Building Resonance Free Subwoofers

@clio09 The swarm is built to a price, but I agree crossover flexibility is an important factor when is comes to integration. The most flexible by far are digital.

@bdp24 You won't get an argument from me. Roger Modjeski was absolutely right. Big flat panels have low frequency resonance issues. Sound Labs deals with it by using, I think it is 10 different sizes of panel. Acoustat used felt pads on the back side of the panels to dampen them. The best solution is to cross out at 100 Hz, but you have to use a very steep curve or you wind up with subwoofer in your midrange which is poison. I use a 10th order filter, 48 dB/oct. The test is listening for voice in the subwoofer system, there should not be any or you smear the image. With shallow filters, even 18 dB/oct, you have to cross out below 60 Hz to keep the sub out of the midrange. Another advantage of having your subwoofer system active from below 100 Hz is increasing the gain on this segment between 10 and 100 Hz allows you to get the feel of a live performance without having to resort to ear damaging volume levels. 

With multiple subwoofers placement is not as big an issue with point source systems. Having a line source system I have to arrange them to form a linear array or they will fall behind the main speakers. Another benefit is the bass response throughout the room is very even with a slight increase at the boundaries. 

Most impressive in all respects. I just finished my bass system consisting of 2 Magnepan DWM's for the lower 3 octaves and an isobaric sub, which I thought was heroically braced until I saw your design. Nicely done!

@terry9 Thanx!  I have never heard the DWM. I assume you are using Magnepan loudspeakers. Where does the Isobaric sub come in?

It is important to note for the gallery that properly made cylindrical subwoofer enclosures do not require any bracing as they are inherently extremely stiff. Another interesting trait of the decadron cylinder is if you travel around the long axis in radians the enclosure wall thickness is constantly changing from 1 15/32" to just over 2 " at the joints. The enclosure wall dampens itself.  

I use four modern Quads, modified. They are crossed over at 18dB, 200 Hz (factory), but they need stabilizing, which is easily done with a platform box of baltic birch with a non-resonant top. Each box is stabilized with a Bryston mono block on the lower shelf, so I use the mass instead of it using me. Soon the DWM's will be further stabilized by Al bracing, a la Mye.

I am building an active crossover, must get back to it.

Subs are also a dipole isobaric, premium 10" Scanspeak woofers, crossed at 50Hz, 18dB. Both DWM’s and Scanspeaks are unusual in having good transients and linearity extending more than 4 octaves above the crossover point, so that the transitions are pretty seamless (to me, another Quad user, and a Magnepan owner). Sub drivers are also Bryston powered - bass doesn’t need more, but it also sounds a lot better than less. IMO.

You are so right about active control over the bass. Would never go back. Both of my bass systems are actively controlled in terms of volume, but only the sub has an electronic crossover.