Question for Rod about Hidden Bass Traps

[Reed Black]

Hi Rod,


You have written an absolutely phenomenal book. It is a consummate design and construction book that spans such an eclectic field that you'd think there would be gaps and seeming inconsistencies everywhere -- and yet, no! Where you leave out info, you explicitly say so and direct us to the proper background reading. An absolutely major contribution for which we are all grateful and indebted.


I even bought a second copy for my iPad, in addition to my hard copy, just so I could always have it with me!


I do have several questions for you regarding your hidden bass trap design. I'm designing a room-within-a-room where optimal dimensions (1 : 1.3 : 1.9) leave me a full two feet above the inner ceiling, and would like to adapt your design for my application throughout the unused ceiling space. The big catch in my space is that there's a neighbor-to-neighbor duct chase running through the center of the ceiling, so I'm trying to get creative (Build thread is here: http://johnlsayers.com/phpBB2/viewtopic.php?f=1&t=17602).

I've marked up the drawing as best I could in order to make my questions clear. Please forgive my ignorance if some of these are no-brainers. I am quite sure it's my own limitations that are keeping me confused.

By the way: I do hope it's "fair use" to re-post this drawing -- here's the full citation just in case, though undoubtably no one needs the reminder! All rights reserved by you and/or your publisher, it goes without saying:

Gervais, Rod. Home Recording Studio. Build It Like the Pros, 2nd Edition. Boston: Course Technology, a part of Cengage Learning, 2011. Print. Page 222.

(If I should take it down, please let me know. Also, the file was shrunk down to 59K and may be hard to read -- let me know if I should try to outsmart the system and post the original, 1.1Mb file ;)

[Rod Gervais]

Reed,

Assuming that it is the uppermost hard ceiling that constitutes the room ratio - then you could absolutely use that interstitial space into for low frequency absorption.....

To answer your questions from the figure in the book (no problem posting it here) in corresponding order:

1) This particular project was a garage conversion - and the garage was attached to the home - the uppermost double layer of drywall in question was the inside surface of an airlock entry to the studio (that also allowed entrance to the living space behind the garage and the house). The airlock also acted to provide a wee bit more isolation to the studio from the living area.

2. That line is the inside edge of the plates for that little wing wall that acts as the inside edge of the bass trap (and is also a continuation of the inside face of the room itself),

3. OK - the horizontal 2x4 members are physically attached (nailed or screwed) into the studs behind them......... The 1x wood finishes over them are a carefully designed room treatment that are not just architectural in nature (although they do look great when all is said and done....... they create a Helmholtz trap as well as acting to add diffusion to the space.

4. Those are 2 wall frames - separated by a 1" air space - the drywall is located outside of the wall surfaces - thus there are only 2 leafs.....

5. Yes

6. Yes that is a hard connection between those walls - the wall to the right hand side of the picture (running north/south if you will) is the outside wall in the front of the garage) the hard connection is not an issues because it does not connect anything to the inside structure - the 2 are totally isolated. The interstitial space you highlighted in yellow is (part of) the air space between the inner and outer walls. The line your arrow points to in the inside face of the outside garage wall - the line immediately to the left of that is the inside face of the foundation wall below - which forced me to shift the studio wall to the left of that inboard from where it might normally be.

7. Yes you are - again - starting from the right hand side of the detail - outside face of the exterior wall frame - inside face of that frame - inside face of the foundation wall below - 1" air space - outside face of the inner wall frame - inside face of the inner wall frame - layer of 1/2" osb, double layer of 5/8" gypsum drywall - 1 1/2" rigid fiberglass between 2x4 horizontal furring - interior wood slat wall finish.........


I hope this helped,

Rod

[Reed Black]

It helped a ton, Rod. Thank you so much for taking the time to answer my (sometimes ""-based) questions!

However, the big "" here was your statement, "Assuming that it is the uppermost hard ceiling that constitutes the room ratio." And very unfortunately, the answer is: no.

Is there any way to use a large airgap ABOVE my inner shell (that constitutes the room ratio) more efficiently, and not JUST as an oversized 2-foot "spring" filled with R13?

Here - I've posted a side elevation of the control room (inner shell dimensions NOT to scale). As you can see, that duct comes across the outer shell laterally. That doesn't matter much anyway: My inner-shell ceiling height, according to my ratio of 1 : 1.3 : 1.9, is about that height. But that leaves TWO FEET of airgap that I'm simply using as a spring, then damping with R13.

Now, a little background -- In NYC, code requires a 2-hour tenant fire separation! So stripping the drywall off the interior of the exisiting ceiling to create a two-leaf system is not feasable, given that we have tenants above. I spoke with another studio designer who works in the city a lot, and he recommended that we simply go with a three-leaf ceiling system, beefing up the middle/third leaf to 2+ inches of type X drywall to make it extremely massy (and massier than either my 1 1/4" drywall inner-shell or the upstairs neighbors' flooring). So maybe that extra space in the airgap doesn't go entirely to waste...

Any ideas? Or am I simply doing the best I can given the circumstances?

Thanks again, Rod.

-Reed

[Reed Black]

Bump--any ideas? Anyone?

[Rod Gervais]

Reed,

There is a certain amount of absorption that takes place within the interstitial space that exists within the cavity of wall or ceiling assembly....

But - as you begin to increase the isolation levels (by adding mass) you decrease the level of absorption at the same time...... Adding mass locks more of the sound within the space it's trying to escape from......

The short answer is "not really to any perceptible extent"

Adding insulation within that cavity will absorb more of what is trying to make it out of the room - but it will not really help in the room itself........

Sorry.........

Rod

[Reed Black]

Hey Rod,

Thanks so much for your thoughtful response -- it's not good news, but that's a fascinating point -- that increasing impedance to sound at the shell also increases the amount that remains in the room, through reflection.

Like everything else -- makes perfect sense once you say it!!! But I'd never have gotten there by intuition.

Be well,
Reed