Hidden bass traps (question for Ethan?)

Posted: Tue Jan 27, 2004 8:01 am

Thanks to everyone who gave me advice on my previous post about corner bass treatments. I have a new question (I'm trying to figure out how to work in real bass traps to my room.)
Question 1)Could I construct wooden bass traps (as per Ethan's plans) and have then hidden behind bookcases as long as the bookcases didn't touch the vibrating surface?
Question 2) If I change the sizes from the original 2 by 8 panels (larger or smaller) does the effectiveness change change? (ie: are two 2 by 4 traps equal to one 2 by 8?
Sorry for all the questions but this forum seems like a wonderful resource for rookies like me. thanks in advance, Lee

Posted: Tue Jan 27, 2004 9:40 am

Hello Lee,

I haven't searched your previous question (which thread).
But what do you mean with Real Bas traps.

It's maybe a question of words, but good corner absorption (angled across corner)act as bas traps (better: broadband absorbers), and when just made of framed wool (glassfiber or rockwool or whatever) they can be very broadbanded and can go rather deep in frequency.

In fact that depends a bit on the outside measures you're willing to offer.

Panel traps also work (other principle).
But in order to get panel traps broadbanded you have to increase the cavity width and make the resonating panel lighter.
Also increasing the amount of wool inside will lower the Q but broaden the bandwidth.

If you hide a panel trap behind a bookcase, it's logical that you screen off the front panel of being excited.

Depending of the backpanel of the bookcase, you will get an interaction between both (rather complicated), but anyhow that's not the way to go.

Splitting a panel trap can have different effect.
I'm not sure exactly.

At one hand you stiffen the frontpanel (depends on mounting method of course)
At the other hand if they are seperated from one another the edge effect comes into play.

As I understood here it's better to use the larger size. So better others jump in here.

Best regards
Eric

Posted: Tue Jan 27, 2004 10:02 am

Lee,

Eric is correct on this.

1 bass trap split into 2 are not the same.

Placing the trap behind a bookcase may cause you further problem.

Your best bet is to construct them as designed - and use them the same way.

You could (however) experiment if you wish and let us know how things turn out.

Rod

Posted: Tue Jan 27, 2004 10:17 am

Hello Rod,

Maybe just some fantasy:

In a bookcase, for certain objects the shelf is too deep, others not deep enough.

Isn't there a way (you'r the construction guy) to use a bookcase as such that one alters the depth of the bookcase back panel(s) (in different depths) to use the bookcase itself in fact a kind of broadband paneltrap (+ diffraction, edge effect of books and objects itself).

As such the same wall surface can maybe be used more efficiently? :p

Best regards
Eric

Posted: Tue Jan 27, 2004 6:21 pm

Eric,

Interesting that you suggest a variable bookshelf, because I just built a 4-foot wide 7-foot tall "bookcase" recessed slightly (4 inches) into the front wall of my control room, with a stiff heavy back of MDF and sheetrock. Each of the five shelves is 11 inches deep. My plan is to use the different shelves as different kinds of acoustic treatment depending on what the room needs, when I finally run some acoustic measurements.

Possibilities for the individual shelves:
- stuff with fiberglass and cover with fabric
- stuff just the back with 2" OC 703 fiberglass and put books of various height and width and depth in front
- cover with slats with various slits, depending on what frequencies need to be tamed
- store my ancient vinyl LPs on the shelves, but in quadratic residue patterns
- store bottles of beer. When finished drinking the beer, partially fill each of the bottles with water to different heights for polycylindrical diffusion AND mid absorption AND humidifcation!

Lee

Posted: Tue Jan 27, 2004 7:58 pm

Let's picture this slightly differently.

Suppose one were to make a series of traps - and use those traps as the back of a bookcase..... making the shelves deep enough so that objects placed on the shelves never had to touch the back and destroy it's ability to operate properly. The shelves themselves could also never touch the back wall.

As we place objects in front of the traps we would begin to reflect some of the sound we're trying to capture.

Agreed that in doin so we also take diffusion into the picture - my guess would be that given the right circumstances you might have an effective system. However it would proably take a while to play with this to make it work right.

Let's hear form some other people on this. Interesting concept.

Rod

Posted: Wed Jan 28, 2004 1:04 am

Hi both,

I was referring a bit to Rods approach.

In fact if the backwall of the bookcase can freely vibrate (loose from shelves etc) then this backwall acts as the membrane of a panel trap.

So putting the bookcase a bit further from the wall, rather than against the wall will adjust the resonance frequency. Wool against the wall behind the bookcase acts as the wool in a paneltrap.

Even when you don't close those open edges, it still will work as a panel trap (law of inertia).

But you can or cannot close (or anything in-between the edges of the bookcase versus the wall.

In fact being airtight to act as a membrane absorber isn't necessary. Those open/closed edges between bookcase and wall is a bit comparable with a vent of a speaker.

The objects on the shelf acts as diffractors, but won't have much impact on the low frequencies.

Since the idea is to have a low Q and broadbanded, in fact all those irregularities will contribute to that.

So the basics were:
If one uses a bookcase, include the back panel as part of the broadband absorption.
It seems relative easy (at first glance)

From there one can extend by deviating depths etc.
The acoustic effect: certainly present, but need a crystal ball for this.

But if the goal is broadbanded, exact tuning isn't that important, just tune the backpanel low enough. Think about the open edges as a vent lowering the resonance frequency.

Lee, of course you can do lots of things with such a bookcase

Just thinking out loud.
Eric

Posted: Wed Jan 28, 2004 1:11 am

Eric

PS:
In fact Rod, once you got a membrane on a airlayer (spring) it will act as a membrane damper.
With a nicely made panel trap it's easier to calculate, but one can broaden the principle.

Posted: Wed Jan 28, 2004 7:51 am

You guys obviously aren't married to my wife - 15 minutes tops, and she'd have all the books "neatened up" by pushing them up against the back panel - which would be fine for the "quasi-quadratic" diffusor part on the front... :=)

Posted: Wed Jan 28, 2004 7:59 am

Make the back panel of the bookcase screen. Then a gap of 1/2" to let the trap panel vibrate. She gets to neaten them up, you get bass trapping.

BTW, the books will provide some diffusion.

You're letting the spouse make aesthetic decisions in the studio? Slippery Slope, dude.

Posted: Wed Jan 28, 2004 8:07 am

If we were ALL married to your wife she shouldn't find the time anymore to neaten them all up Very Happy

Quote:

Originally posted by knightfly:
You guys obviously aren't married to my wife - 15 minutes tops, and she'd have all the books "neatened up" by pushing them up against the back panel - which would be fine for the "quasi-quadratic" diffusor part on the front... :=)

Posted: Wed Jan 28, 2004 8:21 am

OK, y'all caught me - I shouldn't attempt humor before my first cup of coffee (it was brewing at the time) Wife doesn't change ANYTHING in my studio, but if I were foolish enough to try this little experiment in the HOUSE, then that screen Tod mentioned BETTER be the right COLOR... :=)

(Separate spaces - one of the mandatory keys to 38+ years of marriage...)

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