After reading about Infinite Baffle subs for some time, I was keen to try one. The turning point came when someone wanted some information about the Venom 15inch driver available here in Australia through Jaycar.
I have been building up some designs for the Jaycar drivers, but hadn't done any for the Venoms because the low Xmax and high-ish Vas didn't augur well for my preferred EBS ported designs.
It was then that I looked at doing up some IB designs using 4, 8 or 12 drivers.
Having awoken my inner bass monster, I felt that I had no choice but to build the twelve driver design. A pair of six-driver manifolds have been built and installed giving around 22 litres displacement. A Behringer EP2500 amp is used to power this bit of extravagance.
Room Eq Wizard was used to choose
the best location for the manifolds, which turned out to be wall mounted either side
of the current seating position.
The manifold design is a result of reading the advice available at
the Cult
of the Infinitely Baffled
Some of the ideas used are:
- This design gives an opening area roughly equal to the combined Sd of the drivers meaning no potential problems with compression or bandpass effects creeping into the picture.
- The sound arrives from all drivers simultaneously, avoiding the dip in response that can occur with "tunnel" type manifolds.
- Opposing the drivers, and using bracing between the drivers should give some cancellation in vibration.
Each manifold sits on a plinth which takes all the weight, allowing a floating
design whereby the manifold is isolated from the wall opening by 5 -10mm
of silicone sealant. This will further cut down on transmission of vibration
to the house framing.
I'm glad I didn't try to fit these into the ceiling. Each manifold with drivers
weighs 75kg (165lbs)
WinISD predictions
SPL
The blue shaded area is for when excursion is allowed out to 20mm, which is the suggested Xlim. It's the zone where cone breakup leads to distortion, and thus represents 4dB of emergency headroom rather than a normal operating region.
The green lines show what can be expected after room gain is added and some cuts are made to the higher frequencies
Excursion
The grey line is the resulting excursion 1700w applied
Here is the response at the current seating position using the EP2500 amp
The grey curve is with the amp onlyThe red curve is with the BFD applying seven filters
Just for interest, here are the seven filters used to smooth the response
Connection and Calibration
A short RCA to XLR cable (as per the Behringer specifications), connects the AV reciever to the BFD.The long run to the EP2500 in the "engine" room, is handled by a balanced XLR to XLR cable to cut down on induced noise.
Had I wanted the BFD the other room, I would look at building an unbalanced-to-balanced converter
The BFD has a bar graph showing input / output level, including yellow warning and red clipping indicators.
A nice feature of the EP2500 is that you can parallel the inputs using a DIP switch, saving a Y-connection and additional input plug.
With the EP2500 off and the BFD filters active , I experimented with output levels on the AV receiver.
The BFD can work with either a -10dB setting or a +4dB setting. I used the -10dB setting as suggested by the "Cult" experts, and found it was fine. For those with a much higher output level from their AV amp, switching to the alternate setting will effectively cut the signal by 14dB.
If your signal is too low, even on the -10dB setting, you'll need a level shifter, such as the Art Cleanbox. It also handles the RCA to XLR conversion for you. Note that there's also a Cleanbox II, which is just a pair of isolating transformers used for stopping earth loops. That's not what we're looking for here...
Warning:
It seems that the frequency response of the Cleanbox rolls off below 100hz.
Not exactly the best thing for a subwoofer!.
See
this thread at HomeTheaterShack for more details.
They have a response graph, and post the following figures...
| Frequency in hz | Insertion loss in dB |
| 100 | - 0.4 |
| 50 | - 1.5 |
| 20 | - 5.4 |
| 10 | - 10.3 |
| 5 | - 15.9 |
| 2 | - 22.6 |
They've designed a simple modification to fix this. Alternatively, you could compensate using a BFD filter.
I found that my existing scheme for setting the subwoofer trim, as detailed on the Hot and Cold page, caused the yellow warning leds to light occasionally on the loudest peaks - just what I wanted!
With the BFD outputing the largest possible range without clipping, the next
step was to work out where to set the controls on the EP2500.
By slowly increasing the volume and listening carefully, along with some
SPL measurements, I found that the EP2500 attenuators can be set fully clockwise. The maximum SPL delivered
is between 115 - 120dB at the seat. This works out quite well as it means
that the volume can't be inadvertantly turned up too high
So how does it sound now?
Scary!
Certainly it's loud enough to move the house around.
I'll let you know more when I've worked out how to measure the power into the drivers.
Meanwhile, here's a quote from Chrisbee at the "Cult" where he is cautioning against building a huge IB for a small room.
Do not underestimate the brute power of low frequency pressure waves causing sympathetic vibration in building surfaces.
It is not the mechanical vibration of the drivers but the invisible cyclic air pressure changes that will make themselves felt.
You cannot brace against these. My modest 4 x 15" can make the whole roof creak on the REW test sweep and the floor turn to jelly.
The doors vibrate so hard that my full weight leaning on them makes not an ounce of difference. The force is irresistible.
Others have reported waves crossing solid concrete floors.
Walls and ceilings are even more subject to flexure than floors.
The latter are always more heavily built to withstand crowded parties and heavy furniture.
It would be a shame if your installation's potential was limited by the structure of the house.
Start digging for a reinforced concrete cellar?
This was before he converted his array into a manifold setup, which has less vibration. Still paints an impressive picture though!
He is using four AE IB15s giving a displacement of around 9 litres
You can see more of what people write about their IB's at the "Cult's"
notable quotables section
The Jaycar drivers used in this design are unfortunately no longer available.
Alternative solutions can be found on the Tempest-X IB page
- There are some more pictures in the The Gallery
- The IB4 IB8 and IB12 designs on the 15inch Venom driver page
- The driver wiring options page shows how this system can be wired, as well as options for the 4 and 8 driver designs.
