Install subs in a wall?

[gotoml]

I have two Polk 8" subs. The way my room is configured I can install them about 8' up (in the attic) in the wall by cutting out an opening and installing speaker grills.

My questions are:
1. Will I lose sound quality by only having the speakers exposed to the room (the ports will be in the attic).

2. Is it a bad idea to have the subs up in the attic (I live in FL and it gets hot up there in the summer).

3. Will placing two subs about 8' apart and 8' high be good or bad for the sound.

Thanks for any help.

Mike

[burdette]

Originally posted by gotoml

1. Will I lose sound quality by only having the speakers exposed to the room (the ports will be in the attic).

The answer to #1 makes the others moot, in my opinion. Yes, you will lose sound quality. The port is an integral part of the design of the speaker system. It would actually be providing MOST of the output at certain frequencies. You can't "port the ports" into the room, either, because doing so (adding length) changes their characteristics and therefore those of the speaker system as a whole.

If you did anything like you're talking about... build new cabinets in the attic (matched in size to the current) and direct both the drivers and ports into the room.

[Dr. Spec]

What you are describing is almost like an infinite baffle subwoofer.

We just finished one at my friend's house using two Stryke AV15 drivers. It came out great and sounds great too.

If you want to build a sub using an attic space, check out the "Cult of the Infinitely Baffled" website. If you have nominal woodworking skills, an IB is a piece of cake and cheap to build.

Doc

[Dr. Spec]

Here are some pics of the IB:

[Ceruleance]

Doc, why are there two sets of inputs on that driver?

[RuSsMaN]

"DVC"

Dual Voice Coil

[gatemplin]

Gotoml, have you tried stacking the subs in the corner with at least a foot of clearance? It may not look pretty but will probably sound better than putting them in the ceiling. I have found that placing subs in different positions creates weird hot-cold areas in the room.

[RuSsMaN]

Well I guess Redhouse was right, Doc is highly over-rated, and basically full of ****.

Just looking at this post, I can tell he knows little to nothing about *real* subwoofers, and bass in general.

///now back to reality///

DROOL!!! NICE work. What's the plan for the opening? Any sort of grill work? Just some trim on the edges to finish it out?

Cheers,
Rooster

[VR3]

Doc.....can you put subwoofers in my ceiling? :p Nice job! WOW!

Russ, Judging by the spikes they used over the screws, I believe no grill is planned. My guess....

[RuSsMaN]

Sid, I'm asking about the opening to the room, not the drivers themselves... are we on the same page?

Cheers,
Rooster

[VR3]

Yeah....I think so. Doubt we are, continue....I need sleep. :o

[Dr. Spec]

We're leaving the transition box open, with wooden trim around the outside.

Amp is a Samson S1000 and we are running the drivers in series with just one side of the amp for an 8 ohm load. Rated power into 8 ohms is about 365 so each driver is seeing about 180 watts - perfect for an IB.

You can't see it from the photos, but there are already top and bottom openings cut in the transition box for two more drivers and they are blank flanged and caulked with custom covers that have the same exact bolt pattern as the drivers. If we need more output, all we do is remove the covers and pop in another set of AV15's. The other side of the Samson is already wired into the root cellar and ready to go also.

For an IB, you take the Vas of the driver and multiply it by 10 and then multiply that by the total number of drivers. The result is the required volume of the back space. We have way more than enough volume in the root cellar for four drivers; this basement just begged for an IB application.

The HT room itself is 3000 ft3. At the couch 13 feet from the sub box we are seeing 112-113 dB on hot DVDs. At one meter we are seeing 122-123 dB. I don't think we'll need those two other drivers.

The initial sweep was very ragged at the xo of 80 Hz and we need to reverse the polarity at the Samson and try it again. There was a slight midrange bulge at around 50 Hz and then smooth sailing all the way to 10 Hz. Sound quality is typical of an IB - smooth and a bit soft but detailed. Group delay is non-existant and the bass is very cohesive.

Here is a screen shot of the in-room FR:

[Ceruleance]

what are the advantages of IB design? It seems like you lose a lot of SPL, why (or when) is an IB design better than an enclosure that is built into the wall with drivers and ports facing the room?

[Dr. Spec]

From an SPL standpoint, an IB actually plays a bit louder than an acoustic suspension speaker with the same size drivers. So it is hardly an SPL weakling. But you are correct, we could have gotten more output from a conventional vented enclosure. But two AV15's in a 3000 ft3 room seemed like a good start.

The IB drivers operate in a free-air evironment with no enclosure loading, so the motors work very easily and run cool and the amount of power required to drive them is minimal. The AV15 is rated for 500 watts thermal, but only needs about 200 watts to reach its xMax in free air.

Since the motors operate so freely, the output as the frequency drops is not limited by ever-increasing air resistance like a sealed design. And since the enclosure volume is essentially "infinite" with respect to the driver itself, there is no enclosure tuning point where you would require a resonator (like a port or a PR) to take over to extend the response. You can see the IB has as much output at 10 Hz as it does at 40 Hz.

The IB sound quality is just a tad soft, rich, smooth, detailed, and cohesive. It is very good sounding bass.

[Ceruleance]

that's interesting, thanks doc

[tryrrthg]

Man those AV15's are so DAMN sexy! sweet looking drivers!

Nice job Doc, to you and your friend!

How would an AV15 in, say a 250l enclosure, tuned to 18hz compare to the PB2+?

[Dr. Spec]

A single AV15 cannot move as much air as a PB2+ with all ports open and its dual dB12 drivers.

Also, tuning the AV15 to 18 Hz will limit its output by requiring greater cone excursion before the tune point as compared to the PB2+ with a 25 Hz tune.

If you tuned the PB2+ to 20 Hz by plugging a port, it would probably be a close race.

Without looking, I think the AV15 displaces around 3.7 liters at xMax and the dB12 does 2.6 liters (5.2 liters for dual).

The AV15 is still a monster driver and a single would do very well in a room up t0 3500-4000 ft3 in a vented application.

Doc

[hoosier21]

With an IB, is the sizing of the opening into the room critical like it is with a horn loaded system? I have seen several IB’s and they all have different shapes and sizes so I am assuming it is not real important. But there has got to be a minimum, right?

[tryrrthg]

thanks Doc!

someday I picture a Wilson XS clone in my basement. Dual AV15's in a 500l cabinet, tuned to 19 hz with four 6" ports. If I made it with the same footprint as the PB2+ it would be taller than me (6'3") I can hear the wife bitching now! :D

On an IB note, I just read that with an IB the QTC of the sub is the same as the QTS of the driver. If this is true, then will the IB sound like a VERY low Qtc sealed sub? So basically the IB is the best of both worlds, the extension of the ported and group delay of a sealed... VERY cool!

[Dr. Spec]

The general rule of thumb with the transition box is the smaller/shorter the better. Only large enough to accomodate the drivers and retain structural integrity.

The famous "12 Shiva's Dancing" IB uses a very long transition box that in theory will create driver interaction in the box itself. Two shorter boxes would have been preferable in that case.
Yes, the IB is the best of both worlds - there really is no lower limit on the deep extension and there is zero group delay in theory, and transient response is also excellent if the amp has a high damping factor and the woofer motor is well controlled.

Another guy I know is trying one with the Dayton 15" woofers specifically designed for an IB application and they have a Qts/Qtc of around .707, if I recall.

Remember, there is no enclosure loading, so the only thing stopping the woofer from moving after the signal has ended is the woofer suspension and the amp damping factor. I think the Samson is up around 400+ so it was deemed "good enough". A Crown would have been even better because they tend to have extremely high DF, but they ARE expensive. Just don't discount the amp itself as part of the IB design because it is important.

[burdette]

All the talk about IB is interesting, but the original post was concerning the two Polk subs the guy already owns... I think we've agreed he can't put those in his attic and have satisfactory results with the ports opening into the attic itself. And he can't be assured he could just pull the Polk 8inchers and create an IB system with them... so he'd have to buy new drivers to build an IB.. and if he is going that route, he could build *any* type system he wanted and plunk it in the attic...


Whoever said it.. you're right.. those Stryke drivers are.. well.. striking. They really do have a great look. I opened the box of my single AV12 and literally said "holy ****".... I can't imagine what it was/is like to be in the presence of Gonzo's double 15s. The aluminum cone and heavy black rubber surround create a very cool look... and the back of the drivers look as good... I almost wanted to make a plexiglass enclosure, or put in a viewing window or something.