Home Theater Subwoofer

[Paul H]

We're going to be building a home, (now at the design stage) and our preliminary plans call for an approximately 34'x26' x 9' high bonus room over the garage. It will be a home theatre / audio / pool table / lounge room.

I'm thinking of creating a storage room at the end (say 3' to 4' deep, leaving a 30' x 26' room) behind the screen to house all electronics, wiring, possibly DVD's and CD's, and other stuff. The storage room and screen are at the top of the sketch below:



My main speakers are already built - large dipoles described in a previous thread. They are full range for most music purposes; I want the subwoofer for the 40-50 Hz down frequencies.

Questions:
1) Do I put Infinite Baffle sub(s) in the front wall (backs firing into the 3-4' deep storage room), or is there another better idea?
2) If they're in the front wall, should they be centred, or in left and right corners, or lined across the front?
3) How many drivers / how large / suggestions for drivers? I'm looking for clear clean reference level bass down to subsonic frequencies.

Thanks,

Paul

[ThomasW]

If there's an attic above the room put the IB there. It's a bigger space and the SQ will be better than the 'closet'.

Alternatively one could cut a hole in the floor (garage ceiling) and install the woofers there.

This is a BIG room. For an IB you'd want 4-15" or 18" drivers. That would give you high output, low distortion, and plenty of headroom.

Ascendant is coming out with a reasonably priced 18" specifically designed for IB use. Cost will be under $200USD each.

[Paul H]

Thanks Thomas,

The 'closet' at 26' x 4' x 9' wouldn't be big enough?



Paul

[ThomasW]

Depends on the Vas of the drivers. The goal is to have a space at least 10 times the sum of the driver's Vas.

Also I'm not sure you want the rear wave pounding the backside of the gear.

[Paul W]

I like the idea of using the garage to vent the backwave. Where is attic space in this layout? Assuming it would work physically, I would probably put an array on each sidewall between the main speakers and the primary listening position; vented down to the garage.

I watched a couple of concerts last night and still marvel at the performance of IB bass. It will very likely be the last thing I change in my system. Thanks Thomas :T

[Bent]

Being Nova Scotia, insulation becomes a bit of an issue for him - by that, I mean thermal insulation.
That could well eclude using the attic or garage in his case.

[Paul H]

The attic space is over all of the (2nd storey) room, but choosing between the attic and garage I will likely go with venting down to the garage.

Thomas based on your recommendation I'd probably go with at least 4 - 18". I'll be leaning toward overkill - but you probably knew that


Paul

[Paul H]

Being Nova Scotia, insulation becomes a bit of an issue for him - by that, I mean thermal insulation.
That could well eclude using the attic or garage in his case.

The drivers will become a 'cold spot' but it should be workable.

Nova Scotia is cold in the winter, but it's not like it's the Canadian prairies :twisted:

Paul

[Bent]

It might look kinda funny if your chosen drivers have a spun aluminum cone - they'll be covered in frost with you system turned off, but it will dissapear in about ten seconds after powering them... LOL.

Actually a garage as the rear chamber might be really slick - it would be very easy to keep it above freezing nost of the time.

[ThomasW]

Choose whichever location is the easiest to build and live with......

Some cold climate owners have used a removable plug that seals the opening for their attic manifold. One idea would be to glue up several layers of extruded foam insulation, cover it with cloth, and use Velcro to hold it up when the sub's not running

I watched a couple of concerts last night and still marvel at the performance of IB bass. It will very likely be the last thing I change in my system. Thanks Thomas

My pleasure.....

You might not want to replace the IB sub, but if you're feeling flush and want to upgrade, the Thigpen Rotary sub would be a great addition to augment the infrasonic part of the spectrum (20Hz down to 1Hz). You could be the only person in town to reproduce the 3Hz content in "Batman Begins" ....:wink:

[Paul W]

I'll pass on the Thigpen (for now). Guests are already scared by the sofa moving...and I get scared when I see the windows flex :twisted:

[Dennis H]

Has anyone tried insulating their IB with unfaced fiberglass on the attic side? I'd think it would have minimal effect on the tuning -- probably an almost unmeasurable change in Q and F with 6-12" of batting.

[ThomasW]

One fellow with an IB installed in a rather small crawlspace did just that, and really liked the results. He said the bass was tighter with better definition.

One of my next experiments is to line the inside of my small manifold with a layer of felt gasket material. I curious what effect if any that will have....

[Dennis H]

Has anyone tried insulating their IB with unfaced fiberglass on the attic side? I'd think it would have minimal effect on the tuning -- probably an almost unmeasurable change in Q and F with 6-12" of batting.

Okay, so I modelled sort of a worst-case scenario just to see. For a minimal IB, with box volume = 4x Vas, heavily stuffing the enclosure (changing Qa from 100 to 5), Fb doesn't change at all and Qt changes less than 2% from .419 to .412. This would be like tightly stuffing your whole attic with fiberglass. If the enclosure is bigger and/or you use less stuffing the change is even less. With a 10x IB and a foot of fiberglass, the change in Qt is a tiny fraction of a percent. The change in Fb and Qt caused by running the driver hot or cold will be much greater than what a little insulation behind the driver will cause.

Bottom line, go ahead and insulate that attic IB with unfaced fiberglass. You won't be able to hear or measure the difference in the sound and there's no need to have your cones frost up in the winter and your surrounds melt in the summer, not to mention all the energy you waste.

[Paul H]

... Bottom line, go ahead and insulate that attic IB with unfaced fiberglass. You won't be able to hear or measure the difference in the sound and there's no need to have your cones frost up in the winter and your surrounds melt in the summer, not to mention all the energy you waste.

Thanks Dennis. That's good news for those of us in cold climes with a desire for large holes in our attic/garage for those big driver surface areas

Paul

[Paul W]

Considering energy conservation in general, and the prices projected for this winter, fiberglass seems like a really good idea. Any suggestions for holding it so that it doesn't move? (Best thing I've thought of so far is fiberglass window screen inside and out.)

[ThomasW]

It would be a rare situation where the space 1"-20" above a heated room would cause frostey cones.

A 20" cube manifold means ~18" square opening between the room/attic. Using a removable foam insulation plug would mean that only when the sub was operating would there be any heat loss. Since the woofers seal the opening it wouldn't be acting like a chimney even with the plug removed

[Dennis H]

Maybe I'm being dense but why would you want to climb up and remove a plug every time you play some music when you could just insulate the thing and forget it?

Paul's window screen (or maybe stiffer hardware cloth) idea sounds good to hold it in place for a difficult install. Most manifolds you could probably just wrap the batt around and tie it in place with some string. Another idea might be to build a box out of 703 or 701. That would allow easy access to the drivers.

[ThomasW]

If you wrap the manifold with insulation you've created a form of aperiodic loading.

I don't know what would happen if a mesh cage were built separated several feet from the manifold, and the cage was then insulated.

Unfortunately mathematical model changes in things like "Q" don't account for the sonic differences between sealed boxes and a true IB. I really wish they would, because that would make things much easier for people to understand.....

[Dennis H]

If you wrap the manifold with insulation you've created a form of aperiodic loading.

I've heard that before but I have a little trouble understanding it. Seems like the "aperiodic port" is so big and offers so little resistance to the sound that it's effect would be negligible. I guess somebody is just going to have to try it and see if they can measure any difference. The close-mic'd-SPL and impedance curves should tell the tale. In a case like this, I would trust measurements more than subjective listening impressions where it's not possible to do a quick A/B.

[Paul W]

Dennis & Thomas,
I believe you are both right...it is aperiodic loading...and it is not to be feared. For other reasons, I've experimented with aperiodic loaded baffles and it takes a surprisingly high acoustic resistance to make a significant difference.

I picked up some relatively fine-pitch plastic fence (no shorts) and fiberglass at Home Depot so I'll report back after I get the itch to itch.
Paul