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twynns
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Optimal sizing of in-wall backboxes

This was also posted in the loudspeaker thread. Sorry for dupe, but don't know exactly which forum thread more appropriate for this question.

Hoping someone can help with an in-wall speaker installation question. I'm trying to figure out how to appropriately size custom-made backboxes for in-wall speakers, when the mfr doesn't offer backboxes for this particular model, nor sizing (that I can find). I'm going to try and call them directly during the week, but don't know what help I'll get. My local stores offer some generic boxes, but I'm sure there's some science/math to this and I'd like to get as close to optimal as possible. Any help very appreciated! I have 6" deep studs on the walls and ceilings where I'll be installing these and it's new construction, so I have nearly unlimited flexibility in sizing the boxes.

Thx!

ethanwiner
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Re: Optimal sizing of in-wall backboxes

You'll get the best advice from the same company that sells the speakers. I agree you should call them.

--Ethan

Jan Vigne
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Re: Optimal sizing of in-wall backboxes


Quote:
I have 6" deep studs on the walls and ceilings where I'll be installing these and it's new construction, so I have nearly unlimited flexibility in sizing the boxes.

Are the ceilings open to an attic space or is there another floor above this room? If you are installing into an open attic space, you probably don't require a back box. Use the speakers as if they were going into an infinite baffle, which, in effect, they are. You might want to place a thin layer of insulation or preferably some polyfill over the speaker to protect it from dust settling into the assembly (beware of fiberglass which will also "leak" bits of itself into the drivers), otherwise the speaker should require nothing more than unrestricted space behind it.

If there is a floor above this room, you still shouldn't require a back box. Just some insulation around the speaker to keep any backwaves from bouncing back into the drivers.

I'm going to assume any wall/ceiling mounted speakers are not meant to be your best speakers and represent casual, noncritical listening. Even with a 5-8" driver six inches of total space behind the driver isn't much. It's the overall volume of the enclosure that matters but you can't really take an oddball shape and expect it to perform as well as an enclosure that is designed from ground up to suit the particular driver's requirements.

Don't sweat this.

The box is the formed by the studs of the wall. If this is existing construction, you can't really do much anyway. If this is new construction and you want to do something to make yourself feel better, try placing barriers across the studs to make an approximately one cubic foot enclosure behind the speakers.

twynns
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Re: Optimal sizing of in-wall backboxes

Thanks to each of you for your answers to my post. I have contacted the mfr but they have not yet replied.

To reply to JV's post, these will be mounted in two different sloped sections of a tray ceiling (each approximately 27 deg angles (the pitch approx 12" run for each 6" rise)). One slope forms the actual outside roof, and there is no space behind the framing on that section. On the other side, the slope is a false framed section to create the same interior angle, where there is "space" behind it. Hope that helps to clarify--not confuse...

I hear you, and agree, that in-walls are not sonically equivalent to good "box" speakers, however box speakers are not in the available option set for this room. Accordingly, we are trying to get the best sonic performance out of these speakers we can. Given that my two ceiling/wall sections are different--i.e. one can be infinite baffle and the other cannot--hopefully, you can offer an opinion as to whether I am better off matching each side with similarly sized backboxes, which was my original thought. The alternative, as you suggest, is to simply do little other than mount and insulate. I certainly don't want to build boxes if there's no sonic benefit. Any additional thoughts genuinely appreciated!

Thx

RW

ethanwiner
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Re: Optimal sizing of in-wall backboxes


Quote:
I hear you, and agree, that in-walls are not sonically equivalent to good "box" speakers


In theory, having speakers built into the walls is much better than having them in boxes in front of the walls. All professional recording studios have their largest "main" speakers built into the walls. This avoids a problem known as SBIR that creates peaks and deep nulls at bass frequencies due to reflections. However, this is a physics / acoustics issue, not related to specific speaker brands and models.

--Ethan

Jan Vigne
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Re: Optimal sizing of in-wall backboxes

Do understand I was not suggesting you install "box" speakers, just saying you are limited to some extent when you purchase ceiling/wall mount speakers. The type and volume of a speaker enclosure is determined by the Theil/Small parameters of the low frequency driver. If you have these parameters supplied by the manufacturer, there are numerous enclosure calculators on line and free to download. Don't get fancy, you are just building a rough box around the driver, raw numbers are all you need in this case.

If you don't have these numbers available from the manufacturer and you don't want to spend the time and money to determine the parameters for this install, then you drop back, hope for cover and throw a Hail Mary pass.

Either side of your ceiling installation can be an infinite baffle - of sorts. In a broad sense an IB is any baffle larger in all dimensions than the largest wave front produced by the speaker system. Your speakers probably aren't capable of much real extension beneath the mid-fifty Hertz range so the baffle doesn't need to be very large in reality. This should be so for either side of your install.

Another way to look at IB's is any enclosure that does not allow the backwave of the woofer to wrap around the baffle to interfere with the front wave, i.e., a sealed enclosure such as an acoustic suspension system. Once again, your celiling should work just fine for this sort of install if it is structurally sound and without air leaks in the immediate vicinity of the speaker installation.

You can reasonably guess the manufacturer assumed a generic sort of sealed system or an IB open into an attic/closet/garage space for these speakers. The only other real option would be a vented enclosure of some sort. It isn't likely the manufacturer expected you to install a port in your ceiling so go with the sealed enclosure/IB as your best guess.

In a sealed system the enclosure volume determines bass extension and to some extent the system sensitivity. Luckily neither parameter is drastically changed if you mis-guess the ideal enclosure volume in a sealed system. The type of speaker you own just doesn't produce deep bass extension and the electrical sensitivity of the system is mostly determined by the driver itself rather than the enclosure in this case.

Stuffing a sealed enclosure serves two purposes, 1) to slightly damp the backwave of the woofer before it reflects off the back of the enclosure and therefore, 2) to allow the woofer to "see" a physically larger enclosure. The more stuffing you put in the enclosure, the larger the box becomes as far as the speaker is concerned because the rear wave is damped by more material just as more space (bigger space) allows the backwave to dissipate without reflection back to the driver. When the "apparent" enclosure becomes too large, however, the system goes from correctly balanced to overly boomy with irregular bass response.

This is the difference between a driver meant for a sealed enclosure and one meant for a true infinite baffle, the T/S parameters are different for the two types of space the driver will see and the support the driver will receive from the enclosure or actual lack of enclosure. Without those T/S parameters the best you can do is hope you are making the speaker as happy as possible in your situation.

You can easily over do stuffing and take the system from properly damped into what appears to be a box that is too large to give the driver support yet not large enough to be a true infinite baffle, which will take the sound in the wrong direction. The best choice here is to error on the side of too little damping material rather than too much.

If this were my install, I'd do one of two things. First, I'd simply install both speakers in their respective positions with only a few inches thickness of polyfill behind the drivers to help damp the backwave and a bit more polyfill above and beneath the speaker to approximate the one cubic foot box. One speaker might sound a but lighter than the other in this case but combined you probably won't notice the difference since bass is almost monophonic when it is mixed. If there's a dramatic difference that is too noticeable in this situation, I would then build a false back baffle on the speaker that opens into the more open space. If you want to place a top and bottom batton to form your one foot(3) box, this would be the time. Create a dimension that approximates the opposite side and live with the results.

While flush mounting drivers has some advantages you are never going to get the highest fidelity from this installation. Accept that fact and find other work arounds such as a better subwoofer to extend the deep bass response while rolling the bass off to these speakers. (Most surround processors will probably have the roll off rather high for this sort of installation [80Hz?] and you never going to hear any additional benefits of over working the install. The center speaker will be far more important if this is a video install.) An approximately one to one and one quarter cubic foot enclosure will suit most medium sized sealed systems and the change to a two cubic foot enclosure is very likely to make for worse sound quality. You can always stuff a bit more polyfill into a sealed enclosure to increase the apparent size of the box as long as you don't go overboard on the stuffing material. Without those T/S parameters, the proper program to determine enclosure volume and the ability start with a from-scratch enclosure, this is about the best you can hope for.

As a rule, smaller is better here if you use a "box" at all. Look at the size of the box speakers that incorporate similarly sized low frequency drivers in similar enclosure types. If your ceiling mounts have a 5" driver, look at the box size of a sealed five inch driver. Deduct for the thickness of the box material, multiply the thre dimensions and make your best guess as to total enclosure volume of a typical sealed enclosure. Don't expect to get what a designer working from scratch can obtain. Make a nice compromise and live with what you have.

When you install these speakers remember that high frequencies are increasingly directional. Aim the tweeter at your listening position or you will loose high frequency content. All too often it is a poor installation and lack of "toe in" for the high frequencies that dooms ceiling mount speakers to sound like a bad music distribution system in your dentist's office.

twynns
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Re: Optimal sizing of in-wall backboxes

Jan,

Thanks for the very thoughtful, and helpful, response. I did understand your point. In this particular case, I simply have no choice, based on an agreement w/ my wife that wall mount speakers are mandatory. Accordingly, I am attempting to do the best I can within the constraints I have.

The speakers are Paradigm's SA-35's, an 8" x 19", 3 driver design with an 8" midrange and 8" woofer per speaker. Low frequency spec is 34 hz, though I have no expectation of achieving this level of bass from these speakers. Minimum volume spec is 1.41 cu ft, so I will exceed that by a small amount. I do not have individual driver specs so I don't believe that I'll be able to do the calcs you referenced, but that is what I was looking for originally, so thanks once again.

The mfr's tech support has come back to me and stated that the speakers are perfectly comfortable in a "standard wall," by which they mean a 2"x4"x8' cavity, producing roughly (I realize that 2x4 studs are not actually 2x4--but this should be close enough for discussion purposes) 3.5 cu ft. He stated that he thought that backboxes for my installation would be preferable in order to make the speakers on each side of the room "see" similar enclosures. He recommended 2 cu ft minimums, which I can easily accomodate in my ceiling rafter bays. He suggested stuffing each box w/ full height fiberglass insulation above and below the actual speaker and to use 1/2 height insulation behind the speaker, very similar to your suggestions.

Really appreciate the time and energy you, and others, put into the very complete and thoughtful response.

Best regards,

RW

cyclebrain
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Re: Optimal sizing of in-wall backboxes


Quote:
You'll get the best advice from the same company that sells the speakers. I agree you should call them.

--Ethan

You are so trusting Ethan. Getting info from the manufacturer and the distributer is a good start, but also one should take into account the bias and even the possibility that they might not even be qualified to provide usefull information. My apologies to those true experts out there.

cyclebrain
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Re: Optimal sizing of in-wall backboxes


Quote:

Quote:
I hear you, and agree, that in-walls are not sonically equivalent to good "box" speakers


In theory, having speakers built into the walls is much better than having them in boxes in front of the walls. All professional recording studios have their largest "main" speakers built into the walls. This avoids a problem known as SBIR that creates peaks and deep nulls at bass frequencies due to reflections. However, this is a physics / acoustics issue, not related to specific speaker brands and models.

--Ethan

SBIR? Probably not the "Space Based Infrared Radar" that I am familiar with. Seriously I am not familiar with this term. I don't see how a wall mounted speaker could solve the problem of low frequency room peaks and nulls. To my limited understanding, peaks and nulls are a fact of life.

ethanwiner
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Re: Optimal sizing of in-wall backboxes


Quote:
SBIR?


Speaker-Boundary Interference Response.


Quote:
I don't see how a wall mounted speaker could solve the problem of low frequency room peaks and nulls. To my limited understanding, peaks and nulls are a fact of life.


There are two basic causes of peaks and nulls in a room, and both are based on comb filtering. Besides the peaks and nulls the listener hears due to reflections, a loudspeaker creates a different set of peaks and nulls due to its distance from each boundary. So by building the speaker into the wall you avoid that half of the problem.

Imagine a speaker 2 feet in front of a wall outdoors, facing away from the wall. Mids and highs go outward away from the wall, but bass radiates more or less omnidirectionally. So at low frequencies some of the sound leaves the rear of the speaker, bounces off the wall, and collides with the direct sound. This creates peaks and nulls. If the speaker were built into the wall there would be no peaks and nulls.

--Ethan

Jan Vigne
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Re: Optimal sizing of in-wall backboxes


Quote:
He suggested stuffing each box w/ full height fiberglass insulation above and below the actual speaker and to use 1/2 height insulation behind the speaker

Fiberglass does leak and drift, even when used in a stable location. I would still suggest just a thin layer of polyfill or open weave cloth over the back of the speaker assembly to provide some protection to the working parts of the drivers.

twynns
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Re: Optimal sizing of in-wall backboxes

Polyfill, as in the batting used for upholstery, pillows, coats, comforters, etc? Is there some special type or grade of product that would be "better?" than what I might find in a local fabric/upholstery store? One local AV installer said that they use an automotive adhesive-backed insulation matting in their in-wall enclosures. Once again, I don't want to go "crazy" trying to do things for this app that aren't going to make a significant difference--I just don't know what will and won't make a "meaningful" difference. Thanks once again!

RW

Jan Vigne
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Re: Optimal sizing of in-wall backboxes

Yep, "polyfill", the stuff you buy at a fabric store. Different materials have different absorption rates which will mean you can use more or less within a given enclosure volume to get the desired result. How much or how little you use is a rough estimate when you have the DIY programs and TS parameters to work with. Without the programs and parameters you are still just guessing and tuning by ear by stuffing and then taking out until it sounds right over the broadest range of musical material. If the manufacturer has suggested an amount of fiberglass to use, go with their suggestion. My suggestion of polyfill is merely to protect the drivers from any fiberglass that might work its way into the driver's suspension or motor, it was not meant to supplant the manufacturer's suggestion of stuffing.

You're putting too much thought into what you have. Worry about bigger issues within the system.

twynns
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Re: Optimal sizing of in-wall backboxes

Polyfill it is. Many thanks. I probably am putting too much thought into this, but it's a fun project and I don't want to have to redo it later, after the blueboard, plaster, etc are already up.

Appreciate all the posts and assistance.

Rgds,

Twynns

cyclebrain
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Re: Optimal sizing of in-wall backboxes


Quote:
There are two basic causes of peaks and nulls in a room, and both are based on comb filtering. Besides the peaks and nulls the listener hears due to reflections, a loudspeaker creates a different set of peaks and nulls due to its distance from each boundary. So by building the speaker into the wall you avoid that half of the problem.

Imagine a speaker 2 feet in front of a wall outdoors, facing away from the wall. Mids and highs go outward away from the wall, but bass radiates more or less omnidirectionally. So at low frequencies some of the sound leaves the rear of the speaker, bounces off the wall, and collides with the direct sound. This creates peaks and nulls. If the speaker were built into the wall there would be no peaks and nulls.

--Ethan

You are refering to the infinite baffle in an outdoor situation. In our realistic listening room, any speaker will create peaks and nulls regardless of its location, internal to a wall, or external, based on the dimensions of room.

ethanwiner
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Re: Optimal sizing of in-wall backboxes


Quote:
You are refering to the infinite baffle in an outdoor situation. In our realistic listening room, any speaker will create peaks and nulls regardless of its location, internal to a wall, or external, based on the dimensions of room.


I was describing a box speaker placed in front of a wall, pointing toward that wall. That was just to make the point that comb filter peaks and nulls do not require an enclosed space.

You are correct that peaks and nulls are unavoidable in a room, but the frequencies of the peaks and nulls do not necessarily depend on the room dimensions. The graph below is from my Comb Filtering article, and it shows pure comb filtering unrelated to room dimensions. In this case, the measuring microphone was placed 20 inches in front of a bare sheet rock wall, and the loudspeaker was directly behind the microphone, about 6 or 8 feet back.

--Ethan

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