The goal of any subwoofer is to deliver the lowest octaves of music with the least amount of distortion possible at sound pressure levels that match those of the rest of the system.

Since my Martin Logan rig can play very clean at SPL's greater than 105dB, I need low frequency reproduction that can keep up and pressurize the 3K cubic foot home theater.

While I had two Velodyne ULD-18's in my home theater before, they could not move enough air to keep up. Which brings me to the main point of all great subs: they have to displace air in significant quantities.

Most commercial subs have relatively small volumetric displacements, and I'll use the rather large ULD-18 here for comparison, which displaces a total of 2.15 liters (or 131 ci). In contrast, my IB sub displaces 15.24 liters (or 870 cubic inches), which 6.5 times as much one Velodyne.

The other characteristic of Infinite Baffle subs is that they have very low distortion figures due to operating many drivers in their cleanest ranges and not forcing them to compress air in a box. This IB is the perfect match for the ultra-low distortion Martin Logan electrostats.

I innovated a design for in-room manifolds (called an 'outie') with this particular design, as can be read in the install story below.

The system specifications are:

  • System Frequency Response

    • 18 - 120 Hz +/-2dB

  • Crossover Frequency

    • 80 Hz at 24 dB per octave

  • In-Room manifold IB design, opening to 3,500 CuFt below HT

  • Sensitivity

    • 92dB/1 watt/meter

  • Amplifier

    • Sunfire Cinema Grand. - 400x2 into 4 ohms = 800 watts total to IB (more than enough to reach xmax)

  • Drivers

    • Stryke AV15 Mk-II

  • Impedance

    • 2 drivers series wired on one VC to yield 4 ohms , with each pair wired to one amp channel

  • Construction

    • Dual layers of 16-ply Baltic-Birch plywood

    • Glued and screwed together. Glued and screwed to floor

    • Weight (drivers + Box): 40 lbs (not that it matters, as it's bolted and glued to the floor now)

Information on IBs:

IB Install Story

[Circa 2005] In my decent sized home theater (26 x 16 x10), I used to run 2x Velodyne ULD-18’s as my subs. These were very good subs and had low distortion (my main criteria for ‘good’). However, they did not play quite loud enough to keep up with my Martin Logan Electrostats (big Monoliths III’s in the front). Also, after 12 years with them, I wanted something new.

This led to investigating subs and sub options, which eventually led me to the ‘Cult of the Infinitely baffled’ site, which led to a conversion (thanks Thomas).

Having gotten IB ‘religion’, I investigated how to set one up in my situation. Turns out I have a perfect setup, as the region beneath that large HT (plus another 6’ beneath the equipment room) is a large open crawl space/ storage room with about 3,800 cubic feet of air space. No problemo for an IB venting into that ;-)

Hole in the floor

Hole in the floor

The big problem turns out to be that the ideal location for the manifold is the front / middle of the room, which also turns out to be where the homes main power feeds (two huge 200amp service feeds) run right under that spot of the floor as can be seen here on the left.

This lead to a brainstorm that said: an IB manifold can work in reverse as well, so have the box with the drivers in the room and vent the manifold through the floor. This would wind up looking like a ‘normal’ sub box, albeit one with 4 drivers facing out of it on each side ;-)

After a few exchanges with the members of the ‘Cult of the Infinitely Baffled’ forum (thanks guys), it was established that it ‘should’ work in theory.

I selected four Stryke AV15 Mk-II drivers as the ideal candidates, these 15” beauties have excellent characteristics for IB use (long xMax) and solid looking motors. An aluminum cone sounded like a good thing to have between the HT and the crawl space. Not many worries about humidity with aluminum ;-)

Then comes the box. As the IB cognoscenti out there know, dimensions aren’t too critical with IBs, so with the help of a friend (thanks Eric), we settled on a 20” internal diameter (22” external) cube made up of two layers of 16 ply 3/4 Birch plywood glued and screwed together to make an inch and a half thick box that is rock solid. My friend used 2 pounds of screws and 16 ounces of glue assembling this thing. Never seen a speaker box that strong…

Using his shop full of excellent tools, he cut perfect holes for the drivers and added a bottom flange so attachment to the floor would be straightforward and secure.

Here's the raw box, painted and ready for speakers and mounting:


Here's the raw box, painted and ready for speakers and mounting:


And here you can see the flange so we can secure the box to floor with screws:


Next is the install. Again, my friend Eric comes to the rescue, armed with all kids of cool tools to help install the box into the HT.

Having already selected the spot based on both computer modeling and in-room ETF analysis and one of my Velodynes, we marked off the region on the carpet were we would cut open the floor. Here's the test fitting of the box over the hole:

After carpet cutting and sawing out the sub floor in a 20” square at the location, we found that, as expected, at least one joist would be running across the opening. Not a problem acoustically, but we did want to really finish this off with flair, so we built an air deflector for the top of the joist (facing into the box), so there would be no turbulence created at that junction, as seen here:

Air deflector

Air deflector

Air Deflector

Installing the drivers into the box presented a choice: Install facing into the box (as is usual for under-floor manifolds) or put them in the box facing out (as normal for regular sub boxes). I chose to mount them facing out, as we had built a sufficiently large box, there would be no internal interference, and I sensed that the air volume was still large enough that no untoward noises would be generated by that. So far, this has proven to be true. This also improves the in-room look of the IB, as it really does look like any other large sub box, but with more drivers.

To secure the drivers in the box, my friend recommended doing this right and we chose to use fasteners, with t-nuts on the inside of the box, and machine screws from the front. Eric’s suggestion of a dab of blue Loctite to keep them from vibrating out is a good piece of advice


Speakers Loaded

Now that the hole was cut and the box loaded up with drivers, we gave it a quick test to check wiring and the drivers. Everything was fine. Mondo output so far…


Test Drive

Installing it into the floor was a piece of cake. Eric, in his usual meticulous way, measured out a screw pattern for the flange, making sure it did not interfere with the screws he had added to the sub-floor reinforcements he had also put in (belt and suspenders stuff), then drilled out the screw pattern from the back of the flange, drilled counter-sinks for the heads on the top side and then we were set to drop it in.

We put down a bead of Liquid Nails to give both an air tight bond and to kill even the remotest possibility of a rattle, then dropped the box over the hole and screwed it down



The wiring is pretty straight forward. I chose to use one voice coil per driver (the AV15 is a Dual Voice Coil, with each VC having a 2ohm load). To give the amp a 4 ohm load per pair of drivers, I series connected each pair to each channel. Using a Sunfire Cinema Grand, I get 400 watts per channel into 4 ohms, or a total of 800 watts for the entire IB. This is way more than enough to push these drivers to xMax.

So, How does it sound, you ask. It is awesome, simply awesome. Even before tweaking the parametric EQ’s, the thing just floored me with how clean it sounds. I played the new Sting album (in SACD) and you could just feel the texture of each bass note. The extension was also impressive (but my Velo’s also went deep).

The room actually gets pressurized now on the really low notes. This is a sensation that was rarely felt with the previous subs. I attribute that to the higher SPL capabilities at low freq’s of the IB.

Now that it’s all tuned and EQ’d, it is simply amazing. So clean, and so, so powerful. I had no idea a sub could deliver this level of performance, much less at this price point.

The quality of the bass is the best I’ve ever heard, with ultra-low distortion (the combo of Sub and center channel together measures under 0.6% distortion at 90dB SPL).

And here is the proof, some measurements of the IB and the companion 9 cubic foot sealed 18” sub symmetrically situated at the rear fo the room to balance out room modes.

Waterfall dual subs + Left Front

Waterfall dual subs + Left Front

First the Waterfall plot shows the rapid decay thanks to balanced subs, and extensive room treatments. Deep extension into the teens is evident as well.

Dual subs + left front Frequency response

Dual subs + left front Frequency response

Next is the frequency response, note how flat and even it is:

No wonder the people who build IBs are in a ‘cult’, I’m now ‘cultured’ as well 😉

Final picture shows the custom SL3XC center channel, which is bolted to the IB, so super solid mount:

SL3XC Center channel mounted

SL3XC Center channel mounted