution is a very cheap one with about 1100€ for the whole setup. But with more and bigger drivers the costs can easily explode to a multiple of that.
Frequency response of the Simulation (with same lowpass and equalizing like the real one):
1/3 octave smoothed measurement in the middle of my room (looks like a fake, but it is not!
The waterfall shows no sign of first order modes: