Acoustic Treatment Project

[jagman]

Well, I've been working on my acoustics lately.

I have bass traps in the corners and along the rear wall. I used 2'x4x'2" 8# mineral wool panels. In the vertical corners the traps go floor to ceiling and are three panels deep. I am in the process of building the bass trap wall behind the seated position. It is a 4'x8' "wall" two panels deep and will be located about 1' in from the rear wall. It's main purpose is to thwart the main low frequency trough associated with cancellation from the rear wall reflection. I also have 2'x4'x2" 3# rigid fiberglass for 1st reflections (all will be one panel deep). There will be 4'x4' absorbers right and left, a 4'x8' ceiling cloud, and three 2'x4' absorbers on the front wall. Last but not least there will be a 1' wide ring of 3# faced 2" rigid fiberglass going 360' around the room at the wall/ceiling junctions.

The corner traps are up and all the wood has been cut for the structural support of everything else. I also have all the fabric I'll need, so it shouldn't be too long before I'm done.

[Sir Terrence]

jagman wrote:

Well, I've been working on my acoustics lately.

I have bass traps in the corners and along the rear wall. I used 2'x4x'2" 8# mineral wool panels. In the vertical corners the traps go floor to ceiling and are three panels deep. I am in the process of building the bass trap wall behind the seated position. It is a 4'x8' "wall" two panels deep and will be located about 1' in from the rear wall. It's main purpose is to thwart the main low frequency trough associated with cancellation from the rear wall reflection. I also have 2'x4'x2" 3# rigid fiberglass for 1st reflections (all will be one panel deep). There will be 4'x4' absorbers right and left, a 4'x8' ceiling cloud, and three 2'x4' absorbers on the front wall. Last but not least there will be a 1' wide ring of 3# faced 2" rigid fiberglass going 360' around the room at the wall/ceiling junctions.

The corner traps are up and all the wood has been cut for the structural support of everything else. I also have all the fabric I'll need, so it shouldn't be too long before I'm done.

Jagman,
Keep in mind that all acoustical treatment is not absorption. Treating your room is a combination of absorption, diffusion and abbifusion(absorption and diffusion tied together) One of the things I have noticed when people tackle this issue is they tend to think only absorbtion, and not any of the others. Make sure before you add alot of acoustic material to the walls that you measure, measure, measure. Most people tend to overdamp the room when treating it.

[jagman]

I hear you. Aside from the treatments, the only furniture in the room are the TV, the sound system and one couch. The room is a 3,000 cubic ft rectangle. It's crazy... I've never before heard so much slap echo in a room. Since the seated position is only about 4 feet from the rear wall, I figured absorption behind it is best. My understanding is diffusion works better from a distance. I may consider diffusion on the right and left wall at the 1st reflection point sometime down the road. Carlos has suggested diffusion laterally adds more space. Who knows if that's true, but if I don't like the absorption, it may be worthwhile to make some DIY diffusors. Since the ring around the wall/ceiling junction is reflective, 41% of the surface space will be absorptive (including 100% of the floor as it is carpeted). As for abbifusion, I don't know much about it. Does anyone have any good links?

[F1 fan]

jagman,

here is a cool site with a calculator for QRD type diffusers.http://www.mhsoft.nl/user/ac.asp

[Sir Terrence]

jagman wrote:

I hear you. Aside from the treatments, the only furniture in the room are the TV, the sound system and one couch. The room is a 3,000 cubic ft rectangle. It's crazy... I've never before heard so much slap echo in a room. Since the seated position is only about 4 feet from the rear wall, I figured absorption behind it is best. My understanding is diffusion works better from a distance. I may consider diffusion on the right and left wall at the 1st reflection point sometime down the road. Carlos has suggested diffusion laterally adds more space. Who knows if that's true, but if I don't like the absorption, it may be worthwhile to make some DIY diffusors. Since the ring around the wall/ceiling junction is reflective, 41% of the surface space will be absorptive (including 100% of the floor as it is carpeted). As for abbifusion, I don't know much about it. Does anyone have any good links?

If you have only 4ft behind the listening position, I would recommend diffusion instead of absorption. You want the space near the surrounds, and the backwall to be diffusive to enhance the feeling of space. However you don't want the diffusion to be unpredictable, you want some precise diffusivel directional properties so you don't create hot spots. You especially want the rear hemisphere(behind your listening position) to sound spacious, not dead.

Abbifusion is a panel that diffuses the high frequencies, and absorbs the low frequencies. Its a really good panel to use when you have high Q resonances, but do not want you room to sound closed in.

[jagman]

It's interesting because You're the fifth informed person I've received advice from and all have said something different. One says diffusion from close distances isn't effective and heavily advocated high density absorption on the rear wall (to reduce the cancellation that results from lower frequencies bouncing off the wall). Another sells absorption but actually recommends against it but only behind the seated position (especially for two channel music), another only recommends absorption at the corners (wall/wall, wall/ceiling, and especially the tricorners) but not at first reflection points, and another recommends diffusion on all walls and absorption on the ceiling and floor. All are repected people in the business and all provide data that they think backs up their recommendation. Basically, I think it all comes down to taste. No system, let alone a room, allows ultra accurate reproduction of sound; the best we can do is come up with reproduction that we like.

[laserman]

jagman,

Your last post was brilliant. Personal taste is what it's all about. Some like it totally dead and others like a live front and dead rear. Since you are a DIY type of guy, why don't you make some stand alone defussors/absorbers so you can move then around your room to experiment???? Also, don't forget about clouds for the ceiling too. Heck, you may even like them on stands once you find suitable dialed in and tuned spots. I think measurements are fine but one can measure for a flat response only to find out that the sound is aweful due to personal preference. Please keep us informed on what the end result is verses what you planned on doing.

L

[Sir Terrence]

jagman wrote:

It's interesting because You're the fifth informed person I've received advice from and all have said something different. One says diffusion from close distances isn't effective and heavily advocated high density absorption on the rear wall (to reduce the cancellation that results from lower frequencies bouncing off the wall). Another sells absorption but actually recommends against it but only behind the seated position (especially for two channel music), another only recommends absorption at the corners (wall/wall, wall/ceiling, and especially the tricorners) but not at first reflection points, and another recommends diffusion on all walls and absorption on the ceiling and floor. All are repected people in the business and all provide data that they think backs up their recommendation. Basically, I think it all comes down to taste. No system, let alone a room, allows ultra accurate reproduction of sound; the best we can do is come up with reproduction that we like.

Some of these recommendations leave me puzzled. The purpose of the surround speakers in the rear hemisphere is to provide diffuse/non directional spaciousness to counter the tight/dry and damped front speaker/hemisphere combination. The whole principle of the dipole speaker confirms that purpose, as does the fact the Dolby recommendation require that the rear speakers not directly fire toward the ears. If you damp the wall behind the listening position, you defeat that purpose. If we are talking about a large professional theater, or dubbing stage I can understand that recommendation to help control long reverberation times that cannot effect dialog clarity and preserve transient information. 4ft is enough room for diffusion to work, because that is exactly the distance from my listening seat to the rear wall and I use diffusion on my rear wall. Measurements from the listening seat confirm its effectiveness.

In the installation I have done, I follow the principles of Dr D'Antonio from RPG acoustics and he recommends that the front 2/3 of the room be absorptive, the rear 1/3 be diffusive, and the ceiling be diffusive. Its alot more technical than that, but it works with the principles of THX acoustics, and Dolby recommendations. I agree with lining the wall/ceiling junction with absorption. However NOT recommending absorption at the early reflection points just kills image specificity in the front soundstage. It sounds like alot of these guys are into two channel music and not hometheater specifically. The recommendations for music only, and hometheater are quite different. In recent acoustical workshops that I attended, most acoustical professionals are moving away from too much absorption in small rooms, and recommending more diffusion because it makes the room sound more natural and larger. Too much absorption makes a room sound small, closed in, and not very eveloping. Too much diffusion causes a lack of image clarity. The right combination of both in the right places results in a room that sounds natural, spacious where it should, and tightly focused image wise where it should. I prefer science first, taste second because I believe you have to have some point to begin in order to adjust to taste. I don't sell anything, my job is to recommend what makes a room sound natural, and the recommend the products that get you there.

The science is there, so no guess work is really needed on this issue. Every room is different, but there is a solution for all spaces.

[jagman]

You are for the most part right that most of the recommendations I have received are from two channel guys. So basically what you are saying is high density traps for all corners, low density absorption for the first reflection points on the side and front walls, and diffusion for behind the seated position. As for the ceiling, some say absorption is better, others say diffusion is better, and in general that location is the one that is thought about the least. That last bit is my interpretation, but I think it is accurate.

Going back to the idea of absorption behind the seated position. There are two purposes I've picked the type of absorption I have. 1st and foremost is to decrease the 1/4 wavelength low frequency trough that occurs due to proximity to the rear wall. I don't see diffusion having any effect on that as the frequencies involved are typically around 40 Hz. Since I'm using 4" deep 8# mineral wool spaced 1' off the wall, it should have some effect (although it will probably be limited d/t difficulty of traps working that low). The trough will undoubtedly still be there, but if it is minimized there would potentially be a large perceived impact. The 2nd reason is decreasing the volume of the mid to high frequency reflections, and that is where diffusion may come in. If I can make some sort of abbifusion that will disperse the middle to high frequencies while allowing the lower ones to pass through and be absorbed I may be able to get the best of both worlds. This would keep the HT experience more airy, as well as two channel music.

So, that begs the question... how does one build an abbifusion device?

[Sir Terrence]

jagman wrote:

You are for the most part right that most of the recommendations I have received are from two channel guys. So basically what you are saying is high density traps for all corners, low density absorption for the first reflection points on the side and front walls, and diffusion for behind the seated position. As for the ceiling, some say absorption is better, others say diffusion is better, and in general that location is the one that is thought about the least. That last bit is my interpretation, but I think it is accurate.

I say you have it right. The ceiling should not be discounted though. I know there is a school of thought that says to use absorption, but to me that just makes the room sound closed in. My experience with my installations is that a diffuse ceiling makes the room sound spacious, balanced, and sound like its higher than it really is.

Going back to the idea of absorption behind the seated position. There are two purposes I've picked the type of absorption I have. 1st and foremost is to decrease the 1/4 wavelength low frequency trough that occurs due to proximity to the rear wall. I don't see diffusion having any effect on that as the frequencies involved are typically around 40 Hz. Since I'm using 4" deep 8# mineral wool spaced 1' off the wall, it should have some effect (although it will probably be limited d/t difficulty of traps working that low). The trough will undoubtedly still be there, but if it is minimized there would potentially be a large perceived impact. The 2nd reason is decreasing the volume of the mid to high frequency reflections, and that is where diffusion may come in. If I can make some sort of abbifusion that will disperse the middle to high frequencies while allowing the lower ones to pass through and be absorbed I may be able to get the best of both worlds. This would keep the HT experience more airy, as well as two channel music.

So, that begs the question... how does one build an abbifusion device?

First, trough are not really audible, and I wouldn't worry about it as much as peaks. Secondly acoustically there is nothing you can really do about troughs in small rooms. Thirdly most absorption materiel is not really effective below 60hz, and just makes troughs worse because now they extend higher in frequency as a result of the absorption. I would think that close wall proximity would actually increase the bass, and not introduce a trough. Maybe I am wrong, but I doubt it.

Abbifusion devices are extremely difficult to DIY because of the complexity of the diffusion and absorption properties. There has to always be a balance between the absorption and diffusion. Dr D' Antonio told me that it has taken RPG several years of R&D to get this balance correct in their panels.

[bpape]

There are many different ways to deal with a room. I too prefer a little less dead in the back. However, that doesn't mean that diffusion will work when you're only a couple feet away. General rule of thumb is that for diffusion to be effective at 1kHz and up, you'll need to be at least 8' or so away.

In a multic-channel only environment, it's important to have some absorbtion distributed throughout the space. There are ways to get absorbtion and diffusion both in the rear of the room and I personally like that - what was called abfusion before. A series of smaller hard-framed panels hung in an array will provide a bit of absorbtion well distibuted as well as the framed areas and between them acting like a crude well diffusor.

Bryan

[jagman]

I'm much further along in my project, and I've made some changes. The ring of 12" wide, 2" deep 3# rigid fiberglass (RF) around the wall/ceiling boundry was supported with as second 12" wide panel behind it as well as a third panel 8" wide behind that. The space would have been unoccupied, so I thought I'd fill it with more RF. I've decided to stick with the bass wall behind the seated position... mostly because I already had the materials. It's slightly different, though. Now it is 3 panels wide and 3 panels deep. If I don't like it, I can always change it. As for the project, I'm almost done... and it's about time! The only problem is I underestimated how many 2' x 4' x 2" 2# unfaced RF panels I would need... I'm two short . Beyond that, everything is up and I like the sound. It is very clean. I'm a little disappointed that my wife didn't notice a difference... then again, she thinks a home theater in a box sounds great. Oh well. She has been good, though. Adjusting for the two missing panels, I'll have 23 panels of unfaced 3# RF, 6 panels of faced 3# RF, and 30 panels of 8# mineral wool in a 9' x 16' x 20' room... and she hasn't complained. In fact, she says it looks good . I'll try to put up photos later in the week. She asked me on a scale of 1-10 how good/profeessional it looks. I said 8. It's probably a 7, though. None of of the panels are flush with the wall. Other than that, the look is clean.

After I get the last two panels up, I'll to a frequency sweep with the Rat Shack meter.... just to do a before and after. I ordered a BFD on Saturday, too, so the next project will be learning it, REW and taming the dual 18" IB sub.

After that I'll be done for a while. Time to enjoy the movies! But I still want a front projector and screen... it never ends. Maybe next spring.

[jagman]

OK guys... here you go!

Please ignore the need for touch up paint... as well as the water spots on my camera. I threw in a little something extra at the end for fun. You can see I've prepared to get a projector and screen.



















As for the IB's output, I took some SPL measurements with the most recent digital Rat Shack SPL meter. With Sonnies corrections for this version of the meter, the results are promising. For reference, I used Ethan Winer's test tones downloaded from his website. I'm going to have to describe the plot as I don't have a working scanner and the results were manually drawn on logarithmic graph paper.

Pink noise was set to 75 dB. The in room response from the main seated position started at 77dB at 10 Hz and steadily and for the most part evenly rose to 97 dB at 37 Hz (there were a few minor blips). It then dropped to 83 dB at 46 Hz. After that it rose to 100 dB at 56Hz (and stayed there until 63Hz). From there it droped to 95 dB at 76 Hz and then rose to 99 dB at 81 Hz (and stayed there until 100 Hz. Then it slowly dropped to about 75 dB at 140 Hz.

All of this was done with the amp gain turned up 100% and the level output from the integrated amp set to a point where the clip light on the Samson S700 has only flickered once. I normally listen to movies with the volume set at 40 (but the setup and levels are otherwise the same), but in order for the pink noise to be at 75 dB I had to have the volume set to 20. Although I don't have the manual in front of me, I beleive that when I set the volume to 40, the response is 20dB louder than it would be at 20! So, without EQ, I think I'm capable of 97 dB at 10 Hz. I'm sure I could turn it up even more, but I just don't like to listen that loud. BTW, the front left and right, center and surrounds all cross over to the sub at 80 Hz, and the sub also gets all of the LFE signal.

What I gather from this is with EQ, I can cut significant response across the board and still be flat to 10 Hz. Moreover, I'll have room for a decent house curve and not have to worry about destroying my Avalanches by overexcursion or clipping the amp. It'll take some work, but the potential is there!

And... just for a little fun, I thought I'd add this...



BTW, that is a bald man's head at the bottom of the last photo!

[bpape]

Potential is definitely there. That's the only issue with IB though is that you can't move the sub around to help smooth the nulls that you can't EQ away.

Bryan

[jagman]

The null most likely comes from having the IB centered right to left and having the main listening postion centered right to left. Even though there is a significant drop in SPL in the 40's, it's still well above the pink noise level. It could definitely be worse. When I set up the BFD, I'll do several curves. One for center seating for music, one for center seating for movies, one for sitting a foot or so left (and right) of center for when my wife and I watching movies together.

[jagman]

Bryan... I also wanted to thank you for helping me with this project. I've learned a heck of a lot from you and others and appreciate it.

[Vaughan Odendaal]

So how does it sound jagman ?

[bpape]

Glad it's all working out. It's a lot of work but it's worth it.

I too would be interested in getting your impressions of it. I have a few customers that have done IB's and I've never heard a complaint - other than the lack of 'movability'.

Bryan