Bass traps and the like

[Coytee]

I've read about them and seen some pics of some of them. My question is, can they perhaps be located above a dropped ceiling?

My wife is an artsy type, I do NOT see her going along with ANY kind of traps along wall, corner, blah blah blah... it's already all I can do to have the picturesque K-horns in the picture. (which she finds horrid)

So, when we finish our basement (9 foot ceilings) we might use a dropped ceiling to cover up some of the ductwork.

If I end up needing some traps like that, will they work if they are attached ABOVE the ceiling tile? My guess is no, but I still gotta ask because if they DO work above the tile, then it vastly opens the door to me as to what I can tinker with and still hide visually.

Thanks

[artto]

Answer: Yes they can. Will they be effective? Probably not. In fact, it might even make things worse.

Most people considering bass traps for the first time usually misunderstand how bass traps work and what they do. The term bass trap is actually a misnomer. Bass traps are not installed to decrease the amount of bass in a room. They are installed to INCREASE the amount of bass, AT THE REQUIRED LISTENING LOCATIONS, the result of which, may result, in your turning it down at bit to achieve the same, or somewhat louder bass at the listening location. Their primary purpose is to "smooth out" the (peaks & dips in) bass response.

Bass traps do this by absorbing or dissipating acoustic energy in the areas of the room where the long bass wavelengths tend to build up and reinforce each other, typically at room boundary surfaces, especially where these surfaces intersect (corners), thereby reinforcing the bass even more. When these long reflected wavelengths coincide with each other out of phase in other locations in the room, the waves cancel each other out causing a null resulting reduced or very little bass at that frequency, at that location. By absorbing the excessive buildup of long wavelengths in the room corners, you effectively cancel the cancellations.

EDIT; BTW, I just had to put this in. You're wife is "artsy" is she? Not sure how that fits in with bass traps, what the hell. Tell her I'm "artsy" too. In fact tell her I'm so artsy I'm an architect (and part-time musician). And my name is Art too!! And I have all kinds of bass traps and acoustical apparatus. And most fine acoustical auditoriums are considered great works of art. You guys, with the intolerant wives, you know where thats all headed now don't you?

[DrWho]

A dropped cieling would be a perfect opportunity to employ the use of panel membrane traps and helmholtz resonators.

http://www.ethanwiner.com/acoustics.html

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=1363

Installing these on the cieling near the walls would be a great invisible approach. Both of these designs aren't broadband which means you will need to build a few of each tuned to different frequency ranges for it to be most effective (and most natural sounding).

If your woman is the artsy type, then I might suggest trying for the normal bass traps, but try to make them look good in some way. Even the curved masonite panels can look really cool...it's all in the implementation. I'd begin by inquiring what kind of decor she would like to go with and then try to find ways of "sneaking" stuff into the background. Is this a completely dedicated HT? Perhaps you could put up a curtain (like in the theatres) and then you could hide the bass traps in the corners behind it (and along the cieling too).

[formica]

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On 1/24/2005 3:30:35 PM artto wrote:

Answer: Yes they can. Will they be effective? Probably not. In fact, it might even make things worse.

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Just wondering if you think that this would still be the case if using an acoustically transparent ceiling?

I think a lot of people are afraid of their significant other when introducing acoustic treatments... and partially hiding them would please many of them. I was thinking along the lines of dropped "egg-crate" tile ceilings and painting the upper inter-ceiling part (which will be partially visible through the egg-crate grill) a dark colour. This would leave room to install these traps along the four upper ceiling corners.

BTW, I do agree that you can design something which looks visually appealing, but just hoping others won't get discouraged in using them in a family space.

Rob

[formica]

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On 1/24/2005 5:47:24 PM DrWho wrote:

Perhaps you could put up a curtain (like in the theatres) and then you could hide the bass traps in the corners behind it (and along the cieling too).

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That's kinda what I was thinking... similar idea

[Champagne taste beer budget]

Would the "complete egg-crate ceiling" treatment address the issue of the waves wanting to concentrate in the corners? Just thinking out loud here, would it work better to just use those in a certain percentage of the ceiling tiles in the corners of the rooms, or along the wall to ceiling junction? I may be all wet, if so someone please throw me a towel.

Edit: In rereading, I see you did mention the corners and sidewalls. Sorry.

[artto]

Rob, yes, as Mike (Dr.Who) pointed out. But as Mike also pointed out, this is not a "broadband" solution. These type of devices are intended more for use as midrange/treble diffusors, and they are usually 4" or more thick. Even as diffusors their diffraction limit is around 565Hz. These are not intended for use as bass traps

[formica]

opps... should have been more specific when i mentioned "egg crate" in an acoustic thread. I wasn't referring to egg crate foam diffusers... but rather the 2x4' "egg crate" ceiling grills (open cell) used to conceal light fixtures or as ventilation return grills.

Then use regular bass traps above that false ceiling.

Habit...

Rob

[artto]

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On 1/24/2005 7:34:42 PM formica wrote:

opps... should have been more specific when i mentioned "egg crate" in an acoustic thread.

I wasn't referring to egg crate foam diffusers... but rather the 2x4'

"egg crate" ceiling grills

(open cell) used to conceal light fixtures or as ventilation return grills.

Then use regular bass traps above that false ceiling.

Habit...

Rob

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"Then use regular bass traps above that false ceiling."

NO!!!!!!!!!!!!!!!

That would probably cause exactly the reverse effect desired! The "bass trap" would then be absorbing more energy precisely where there is very little to be absorbed! In that context the bass trap would most likely be located exactly where there are already bass cancellations taking place.

[Champagne taste beer budget]

I defer to my previous question re using them around the edges of the walls or in the corners. I MAY actually be learning something here!!!

[formica]

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On 1/24/2005 7:57:31 PM artto wrote:

That would probably cause exactly the reverse effect desired! The "bass trap" would then be absorbing more energy precisely where there is very little to be absorbed! In that context the bass trap would most likely be located exactly where there are already bass cancellations taking place.

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I'm not sure if i understand your explanation perhaps my description wasn't clear, or there is a basic principal I'm missing.

The false ceiling I tried to describe consisted of standard Tee's with an open grill (eggcrate louvre) system. Basically when you'd look up, you would be able to partially see through it. Acoustically I expected it to have minimal effect on the bass frequencies... and probably minimal higher frequency diffraction as well.

With that thinking... the original ceiling at 9' would still dominate the room mode... and I suspected that treating those corners (wall and ceiling at 9') with bass traps would result in the best result. Isn't it irrelevant whether they are wall/ceiling rather than wall/wall corner?

Take a look at these larger grill systems used to conceal mechanical, electrical, and acoustical treatments in commercial spaces. Same idea, but on smaller scale.

deltacell_2001.pdf

What did I miss?

Rob

[artto]

I understand where youre coming from Rob. Ive discussed this in depth with Jeff Szymanski, acoustical engineer at Auralex, some time ago as I had a similar idea about building some bass traps in-the-wall and/or behind the wall, so the bass traps wouldnt take up so much space in the room.

There are several issues at work here. Im going to make a judgment call and take it that, from his initial post, Coytee is trying to put some bass traps in to reduce the level of bass transmission to other areas of the house, without having large corner bass traps installed inside the room itself. If this is true, then that is not a good idea. That is NOT what bass traps are used for. With something as light, and structurally not rigid as a drop ceiling, the long bass wavelength will probably see the floor above as the room boundary. And then of course, you lose sound isolation because you dont have a strong, hard, dense room boundary surface for the room itself.

If on the other hand, Coytees objective is to create a bass trap, in-the-ceiling, with the open louvered tiles you suggested, at the location of a room corner, then yes, there may be some amount of bass trapping as a trap would normally be used. But lets not forget how a bass trap works and what it is supposed to do. First of all the space between the dropped ceiling and the floor above is probably not going to be very deep or large. Bass traps need to be located along room boundaries at corners in order to be most effective. The trihedral corners (wall, wall ceiling/floor) are the most effective locations (just like a Khorn).

If these traps were placed in the ceiling, further out into the room, any effective bass trapping they might provide would probably have the reverse effect, absorbing bass where there may already be a null, thereby increasing the effective null instead of reducing it.

I have seen ceiling & rear wall (even front wall) bass traps installed in recording/mastering studios, but these are typically very large and deep (>3). Therefore Im not sure as to how effective Coytees results will be under the circumstances he has to deal with, and either way hell certainly lose sound isolation in the process.

[Q-Man]

Would it be worth trying bass traps in a room where all the corners have Klipschorns in them?

[artto]

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On 1/25/2005 6:28:59 PM Q-Man wrote:

Would it be worth trying bass traps in a room where all the corners have Klipschorns in them?

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I would think so. I haven't actually tried it myself, but knowing that Khorns are capable of exciting any and all room modes to the max there should still be substantial benefit.

One of my next projects is to install Roxul RHT80 compressed mineral wool, full depth, behind the Masonite polycylinders around the upper half of the walls. The polycylinders are slightly resiliant and I believe damping them with Roxul (Owens Corning 703/705 should provide similar results) will help them function as better bass traps (they already absorb more bass than treble, but I wouldn't really consider them "bass traps" at this point). Its also a way to keep the room "live" (smooth hard surface of the Masonite), but still diffuse early reflections from around the speaker. My experience has been that Khorns prefer more of a "live" reflective area around them. The other advantage to this is that since the sides of my polycylinders are open, reflected sound that gets behind them will be instantly killed, thereby maintaining simliar reverberation time, regardless of frequency, even as the SPL gets very high.

At Klipsch/Indy, they use Auralex MegaLENRD bass traps sitting on top of the Khorns, which I suspect is about the only place they will be able to go if you have Khorns in all corners. Another approach would be to line the horizontal corners of the room with corner bass traps.

[Olorin]

That LENRD looks like a really cool product. Suppose a guy had a really tiny HT room -- say 11 x 12 with a ceiling that sloped from 9 to 13 ft. In your estimation, how many LENRD blocks (and where) would be needed to make a good difference? I know a room like that is never going to be really good, but better is always a step in the right direction.

[artto]

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On 1/25/2005 7:01:37 PM Olorin wrote:

That LENRD looks like a really cool product. Suppose a guy had a really tiny HT room -- say 11 x 12 with a ceiling that sloped from 9 to 13 ft. In your estimation, how many LENRD blocks (and where) would be needed to make a good difference? I know a room like that is never going to be really good, but better is always a step in the right direction.

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There are two main LENRD products. The regular sized LENRD (with different cosmetics and slightly different absorbtion coefficients) and the MegaLENRD which is about 4x larger and absorbs much lower frequencies.

Don't forget that the length of the traps you installed is also important. For instance, the standard "12x12x24" angled corner bass trap, when installed side-by-side across a horizontal corner, with 8 sections (16' long), has an absorption effect down to about 1/4 of the frequecy wavelength (16'=70Hz), although, because of its cross-sectional size it will not perform as well to those lower frequencies as say, a MegaLENRD, of the same length would, but it does have some effect.

It shouldn't take too much of the regular sized LENRD (personally, I think MegaLENRD would be way to large for your space). One of the Auralex "kits" will probably serve your needs well ($200-$650) or you can buy individual 2' long bass trap sections for $28 each from

http://www.samash.com/catalog/search.asp?STRID=192615&Method=2&SortMethod=3&Word1=&CategoryID=0&BrandID=1827&Search_Type=SEARCH&DepartmentID=6&DepartmentKeeper=&groupcode=nonetodaythanks&PAGENUM=0&pagesize=10

or http://www.americanmusical.com/sort--m-h--s-Auralex.html

Auralex has a free consulting service. I suggest that you send them a plan/description of your room, & Jeff will let you know what you need.

[WMcD]

I'm in agreement with Art here, with some additional thoughts. Some theoretical, some practical.

The primarly question may be whether a suspended ceiling will interfer with the functioning of the bass trap, or transmissions of bass frequencies to any object on the other side, or back.

I suspect not. Suspended ceilings are fairly low in mass and floppy. So they don't get in the way. This is consistent with issues of how to keep bass from going to the next door neighboors.

You need lots of mass, rigid damped structures, walls on different studs, and an air tight seal. A suspended ceiling has none of these qualities.

Please note that complaints about the neighboor's system, or ours as the case may be, is that the thumping bass is coming through. This just shows how transparent some structures are in fact.

Again in agreement with Art, standing waves are an issue and bass traps may reduce them. They arise because of the reflections being 180 degrees out of phase from the source at some frequencies. You hear silence because of phase cancellation. But they are in phase and at other frequencies and add up.

There is a joker in the pack in what I've typed immediately above. It is that nulls go to zero, but peaks go up only 3 dB. That is to say that the mountains are not very high. the nulls are very deep valleys. Bass traps can reduce the nulls.

When there is a phase cancellation, the sound level can go to zero. Your speaker can be pumping out 90 dB or any level. If you're sitting at the null of cancellation, the actual level can be close to zero. Realistically, it might be 30 dB down. OTOH, the adding is only a 3 dB addition.

It is difficult to convince anyone of the standing wave phenomenom until they do it themselves. You can if you have a test CD or computer program which will allow you to feed pure tones to your system. In theory, it is more simplified in wave mechanics if you drive one speaker. But try both ways

If you set the tone to about 1000 or 2000 Hz or more you will find annoying change in every step or head movement.

At 100 or 50 Hz you will find some locations where the level drops to zero. It sounds like someone turned off the amp. A few steps away, it sounds normal. Change the freq an the nulls move around.

Try it. It is the first step to understanding, and believing, just how much the room and location is the biggest part of our hi fi systems.

Best,

Gil

[colterphoto1]

NO NO NO

DO NOT use 'suspended' type ceilings in a room with Klipschorns. They will vibrate and buzz and drive you crazy! Even with the heavier panels (not the lightweight fibreglass) they will vibrate, ESPECIALLY with the EGG CRATES.

Unless you need access to something like a plumbing trap, make the ceiling SOLID. Here's how we used to cover up air ducts (and I was a home remodeler for years- did LOTS of finished basements). Create a bulkhead around the ductwork by using this technique. Usual bulkheads are constructed out of flimsy 2x2,s that won't span the ducts, but here's an elegant solution we worked out.

Next to the ductwork, nail a 2x2 'nailer' into the ceiling joists, then install the vertical parts of the bulkhead out of 3/4 plywood nailed to the 2x2, it should hang 2" below the lowest part of the ductwork. Now install 2x4's flat (horizontal) between the two plywood side walls. You can put one about every two feet and it will hold drywall even if you're spanning double-wide ductwork. You should still drywall over the plywood side boards so they'll paint up like the rest of the ceiling, you'll see the grain in plywood.

Michael

[colterphoto1]

Re bass traps,

I was recently in a studio where the 'lernd' (sp?) bass traps in the corner were covered with shirr fabric gathered very tightly, hung on a quarter-round circle, and then uplit with a small can light. Very elegant, looked almost like a limestone column.

'artsy' enough for the lil miss?

[artto]

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On 1/25/2005 10:19:43 PM William F. Gil McDermott wrote:

If you set the tone to about 1000 or 2000 Hz or more you will find annoying change in every step or head movement.

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Not only that, try changing the elevation of your ears while doing this. It's a rude awakening. You'll hear the frequency "appear & disappear" as you up move up and down too! A real lesson in how three-dimensional room modes affects what you're hearing in the relatively small rooms we listen/live in.