EOH Acoustical Treatment

[DrWho]

Apart from building a robot for engineering open house this year, I also put together a listening room for the Audio Engineering Society. The class room was an acoustical nightmare being about 14 feet cube, but we managed to make quite a huge difference on the sound. I just thought I'd post some pics to show off what about $50 of treatment looks like:

[DrWho]

And then here's a shot of the front of the room where we have a Helmholtz Resonator, my computer, and a screen for watching movies.

[DrWho]

Here's the right wall of the room where we have an exhibit on the subwoofers we built and the insides of a Heresy loudspeaker (talking about compression drivers, horns, and crossovers).

[DrWho]

And then continuing around the room, here's a shot of the back right of the room. If you look closely, you should see another yellow tube acting as a helmholtz resonator on the floor.

We picked the locations of the resonators by listening for frequency bands that were crazy style in the room and then moving the tubes around until we could hear that the tube was resonating a lot when putting your ear near the opening. You could then notice a change at the frequency at the listening position. The idea was to soak up reflections that were causing interference at the listening position.

[DrWho]

Ok class, pay attention. Here's the measured output at the listening position at various positions on the volume dial. We were supposed to stay under 75dB for EOH, but we totally let her crank whenever EOH wasn't in session - man it sounded good once all the treatment got put up. [Y] We were limited by the output of my crown amp for the subs....at 500W / channel we were clipping at 0dB on the receiver volume level, which was about 110dB at the listening position [H] The subs are only rated for 250W, but man they coulda taken even more juice.

[DrWho]

And then here's the left wall....no treatment because there was no way to mount anything over there with all them pipes and radiators and stuff []

Showing in the picture is a torn apart digital amplifier (one that I haven't finished building). Friday morning we added a pair of BR-1's and a finished amplifier to show off other projects we do in AES. In the bottom left of the picture you see my Klipsch backpack being used as a bass trap next to a Marantz 940 that we were using as surround speakers. Oh, and behind the couch is the measurement mic that we would pop up over the couch to take measurements from the main listening position.

[DrWho]

That's pretty much all the good pictures of our exhibit. We started setting up at 7pm Thursday night and finished at 4am. I didn't get to take many measurements (none of the empty room), but I know some exist and I wanted to post them. I was off at the robot competition for the majority of the time so I'm not sure what all was saved and whatnot.

It was unfricken believable the difference this little bit of diffusion and the couches made. I'm not sure how effective the helmholtz resonators were, but at least we were able to talk about them and show off []

Anyways, I'll post more stuff as it comes back from the other people. There were judges going around rating the exhibits and they spent more time at our booth than they did all the other booths combined in the rest of our building. They were having a blast. We didn't win any prizes, but that's ok. It was a very popular booth (who doesn't like music?) We were able to get a lot of incoming freshmen interested in AES and we got a lot of new contacts in the local audio industry - which means more trips.

Oh, and it was great advertising for Klipsch....them Chorus II's just sang (and I loved the horns keeping the sound off the walls).

[tenzip]

Cool. [Y] Nice job, I can imagine it's hell trying to tame all those hard surfaces.

If you'd had 2 'sets' of speakers, you could have set up 2 rooms, one treated, one not. That would have been an interesting comparison for folks touring through.

Is there more info on your 'cardboard subs'?

[PrestonTom]

Helmholtz resonnators are a bit tricky. There is a trade-off betwen the bandwidth and the degree of attenuation at the targeted frequency (the Q). Some of the designs can be extraordianrily narrow in their tunning and of course that would need to be matched to the frequency of the room resonance or ringing. On top of that they need to be placed where the pressure (or is velocity?) is at or near a maximum.

Good Luck,

-Tom

[DrWho]

We just told people to walk next door and listen how "pingy" the non-treated room was. We wanted two rooms, but they didn't give it to us []

What would you like to know about the cardboard subs? I think this is the thread where I talked about designing them....but n the process I learned a bit more and slightly modified the design:

http://forums.klipsch.com/forums/post/717845.aspx

There are prettier versions of these subs too...I should try to find a picture of one of the better ones - since aesthetics seem to be a big deal with sound quality for some reason []

For what it's worth, we had an F3 of 11Hz in this room...probably because of the insane room gain from all the brick walls.

[colterphoto1]

Thanks for the pix Doc, bet it was fun taking your system for a little spin. Nothing like showing off for the guys, huh? What's it like to be BMOC? [H]

How did that Sub building seminar go, btw?

[DrWho]

BMOC? Big man on campus? It rocks....now to use it to find me some lady friends []

The subwoofer workshop was a catastrophe...two of the days we set aside for cutting wood, the router bit broke apart during the first cut. It was the wierdest thing ever. The return process went smooth and all, but it put us over 2 weeks behind on having parts available. So pretty much I had to schedule sessions with everyone individually to build their subs since we weren't able to build everything during the workshop.

The lectures went well though - I didn't know there was so much stuff that could be talked about. I think my first lecture went 6 hours...and here I thought I'd be struggling for info! They asked for another lecture and so I went for another 4 hours or so.

The performance of the subs for exceeded expectations though, which was extremely comforting. I was going into this workshop never hearing anything and trying all sorts of new ideas and telling people that it's going to sound good without having any reason to really believe it. Everyone was more than happy in the end and it sounded way better than I thought it would.

[DrWho]

Oh, one more thing....the carpet on the floor didn't make a hill of beans of difference...we had it laying around and figured I would try it just to satisfy a curiousity.

[tenzip]

Oh, one more thing....the carpet on the floor didn't make a hill of beans of difference...we had it laying around and figured I would try it just to satisfy a curiousity.

I was actually going to ask about that. My wife resists the idea of carpeting the embryonic HT room. I assume that "real" carpet with pad would make a difference?

[mikebse2a3]

Helmholtz resonnators are a bit tricky. There is a trade-off betwen the bandwidth and the degree of attenuation at the targeted frequency (the Q). Some of the designs can be extraordianrily narrow in their tunning and of course that would need to be matched to the frequency of the room resonance or ringing. On top of that they need to be placed where the pressure (or is velocity?) is at or near a maximum.

Good Luck,

-Tom

In a tri-corner of the room pressure reaches a maximum and products such as ASC Tube Traps which work off of these pressure variations are most efficient in that location.

Hey Mike good work with the room treatments.

Was the Helmholtz resonnators cut to length and one end left open or did you use some fiberglass or such over the opening? Many years ago I cut several different lengths of 4" PVC Pipes tuned to my frequencys of interest and it actually worked pretty good. I basically sealed one end and located the open ends solid into the tri-corners with just enough air space to allow the pipe to resonate.

mike tn

[mas]

Are those the optimal panel colors for the most effective scattering and diffusion?

Did you have to special order those particular colors? A color/frequency attenuation chart would be most useful. ;-P

I really like how the panels fit into the chalk board rails and between the wall tray and ceiling! Great examples of turning lemons into lemonade in a room where attachment would be difficult at best![Y]

[DrWho]

There really was no science behind the "helmholtz resonators"...I didn't have the time to find the calculators online and figure out what frequencies they would attenuate and where thy would need to go. I just did it all by ear by getting the tube to ring (with the other end closed off) and then trying to memorize the frequency and walked/crawled around the room until I heard that same tone being built up in the corner. When you put the opening of the tube where you heard the boom, it suddenly went away in the room - coolest thing ever. I would venture to say that the frequencies being addressed were around 50Hz for the large tube and 80Hz for the small tube in the back.

The response in the room was +-5dB with the only exception of 10dB peaks at 25Hz and 50Hz with a null at 35Hz.

Because of the wide spread of the LR mains, we were able to keep the sound completely off the side walls. The first reflection at the listening position was off the rear wall at 12ms. After adding the diffusion, we were able to drop the first peak around 9dB, which is very close to the 10dB Mark tells me is the goal in most rooms. Beyond that, the reverberant field had a fairly linear rolloff. Looking at the waterfalls we could see that all the panels were vibrating a bit, but later we added blankets and cushions and stuff to dampen that (and never took other measurements). I would highly recommend anyone with curved masonite to dampen the rear of the panels - it's pretty obvious once you dampen their vibration.

If we were to spend a little bit more money, I think we should have invested in a few more bass traps in hopes of knocking out that 50Hz resonance and then we needed a bit more absorbtion to tame the 1-6kHz region. We probably could just use some of that crappy foam by mail stuff and attach it to the face of the panels - it has a pleasantly low absorbtion coefficient which would be good for not over doing it.

If Wuzzer is out there reading this....I think you should try putting some of your foam on curved panels in your room to help break up the parallel walls even more. The stuff is dirt cheap and easy to work with.

Btw, the colors were actually chosen for the sonic attributes! The yellow roll in the back made the sound more sunny while the browns everywhere else helped bring out the natural timbre of all the wood instruments. I play viola and can say that I've never heard a recorded viola sound so realistic! lololol

[DrWho]

Oh, one more thing....the carpet on the floor didn't make a hill of beans of difference...we had it laying around and figured I would try it just to satisfy a curiousity.

I was actually going to ask about that. My wife resists the idea of carpeting the embryonic HT room. I assume that "real" carpet with pad would make a difference?

Probably not too much of a difference. Half the carpet we put down was of the thick padded variety. The problem is that the wavelengths of sound are simply too large and don't get effectively attenuated. In fact, the rule of thumb is a surface at least 1/4 wavelength in thickness. So to attenuate "down" to 10kHz, you're going to need a material at least .3" thick and twice that for 5kHz. You're talking a really plush carpet to get down into the audible range - and even then you gotta hope the carpet doesn't reflect much.

[tenzip]

Oh, one more thing....the carpet on the floor didn't make a hill of beans of difference...we had it laying around and figured I would try it just to satisfy a curiousity.

I was actually going to ask about that. My wife resists the idea of carpeting the embryonic HT room. I assume that "real" carpet with pad would make a difference?

Probably not too much of a difference. Half the carpet we put down was of the thick padded variety. The problem is that the wavelengths of sound are simply too large and don't get effectively attenuated. In fact, the rule of thumb is a surface at least 1/4 wavelength in thickness. So to attenuate "down" to 10kHz, you're going to need a material at least .3" thick and twice that for 5kHz. You're talking a really plush carpet to get down into the audible range - and even then you gotta hope the carpet doesn't reflect much.

How much is my wife paying you? [] So, it would be ankle-deep before making much of a difference.

Can you give some detailed info on those subs? Dimensions, construction techniques, etc.? Looks like something that could be put together without too much trouble, and I might.

[DrWho]

The sub costs around $300 to build depending on how pretty you want to make it. Finding a local sonotube supplier will be the hardest part.

The baffle is simply MDF cutout to press fit inside the 18" sonotube. After stripping the wax from the interior of the sonotube, liquid nails were used to attach the baffle to the cardboard. You'll want to run your finger around the edge and push the glue into the space between the wood and the tube and do the same from both sides. It needs to be airtight as well. You do the same thing on both sides, only that one side has the cutouts for the driver and port, while the other side has the cutout for the amp.

If you wanted, you could go with an external amplifier, which leaves open the option of a rear port - the advantage being that you can flare it and reduce the port chuffing.

The 18" sonotube should be cut to 24" tall which yields an internal volume somewhere around 3.5 cubic feet. The 3" port should be cut to a length of 12" for a port tuning of 20Hz.

The driver is the Ascendant Audio Assassin 12" which doesn't seem to be available anymore?

http://www.ascendantaudio.com/

It's basically a Shiva-like driver for $90 that performs better...

And then the amplifier was a customization from Rythmik Audio:

http://www.rythmikaudio.com/nonservo_product.htm

The A250-Basic amp with the $5 bass boost mod. I asked them to do a specific filter that they don't include in their charts which better matches the alignment of the driver in the system. If you're interested, I can get you the exact numbers I asked for since they turned out nearly perfect after measuring the results. It's less parts too.

Overall, the subwoofer system should do a clean 110dB +-3dB from 20Hz to 200Hz in a medium sized room at the listening position.

We actually have a leftover kit from the workshop we ran so if you're interested in it let me know. I've already got a bunch of motorboards and sonotube cutup and ready to go too. Some of the guys went with down-firing designs for a more SVS style look, while others like myself just prefer to put em on their side without finishing them []

The only thing I would do differently if I were to do it again would be to figure out a way to do flared ports....perhaps just making everyone go with an external amp and building a small enclosure for it, which would provide room to put the port on the rear panel instead. Also, you should add some dampening material to the insides of the enclosure - the tube tends to ring a bit, moreso when cranking it.

I think that pretty much covers the basics of the design? Let me know if you need more info.