Soffit bass traps

You can use your 703 or loosely pack it with insulation.

CJ

Is there any data to show how the sound "up there" affects the listening position? I don't know if it could be/will be reflected down? Isn't this why we only treat upto ear level?

Treating the wall to ear level is for mids and highs ... what is being referenced above in the soffit is for bass traps ... you can never have too many bass traps is the theory

Not even the experts agree. I've seen both sides argue the issue here, and personally I used two styles in my super chunk traps. rigid 703 equivalent 16" up, and lighter desnity (but still rigid) in the middle. I used Roxul Rockwool.

Slowing air and disrupting the pressure wave is the goal of any bass trap, and the longer wavelengths (those below 150hz) take just a great deal of size to trap properly. It's the reflection off a hard surface that acts as the "trap", not so much how much or dense it has to travel through. NRC talbes show the high density stuff works better, but it's the distance from the wall that is MUCH more important to the performance than anythign else.

if your soffit is 703 on two sides, hollow in the center, and then reaches a rigid backwall, i suspect it will perform exactly the same if you fill it with fluffly, nothing at all, or ridgid. If there is a difference, I doubt your ears or any measuring equipment you have can detect it.

That's strictly my opinion though.

ETFAcoustics can compute the RT/60 times in a decay pattern. The theory escapes me, but there is certainly the risk of making a room too "dead sounding" by trapping too much. That's the reason for the diffusion devices many places sell, which scatter the sound to achieve the ideal RT/60 balance.

Again, like my last post, I've heard conflicting things. I have heard a fairly consistent opinion that the ceiling really ought to be more dead than live, to give the impression of a higher ceiling (audibly).

The only piece of consistent advice i've heard is put a bass trap in every tri-corner. The traps aren't very effective (at least the DIY ones) on a single plane wall or ceiling, it really takes a corner to get that reflection effect.

In any corner, the farther you can space the trap from the wall, the better. A trap 16" diagonally over the corner will way outperform one 4".

You could potentially debate this till the cows come home. I have a very similar setup in our dedicated home theater room. I actually hired an acoustical engineer to design the acoustics for the room. FWIW, I will share with you how he told me to finish out my soffit.

Mine ran the entire length of the rear of the room, app. 21' long x 12" tall x 6" deep. Bottom was finished out in drywall, like yours.

Front was faced with 1 1/2" rigid fiberglass, and covered with Gom Anchorage (I forget the number). It was not lined on the inside with the 703. Inside was filled, according to his spec, @ 75% with loose R-13 bat insulation.

Of course, this was utilized in conjuction with other acoustical components, panels, etc. It seems to have been just what the doctor ordered...for my particular application. Good luck!

Wonderful looking room you have by the way!

Audioguy-
Where did you get the diffusors on the back wall?

Typically, the bigger LF response problem in small rooms is as a result of modal issues. A characteristic of modal frequencies is they always peak at a room boundary ... always. Thus at the floor/ceiling, front/back wall, and side walls the modal frequencies are at their highest pressure, lowest velocity. From this we get:

1. Pressure type (not velocity type ... such as fiberglass panels) are effective; and,
2. Since all modal frequencies peak at boundaries and since all the boundaries come together at wall/wall/floor or wall/wall/ceiling intersections, the most effective placement is therefore at a right tricorner.

That is why you find bass absorbers more commonly disguised as softs or placed in the corners of the room. (Nope, we don't care about the "sound up there"; but, "up there" is a very effective location and it doesn't take up floor space.)

The type of ceiling devices AudioGuy has are a typical Rives method to address not only some bass absorption but is used (when exposed) as a diffusive device. While this method does provide some absorption in the LF domain, what is actually going on is the wall impedance is reduced meaning that most of the LF is being allowed to move into the space above of the room. Basically, you're getting better bass response in the room at the expense of sound isolation ... a choice you, as the home owner, get to make.

Yes, 1-2" of 703 on the surface and lighter material like uncompressed fiberglass batts for the several inches of inside thickness gives somewhat more absorption below 200 Hz compared to solid 703 for the entire thickness. The difference is not huge, but gets more significant with greater thickness.

Regards,
Terry

I have seen where some people have the soffit bottom open (cloth covered), some the sides, and some both. Since both wall/soffit and ceiling soffit form right angles I would think both should be open, but does that provide too much absorption?

Too much absorption?

Maybe you are referring to high frequency absorption. This can certainly be an issue. It is generally agreed that uniform reverberation time across all frequencies, regardless of the desired liveness of the space, is best. That means acoustical measurement of your room and/or estimating the reverberation times across frequency bands using the standard tools of acoustics, such as the Sabine formula.

So there is no all-purpose answer. It depends on what your room needs.

Regards,
Terry

I've seen tons of posts about superchunks and the like. Is there a similarly simple design for DIY pressure type bass traps in soffits or corners?

Hi Roger,

Dennis is technically right. However if the bass trap is deep enough, as in straddling a corner, it can catch enough of the velocity wave at low frequencies (1/10 of a wavelength, for example) to be effective.

There is also the technique of converting a velocity absorber to a pressure absorber. A membrane or thin panel attached to the front of a porous absorber does exactly this. Sound pushes against the membrane, the membrane moves, and now there is air motion for the velocity-type absorber to directly work on.

Membranes absorbers can be tricky, though. If the membrane has too much mass, it creates a tuned mass-air resonance which only works over a limited range of frequencies. The bass trap has a "Q" which can be high and therefore very effective, but which may not be tuned to the frequencies you need.

Fortunately, the process of moving from pure velocity absorber to a tuned pressure absorber is a continuous one. Using a small amount of mass, say, from the paper facing which comes attached to ASJ or FRK type fiberglass, results in an absorber with a very broad and bass-extended frequency range. 1/8 inch of mass-loaded vinyl, however, would create an absorber tuned to a pretty specific and limited range of frequencies. The latter is what is found in a product such as the RPG Modex Corner Bass Trap.

Regards,
Terry

I also am considering soffit bass traps. I'm still a little confused on the construction of them. I plan on having recessed lights in the soffits also.

Will this affect anything? I'm guessing I drywall the bottom, add the lights and then put fiberglass along with 1-2" rigid on the side facing the room. Does it matter what type of fabric? Does it have to be fabric?

Do the traps have to run the entire length of room or maybe just at the back?

Any detailed pictures would be appreciated.

Thanks much, Terry, for the detailed reply.

Based on all I have read here, and in Floyd Toole's book, and just yesterday in the AES paper Floyd referenced by Voetmann and Klinkby (an excellent and digestible read), I have concluded that one cannot make DIY membrane devices with any hope of predicting their acoustic behavior. It's a process of measurement and design iteration even for the experts.

The AES paper shows data on a their own design of a Soft Layer Absorbent Membrane (SLAM) device, and superimposing it over the RPG Modex unit shows them to be quite similar, even though the construction is different. I could not find any commercial sources for a SLAM type product--do such things exist?

I feel a half dozen RPG units are in my future.

Hi Roger,

I don't really know much about the SLAM, though it appears to have damping within the membrane rather than (in addition to?) damping from an internal porous absorber.

Here is Jan Voetmann's company website:
http://www.akustikjav.com/

SLAM is under the Resources tab. The site is in Danish only, but Google can translate it into comprehensible English. It does, however, change SLAM to SLUDGE!

Helmholtz resonators are indeed quite hard to design predictably. The center frequency can be gotten reasonably accurately from the original formula given by the good Dr. H. back in 1860. But the Q is quite complex to predict. I've been refining my own math software for this over the years.

Regards,
Terry

Markek,

Here is a shot of the under side of my soffits before the compressed fiberglass was installed.



Here is a link to a cross section

Hope it helps.

GP,

Thanks. So it looks like you will have about 3/4" of some sort of fiberglass and then will you cover this with fabric? What kind of lights will you install in those holes?

Mark

If you have a seating platform which extends across the entire width of the room and to the back wall, you can install fiberglass batts in the platform. If you then open up the platform in the back corners (using a floor register), you can create a pretty effective trap for modal frequencies. How effective depends on several factors such as size of opening and the cavity space in the platform. One of the problems with corner traps, becomes their size. 1/10 of an 80Hz wavelength is 1.4'. If the corner trap is triangular, only a very small portion of the trap has any significant depth.

There is a 2" thick fiberglass product available where the lab reports shows a 0.73 coefficient at 125Hz and .0.62 at 100Hz. Not bad. It is of the design Terry mentions where a 'skin' is installed over the fiberglass. Results based on Class "A" mounting.

Hi Dennis,

There seems to be something more complex going on with a corner trap. Here are some tests done with a few of them:

http://forum.studiotips.com/viewtopic.php?f=8&t=536

The peak absorption was at 100 Hz, with almost as much at 80 Hz. Eric Desart has puzzled over this, and I have as well. I suspect that mechanical properties of the absorber are playing a significant part.

The simplest model of a porous absorber has a perfectly rigid structure, and neglects mechanical effects within the absorber. It just considers the fluid flow of air and the impedance it encounters. But when mechanical properties of the elastic solid structure are added, there are stress and strain effects which under some circumstances become important. This was analyzed half a century ago by Maurice Biot. Biot theory is pretty complex, and my grasp of it is limited.

Regards,
Terry

I don't care about sound transmission out of the room either.

The single layer of drywall is either it works or it doesn't. You ended up on the right side of that experience.

---

I put Biot down as well. Tried again, fell asleep again. I agree mechanics are playing a role. The problem then becomes that one corner trap is not another corner trap when the construction varies by more than just a little. In looking at that 2" piece referenced above, and some other past (odd) experiences, I also suspect the performance is also impacted by more than a little bit by the order of the laminates.

The cavity is actually 1" deep. Notice the 3/4" MDF was cut in to strips one inch wide. Then placed horizontal and glued to the underside of the soffit.

They are low voltage recessed lights that were a special order at Lowes.

Sea Gull Lighting: Black Miniature Recessed Light
Item #: 9426-12
Finish: Black

I hope you won't mind if I pile in here too. My workers are just about to finish framing a bulkhead we've applied to my ceiling. I'm wondering if there might be a decent opportunity here to apply some acoustic absorption in the portion of the bulk head running along the top of the screen wall. (I'm hoping to tame a bit of bass bloat, ideally, but otherwise I would hope this would do something beneficial).

The bulkhead portion above the screen is 12 foot 9 inches long, 24 inches wide and 6 inches thick from the ceiling. I wonder if we can make a frame that shape and, since I'm going to cover the entire bulkhead with fabric, just fit some sort of acoustic material in the portion over the screen.

Any suggestions? Here are to depictions of my screen wall, looking up so you can see the bulkhead that is being built from the ceiling. In the second image I've indicated a sort of frame idea (to be covered in fabric) in the bulkhead in front of the screen:




With the bulkhead trap indicated above the screen:

Bump ^^^^

I have a platform thats half the width of my room and I was hoping to turn it in to a bass trap. After reading your comments above I'm wondering how it will impact its effectiveness being only half the room size. Also all I need to do is add a couple registers in the rear floor area to turn in to a bass trap? That sounds too easy. Thanks in advance