Infinite Baffle Subwoofer Install Story
In my decent sized
home theater (26 x 16 x10),
I used to run 2x Velodyne ULD-18’s as my subs. These are very good
subs and have 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 ;-)
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:
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 IB’s, 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:
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:
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
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…
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 ad 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
my Crown XLS602 pro-sound amp, I get 600 watts per channel into 4
ohms, or a total of 1,200 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).
No wonder the people who build IB’s are in a ‘cult’, I’m now
‘cultured’ as well
