Building Resonance Free Subwoofers

@bdp24 Good for you! Keep it up.

Now, put on a bass heavy tune and turn it up. Put your hand on the sub. Any vibration you feel is audible. Ideally, you should feel none. I went through 3 other designs before developing this one including extreme mass (sand and in my case solid surface material, Corian) which did not perform at the level I was looking for. Two important concepts are the cylindrical enclosure which does not require bracing and balanced force topography, opposing drivers. You can get cylinders in all sorts of materials like Aluminum. You can get 15" aluminum pipe cut to length, figure out a way to finish the ends and plant two drivers in it, one at each end. I'm not a metal worker, but I think you could weld aluminum caps on the ends and cut  holes for the drivers then have the whole thing anodized. I thought about coopering cylinders out of hardwood and turning them round on the lathe but because my system doubles as a theater the subs had to be black and I wanted a more dramatic shape then a plain round cylinder. Size is always an issue for aesthetic reasons. I was thinking of using 15" drivers, but the enclosures would be almost double the size and since I am using 8 drivers 12" is more than enough. Because the drivers brake each other you want a driver with a BL product certainly above 20, the higher the better. This also allows you to keep the enclosure small. Rhythmic drivers are made by Dayton by the way. 

Another important concept for assessing bass performance is AB ing the system with headphones. This takes the room out of the equation and lets you know how much detail is in the recording and what the system is glossing over or adding in. 

Great stuff @mijostyn.

It is my opinion that most hi-fi’s I hear are lacking the "gravitas" of live music, the massive bass foundation of music heard in concert halls, and even smaller venues like bars and clubs. It is the bass frequencies that provide the physical sensations that are felt rather than heard in live music. When I hear a grand piano live, it makes the reproduction of that instrument on many hi-fi’s sound like a child’s toy piano.

I have a fair number of recordings of pipe organs made in cathedrals (some of them David Wilson recordings), and sufficient bass reproduction is required to create the "shuddering" sound those pipes make when the bass pedals play those very low notes. The bottom note a 32’ pipe creates is located at 16Hz! The lowest note of a standard 4-string bass (whether acoustic or electric) is located at 41 Hz, and many loudspeakers are incapable of reproducing even that frequency at live music SPL.

Before I built my subs I had a pair of HSU’s original sub (the SW10), which was a single 10" woofer mounted on one end of a Sonotube---a cardboard round tube, like those used in making cement pillars. The tubes made production costs inexpensive (the most expensive part was the real wood-veneered top end cap), and the round shape was effective at preventing enclosure flexing, as you noted. Unfortunately, Dr. Hsu used cheap woofers with foam surrounds, and the SoCal air pollution resulted in those surrounds disintegrating in relatively short order.

Before that I used the woofers in the old ESS Transtatic I loudspeaker, which was a KEF B139 woofer (which David Wilson employed in his original WAMM super-speaker of the 1970’s) mounted in an excellent transmissionline enclosure, which as you know are pretty hard for an amateur to make himself. For my sub builds, I didn’t have access to the necessary woodworking machinery (or the skills and experience to use it safely), so I drew up my sub design plans in the manner I learned in mechanical/architectural drafting in high school, and had a cabinet maker cut the MDF and plywood as specified in my drawings. He had a full wood shop, with a table saw, CNC machine, router, etc., and was very reasonably priced.

For anyone considering adding a sub or four to his system (very highly recommended), take a serious look at the Rythmik DIY kits. If I can do it, most anyone can! By the way, I got the ideas for my enclosure bracing after seeing the interior of the subs Jim Salk made using the Rythmik F12 and F15 sub kits. To see them just do a google search for the Salk line of loudspeakers. Unfortunately Jim has retired, and his custom Rythmik subs are no longer available.

@bdp24 Exactly! If you want to get anywhere near a live performance a powerful subwoofer system is mandatory. I might add that approaching realistic levels requires more gain than one would expect. Standard speakers, even large ones can not handle this. Room control, digital EQ and crossovers are important additions. I would even go so far as to say they are mandatory for the best performance. They also make integration a breeze. 

My last set of commercial subwoofers were early Velodynes and like yours the foam surround disintegrated in 5 years. Garbage. That is when I started building my own. As you note there are many roads to Rome. Kits are a wonderful and inexpensive way to build a subwoofer system. Dayton also offers subwoofer kits and they perform just as well as most commercial units even if they are not as sophisticated in the finish department.  Box enclosures do require extreme bracing. You should see how Magico builds their Q series subwoofers, wild. They are also balanced force like mine. KEF and Martin Logan also make balanced force subwoofers, but continue to use box enclosures. Aside of cylindrical enclosures I would also suggest that like Audio Kinesis, they make their electronics packages outboard. They also need to add high pass filters. 

@mijostyn IIRC the Dayton Audio amp that Duke LeJeune recommends for his swarm has a high pass filter but it's fixed at 12 dB per octave. So not very flexible. It's why I decided to use my own active crossover for the swarm I put together.

When I spoke with Roger Modjeski (of Music Reference) about mating subs with the old QUAD ESL loudspeaker---of which he was a huge fan and owner, he recommended employing a crossover frequency of 100Hz, with 4th-order filters in both directions (high pass and low). He maintained that the bass panels of the ESL had a rather pronounced resonance in the 50Hz-100Hz frequency band, and benefited from not allowing the panels to reproduce those frequencies.

The subject of whether or not to use a high pass filter with the main speakers is a matter of some disagreement. Making a seamless transition from speakers to subs is not easy, but having as good a set of subs as possible is of course the place to start, whatever crossover filter characteristics one prefers.

It is my opinion that relieving the loudspeakers the duty of reproducing very low frequencies can greatly benefit the loudspeaker’s reproduction of the higher frequencies the woofer must also reproduce (commonly into the midrange). Employing a crossover frequency of 100Hz with 4th-order filters is a good general recommendation. Finding the best room locations is the next order of business.

Place them where they best address the room’s high and low pressure zones, where bass frequencies either disappear into black holes or "ring" far past the point where the signal has ended, the result of the room’s dimensions creating those zones, referred to as "eigenmodes". Tho location of the "modes" of any given room can be found by entering the room’s dimensions into one of the mode calculators findable via a Google search. If at all possible, do NOT place your subs in those locations.