# experimental on-wall design



## solid7 (Jan 31, 2008)

Here is my concept for my new, large on-wall speakers. This concept came about from the desire to have big(ger) sound, but the lack of real space to build large speakers, or to be able to properly place them.

I have begun the build, but was recently laid up with a torn ligament in my shoulder. I am experimenting with some (very) different build techniques, and will post more pics of the real product in the next few days. The center channel is mostly built, the mains are in work.

For reference: The TV is a 52" Samsung, and the drivers are Peerless Mids and tweets with Dayton woofers. (6" on the Daytons, 5.25" Peerless and 1" Peerless tweeters) The finish will be a piano black.


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## Anthony (Oct 5, 2006)

Interesting design. The benefits are obviously no baffle step (or very limited), downside is that placement is limited.

Another benefit is that only 3/4 of the sides need to look good 

Good luck and keep us posted.


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## mosconiac (Jan 31, 2009)

I experimented with such an arrangement recently and ended up leaving the center channel out. The image "horseshoe" as sound travelled from L-->C-->R was VERY distracting, so I ended up with only L-R. This keeps the image stable on the vertical plane and still images well even off-center. Naturally, an L-R only setup will breakdown as you move extremely off-center.

I ended up opening the walls so the speakers could be recessed (roughly 4"...1/2" for the drywall & 3-1/2" for the stud). The enclosure I chose is roughly 11" deep with grill, so about 7" of the enclosures are visible.

The opening in the wall is adjacent to a stud, so I could afix the speaker to it. In fact, I used "L" brackets on each side of the speaker to afix it the stud. I glued a section of 2x2 behind the drywall for the other side of the enclosure so I support the drywall on that side.

I'll whip up a quick sketch so you can see what I am describing. Photos are at home and I don't have access to them here at work.


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## mosconiac (Jan 31, 2009)

Here's what I did for the front stage. Naturally, I have covered the "L" brackets with trim to hide them, but the sketch is cleaner without that shown.


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## solid7 (Jan 31, 2008)

Anthony said:


> downside is that placement is limited.


Yes, but unfortunately, that is my overall downside, as my room is so irregular in size and shape. Under normal circumstances, I would never have tried to build an on-wall.


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## solid7 (Jan 31, 2008)

mosconiac said:


> I ended up opening the walls so the speakers could be recessed (roughly 4"...1/2" for the drywall & 3-1/2" for the stud). The enclosure I chose is roughly 11" deep with grill, so about 7" of the enclosures are visible.


Yes, I am still considering doing this, as it would allow me to make the mains even deeper. I'm running on the small end of the recommended volume, and doing so would correct this. My CC is more than large enough.

I would most likely just end up making a couple of nooks in the wall that would "coincidentally" be the same size as the speaker. After opening up the wall, it would be easy enough just to shim up to the studs on both sides. The mains are 10" wide, and the wall is a partition wall, with studs on 18" centers. I purposely designed them in a way that allows me to utilize this option. (meaning that I placed them in the design according to where the wall studs fall) Some decorative trim, and off we go...


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## solid7 (Jan 31, 2008)

These pictures depict some of the early methods, tools, and prototypes that I created. They were completely hand built, and were meant to be a proof of concept, so to speak. I wanted to make sure that I could build a small scale version reliably enough to dive into the whole design. It works quite well, but I had to leave the router template behind, and instead, had details cut by CNC router from a sheet. (with an open profile - I have to glue the back on, but I get more details now per sheet)


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## mosconiac (Jan 31, 2009)

Excellent work. I love the idea of using MDF wafers to make unique & interesting enclosure shapes. It's a significant time investment (not to mention the extra MDF), but the result are worth it. Bravo!


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## fbov (Aug 28, 2008)

mosconiac said:


> Here's what I did for the front stage. Naturally, I have covered the "L" brackets with trim to hide them, but the sketch is cleaner without that shown.


If at all possible, try to avoid this kind of step around the speaker - install them flush with the wall or free-standing in front of it. Anything else will result in a very lumpy frequency response, even a gap has bad effects. 

Frank


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## solid7 (Jan 31, 2008)

fbov said:


> If at all possible, try to avoid this kind of step around the speaker - install them flush with the wall or free-standing in front of it. Anything else will result in a very lumpy frequency response, even a gap has bad effects.


I have intentionally drafted the front face of the speaker at 10 degrees off normal, and added 1 3/8" rounded corners to avoid this. The added depth shouldn't take anything away from the compromises that have already been made, and compensated for.


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## solid7 (Jan 31, 2008)

mosconiac said:


> Excellent work. I love the idea of using MDF wafers to make unique & interesting enclosure shapes. It's a significant time investment (not to mention the extra MDF), but the result are worth it. Bravo!


Thank you for your words of encouragement. As with all things, I have a few "lessons learned" to share:

1) MDF must be reinforced when stacked. If you notice there are 12 holes around the perimeter of the sections. This is because the MDF stacked - while lacking a true "grain" - still has the effects of cross-grain shear in this configuration. There is a lot of really [email protected] MDF on shelves these days, and the effect is a "layering" or "delamination" effect when transverse forces are applied. (when cut, if you bend it, it breaks almost as if perforated. To overcome this, dowels are placed to bridge the layers, and add strength against bending. (overall stiffness)

2) reinforcements must be staggered. If you don't stagger the dowels, the material will break where dowels end. I used 1/4" all round from Home Depot. It says that it's hard wood, but I don't believe that it is. But that's another story. (it's much more round, and dimensionally stable than Oak) At any rate, half of the dowels should be a couple inches shorter than the other half, to avoid creating a perforation on the same plane. The overlap will correct this condition.

3) Long lengths must be made in sections and joined later. Long bar clamps are a must - but beware, as it's VERY easy to start building a bend into glued sections longer than 20"

4) No matter how nice it turns out, the whole assembly must be sealed after assembly, inside and out. This adds strength, as well as "surface tension", so that router bits don't sink in as easily when joining end pieces, backing, or making corrections. I use 50% elmer's glue and 50% water to seal, and then Bondo to fill (as it sticks nicely to the glue)

As for the time premium - I would only say that you pay a time premium where it's actually possible to make the shape in some other way. The only other way that I could make this shape with available tools, is to stack the material, laminate it, and hog out a solid laminated "billet" on a CNC machine. That's one option, but it's actually more difficult than than the glued sections. Plus, this is at least somewhat "DIY", where that is not so much. (although I do have the option to do it that way, but I wanted to prove that this could be done - the sections can be sourced cheaply enough to make it worth while)

Still need to get those other pics up. I will show more of the assembly technique.


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## fbov (Aug 28, 2008)

solid7 said:


> I have intentionally drafted the front face of the speaker at 10 degrees off normal, and added 1 3/8" rounded corners to avoid this. The added depth shouldn't take anything away from the compromises that have already been made, and compensated for.


Not to be argumentative, but you're thinking of the wrong effect and have not compensated for wall spacing. This is a wave interference effect. 

The reflection from the wall next to the speaker will be out of phase at some frequency, so you'll get a dip in frequency response. In round numbers, 1 Hz has a 231.5 ft. 1/4 wavelength. If a speaker is 1 foot from the wall, you'll see a dip at 231Hz. Yours profile looks like this on one side and bit more on the other, so the first peak will be in this range toward the center and a bit lower outboard. 1/2 wave differences come in as peaks, starting at twice this frequency. About the only thing you can do accoustically is to damp the reflection; front wall damping nearly always sounds good.

Frank

Ps Floyd Toole's book, Sound Reproduction goes into what's happening in detail. It should be required reading for any one doing their own home theater IMO.


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## solid7 (Jan 31, 2008)

OK, point taken. But what I was trying to say, is that any depth that I add to the speaker would be the amount that I sink it. So the front face would be in exactly the same place. Speaking in relative terms, I shouldn't really be losing anything significant in doing so, other than whatever I have already been forced to compromise. (yes, I'm well aware that a wall mount speaker is not ideal, but it's what I'm forced to work with)


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## GPM (Jan 14, 2007)

fbov said:


> In round numbers, 1 Hz has a 231.5 ft. 1/4 wavelength. If a speaker is 1 foot from the wall, you'll see a dip at 231Hz.


Hmm, the speed of sound (SoS) is predominantly temperature dependent, so unless the listening environment is on a polar icecap as a 231.5*4 = 926 ft/sec indicates, a typical sound reproduction app will be around ~344.5 m (~1130 ft)/sec. and reflections/eigenmodes (standing waves) are 1/2 WL resonant in nature, not 1/4 WL, so the fundamental is a null (dip) with both even (peaks) and odd (nulls) harmonics as opposed to the 1/4 WL resonator's odd only harmonics. This makes for a fundamental (f) null around 565 Hz/1 ft. with peaks at ~1130 Hz/f2, etc. and nulls at ~1695 Hz/f3, etc..

Follow the Sound and Hearing link to find in-depth SoS and various reflection tutorials among many others: http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Anyway, since acoustic energy falls at 1/f, speaker baffles that are near/at a boundary generate weak near-field reflections that may/may not be audible, though by the same token have a much lower, ergo stronger, set of reflections/eigenmodes to other boundaries such as the floor, etc. which will be audible and why little/no baffle step compensation (BSC) is required.

GM


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## fbov (Aug 28, 2008)

I'm not at the North Pole, I just did the math in my head... poorly. 

You're right about standing waves resonating at 1/2 WL, but this isn't a resonance I'm talking about but rather a wall reflection. 

Speakers mounted on a wall experience destructive interference starting at frequencies whose wavelengths are 1/4 the distance from speaker to wall (and harmonics at (2n+1)/4). Since the distance is traversed twice, you get the 1/2 wave required for destructive interference. 

I wanted to be sure the OP was aware of this phenomenon. It appears he is and has made the tradeoff consciously, which was my only goal. 

Frank


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## mjg100 (Mar 12, 2008)

For small rooms I think this is a pretty good way to go. I would suggest crossing high (100Hz.) and use multiple subs. I use on wall speakers (not DIY) in my small dedicated room. I found a good deal on some RBH WM30 www.rbhsound.com/wm30.shtml speakers. I have my center mounted on the wall behind my AT screen. I have 3" thick sound panels on my front wall (except where the center speaker is located) and bass traps in the corners. Sound panels are tight against the center channel. I made some brackets so that I could mount my mains in front of the sound panels. I have a DIY sub located under each speaker (5.1) and a sub in the middle of my back wall. This allows me to cross to my subs at 100Hz and gives my mains more headroom. Here is a picture of the front of my room.







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There are grills on the sealed subs in the front of the room, but the flash from the camera shows the Aluminum NHT 116 drivers. My subs in the back of the room are larger. Real pleased with the sound in this room. I have a lot nicer (more expensive) equipment in my downstairs room, but the dedicated room is a much better HT experience, sound and image wise.


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## GPM (Jan 14, 2007)

fbov said:


> .........but this isn't a resonance I'm talking about but rather a wall reflection.
> 
> Speakers mounted on a wall experience destructive interference starting at frequencies whose wavelengths are 1/4 the distance from speaker to wall (and harmonics at (2n+1)/4). Since the distance is traversed twice, you get the 1/2 wave required for destructive interference.


Reflections are 1/2 WL in nature, so there's no 1/4 WL interference, though sound is reflected when it hits an object that's > 1/4 WL in size, which is what I guess you're trying to say. 

GM


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