Building Resonance Free Subwoofers

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.

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.

By the way: Rythmik sells the factory-built F15HP with a 3 cu.ft enclosure, but for DIY buyers recommends a 4 cu.ft enclosure for greater minimally-greater output at very low frequencies. I did 4 cu.ft., which ended up measuring 24" H x 18" W x 24" D. Those dimensions may be manipulated in any way one chooses to create the 4 cu.ft. internal volume. I chose mine purely on aesthetic grounds.

I also built the Rythmik Audio/GR Research OB/Dipole Subwoofer, again with a double-wall MDF/Baltic Birch plywood construction. I built the OB "frame" in the "W/M" style, rather than the more common "H" style. Siegfried Linkwitz also chose the W style frame for his OB sub. I added a brace across the "open" side of the frame, ’cause why not?

Wow, and I thought my design and build was extreme!

I originally was going to make the enclosure for the Rythmik F15HP DIY kit employing the design Danny Richie shows on his GR Research website: a double-wall box, with the space between the two walls filled with sand.

After considering the resulting weight, I instead built a dual-wall box, the inner layer MDF, the outer Baltic Birch plywood, with no space for sand. I then braced the Hell out of it: a 1.5" x 1.5" BB ply brace every 5" in every plane---front-to-back, side-to-side, and top-to-bottom. The bracing prevents the enclosure from "expanding" in reaction to the low frequencies contained in recordings, minimizing the resonance of the walls and raising the frequency of that resonance to way above the frequencies the sub reproduces.