In-the-Wall Back Loaded Horns - diyAudio
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Old 29th June 2013, 05:56 AM   #1
howardg is offline howardg  United States
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Default In-the-Wall Back Loaded Horns

IN-THE-WALL BACK LOADED HORNS

-The enclosures will be behind decorative wood panels on the front.

-Each .75” plywood enclosure is about 21”x 87”x 3.5” inside.

-The drivers will be Fostex Fe166e.

-The horn compression chamber size, throat area and expansion rate will roughly resemble the Austin A-166 by Ron Clarke. But horn length will be about 160”.

-Due to the length of the cabinets there will be just one major fold, putting the horn mouth near the driver cone, and these both will be in plane with the wall of the room on which they mount.

-The distance to the ceiling is: 5”(driver) and 20” (horn mouth); with each about 25” from the side walls.

-Below the speakers are to be 68” high wardrobe cabinets, that is why the speaker drivers are so close to the ceiling.

- Bedroom usage and appearance are driving this design, but I do want the sound to be great, if possible.

I figure that there will be little or no baffle step loss, and good planar support for the bass response---hopefully not too much.
On the other hand, phase coherence/imaging may suffer due to all of the reflective surfaces near the drivers.
I will likely want to point the drivers down at an angle into the room, and maybe toe them in a bit.
I am picturing wooden mounts, not much bigger around than the drivers themselves, that would bring the drivers out from the wall (panel) surface.

I am wondering if it would help imaging to bring the drivers out from the wall a few inches, any ideas?

How far would they have to get from the wall to benefit imaging?

Theoretically: Would flush mounted speakers in the middle of large wall, without closely adjacent surfaces tend to image OK?

Thanks, Howard
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Old 29th June 2013, 08:14 AM   #2
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Great project Howard... I don't have any experience with horns but I look forward to your results.

Regards
Dean
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Old 29th June 2013, 10:31 AM   #3
xrk971 is online now xrk971  United States
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Howard,
This sounds really interesting and I have often wondered about embedding a nice speaker between the drywall and studs in a home. We are about to build a new home so I may actually have an opportunity to do this - drywall board is actually a great material to build a horn speaker with. I want to take a crack at simulating this in akabak with the angled ceiling placement even. I just looked at the Austin II plans and see that the horn mouth is 8 in x 15 in but your size constraint is 21 in x 3.5 in. Is the 3.5 in dimension the depth of the speaker in the wall due to the joist depth (2x4 timber) or is it 21 in? I am assuming 3.5 in deep x 21 in wide x 87 in high. The mouth aperture will have to be 21 in wide x 7.1 in tall. The expansions will have to be along a direction parallel to the wall since you are limited by the wall depth of 3.5 in. Basically I am envisioning a flattened wall horn. One thing to keep in mind is that the max dimension of 3.5 in may limit the lower bass extension as it can only accommodate a maximum wavelength size in one dimension. It may result in a cupped sound. Before I start, I just want to confirm that I am correct in assuming that your max speaker depth is 3.5 in.
Regards,
X
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Old 29th June 2013, 12:38 PM   #4
xrk971 is online now xrk971  United States
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The Austin A166 only has a 127 in path, do you really want a 160 in path? Also if 3.5 in depth is max and 21 in width is max, the largest CSA for the mouth will only be half of the Austin due to 3 in wide clearance required for throat passage from driver chamber above the mouth. So bass gain will only be about half of the Austin. You will have a +9dB gain due to corner loading so I don't think it will be an issue but I can already see that this won't look anything like the Austin due to geometrical constraints. Basically I will design a custom in wall horn with FE166 driver.
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Old 29th June 2013, 01:08 PM   #5
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Just a minor reminder guys: as you are no doubt aware the Austin and Dallas horns were not designed using the 1-dimensional wave equation (for that matter, FH3 wasn't entirely either). If you are looking at them purely from that basis, you will not actually understand why they are the way they are. Changing the aspect ratio for e.g. is a significant performance departure / compromise.

Last edited by Scottmoose; 29th June 2013 at 01:10 PM.
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Old 29th June 2013, 01:15 PM   #6
xrk971 is online now xrk971  United States
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Understood, that is why I said the 3.5 in depth will be a big effect on performance.
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Old 29th June 2013, 02:23 PM   #7
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Quite, but it's not merely about limiting the terminus size, as seemed to be implied & why I pointed it out. It's about the shape of the expanding wavefront which by definition is not accounted for under plane-wave assumptions.
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Old 30th June 2013, 01:48 AM   #8
xrk971 is online now xrk971  United States
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Default Initial Design of In-Wall BLH for FE166EN

Here is what I came up with for a design so far: A 4.33 liter driver chamber connected to a 129 in long conical horn with a throat area starting at 10.15 sq in and expanding to a 70 sq in mouth area. The driver is canted 15 deg toe-in and 15 deg down and is located 5 in from the ceiling. The horn mouth is located immediately below the driver box and has a 4 in tall x 17.5 in wide opening.

The simulation in AkAbak assumes corner mounting 25 in away from the side wall. The corner loading provides a huge bass gain of +9 dB. First plot is frequency response at 1 m, the bass peak is about 105 dB at 1 watt input. The next plot is the zoomed into 1 kHz, then next plot is impedance, then the cone displacement (exceeding xmax of 1 mm already so I don't think there is enough horn loading due to lack of expansion ratio), final plot is the impulse response showing a significant group delay beyond the audible 8 ms range. The bass extension is pretty deep (90 dB at 27 Hz) - but it is not flat.

This design can definitely use some tweaking - and I would love to receive feedback/constructive comments to improve design. It probably would sound not too bad given the size constraint and that it is in-wall, certainly better than any box-store in wall speaker you can buy. The sensitivity with the FE166EN is phenomenal - perfect for fleawatt tube amps.
Attached Images
File Type: png Wall-BLH-Plan.png (484.3 KB, 557 views)
File Type: png Wall-BLH-Freq-20kHz-1m.png (36.4 KB, 529 views)
File Type: png Wall-BLH-Freq-1m.png (29.3 KB, 517 views)
File Type: png Wall-BLH-Impedance.png (22.9 KB, 513 views)
File Type: png Wall-BLH-Displ.png (22.2 KB, 510 views)
File Type: png Wall-BLH-Impulse.png (16.3 KB, 24 views)

Last edited by xrk971; 30th June 2013 at 01:52 AM.
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Old 30th June 2013, 03:08 AM   #9
howardg is offline howardg  United States
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I am glad to see that there are others that find this idea intriguing. For some it might be the only way their spouse would accept a large horn in a shared space. I will try to remember to take pictures of the insides of the horns to share later. One will lay horizontally, and one will stand vertically, due to space/design constraints. So they will be different, but will hopefully perform similarly.

I know that I am not conforming to the A-166 design closely, but it is good to have a guide for the CC and throat sizes. That said, I am very interested, if not technically educated, about what makes a good horn do what it does.

In the past I have mostly taken hints from successful/popular designs, and then done rough builds to test out my designs. My successful tweaks have largely come by trial and error.

Xrk971 , If I was working with a deeper stud bay, like 5.5”, that might make things easier-maybe you could make yours deeper. Probably best to avoid exterior walls, due to heat and moisture issues. Maybe if the FE166e's don't work out, I could use a pair of 126e's that I have-but I would not want to do a total redesign at this point if I can avoid it.

The mouth of the A166 is about 8x15, but the cross-section immediately before the mouth is 7.3x7.9, which is about the same as my 3.5x17 (21 minus 3.25-throat- minus .75-divider). So maybe my horn's mouth could somehow have a rapid expansion like that of the A166. Also the horn mouth will be unloading right onto the top of a 20” deep cabinet top-which I think will effectively be an extension of the horn mouth.

It would not be hard to shorten the horn, if it came to that. Does anybody know what one might expect with a horn that is 30” longer than the A166? I could experiment pretty easily I think, because the horn is like an open flat box on the workbench, and the top that closes it up has the mouth cut-out in it. So just move the cutout, and close off the last 30” of horn. If the final piece were to be shortened, the internal baffling, which is super simple, could be adjusted/optimized.

SCOTTMOOSE SAID: “Just a minor reminder guys: as you are no doubt aware the Austin and Dallas horns were not designed using the 1-dimensional wave equation (for that matter, FH3 wasn't entirely either). If you are looking at them purely from that basis, you will not actually understand why they are the way they are. Changing the aspect ratio for e.g. is a significant performance departure / compromise”.

I will have to look up “1-dimensional wave equation “, as I don't know about it! I actually started this project less hopeful about getting actual horn performance, thinking that I would end up with some sort of restricted mouth, mass loaded transmission line (MLTL).

Here is an interesting question: What would happen if the aspect ratio was made roughly 1:1 by creating 4 separated channels in last 60” of expansion(which would also greatly reinforce the larger panels in this area)? The channels might be 3.5 x 4, or 3.5 x 3.5, or 3.5 x 3, for example, and would expand as they progressed towards the mouth. Kind of like a sectoral horn.

Sounds like maybe each of the small pathways would prevent the propagation of larger wave-froms, and thus the lower,louder bass frequencies, is this the concept?
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Old 30th June 2013, 03:10 AM   #10
howardg is offline howardg  United States
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Ha! you got that simulation posted while I was writing mine, I will have a look at it later, dinner time!
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