Best thickness for basstraps

serge instrumental · Post by **serge instrumental** » Sat Oct 02, 2004 9:49 am

I guess that 1/4" or 3/8"plywood are good average thickness for panel basstraps. But does aspenite or MDF(both less expensive) could do the thing.

I have JBL LSR28P studio monitors that are angled from the nearest point at 12" from the wall (rear firing port).

They are placed on stands at 35" high (the bottom of the monitor).

I have a desk (console, monitor, keyboard) facing the monitors that is 72"X 30" X 30"(Width X Height X Depth). The back of this desk is partially opened (from the floor up to 20" high) so this makes this space breathe.

My idea is to built a big basstrap (4'high X 8wide') 10 inches deep (outside measures) between the monitors and the wall. And I guess that suspended panel(s) should be the most efficient.

knightfly · Post by **knightfly** » Sun Oct 03, 2004 7:32 am

First, while any panel will resonate and act as a panel absorber, some are more likely to deteriorate from moisture or be more difficult to work with - MDF is more likely to allow screws to pull through if it gets wet for any reason, and has less strength (and more mass) than plywood. I'm not saying it won't work, just that there are possible problems.

Second, panel absorbers, while they are more efficient at their specific frequency, tend to be peaky at resonance and difficult to predict at other frequencies - their response changes with type and thickness and placement of insulation inside, and I've yet to see anything that allows you to predict accurately what a panel absorber will do until you BUILD it and test it.

This means, for one thing, that it's possible you could build an absorber that would WORSEN your particular speaker/wall spacing situation instead of improving it. Your example of using 3/8" MDF, which is about 22 oz/sq ft density, when placed over a 10" cavity, would peak at around 45 hZ, and might have a usable bandwidth as narrow as an octave or as wide as 3-4 octaves, and an absorption coefficient as high as 1.0 at resonance.

From a practical standpoint, using that space for a more broadband absorber would give a much smoother response IMO - something like 4" of rockwool or rigid fiberglass with a 6" gap behind would give you a more uniform result, and definitely a more predictable one -

IF you've not already played with Thomas' wall bounce calculator, you should do - enter in your speaker's specs, change the absorption values (try the peaky panel values first, using 1.0 @ 45, (don't think the calculator goes down that far, you'd need to "fudge" a bit) and using lessening absorption (maybe .1 or so) for all other values - ) - Then, enter in the more uniform, higher in most cases, values for 10" wideband absorber (these will all be between .8 and 1.0, anything close will illustrate my point) - you'll see what I mean by which absorber will suit your purposes... Steve

serge instrumental · Post by **serge instrumental** » Sun Oct 03, 2004 10:13 am

Thanks for your reply knightfly

knightfly wrote:From a practical standpoint, using that space for a more broadband absorber would give a much smoother response IMO - something like 4" of rockwool or rigid fiberglass with a 6" gap behind would give you a more uniform result, and definitely a more predictable one -

Steve

Using a 4' 703 at 6" from the wall for example, does it have to be covered with fabric in the front(the visible side) or it has to be made of a sealed box with a 3/8 plywood on the front(covered with fabrics) with the 4 pieces of 703(4" thick) glued to it(with 6" gap from the wall), then making it in a 4'X8 frame as I told before?

By the way the configuration of my studio is quite special, it would be hard to calculate. Maybe I should make a diagram out of it, but I'm not very gifted in schematics

AVare · Post by **AVare** » Sun Oct 03, 2004 3:21 pm

Using a 4' 703 at 6" from the wall for example, does it have to be covered with fabric in the front(the visible side)

The front should be acousitcally transparent. The most common way this is achieved is with fabric. The fabric does not have to be acoustically transparent, just not reflective.

Andre

serge instrumental · Post by **serge instrumental** » Mon Oct 04, 2004 2:54 am

AVare wrote:
Using a 4' 703 at 6" from the wall for example, does it have to be covered with fabric in the front(the visible side)
The front should be acousitcally transparent. The most common way this is achieved is with fabric. The fabric does not have to be acoustically transparent, just not reflective.

Andre

Thanks!

It will be cheaper to built!

knightfly · Post by **knightfly** » Mon Oct 04, 2004 3:52 am

Yeah, Andre's got you covered (sorry for the pun) - in fact, if not for looks you could just prop the stuff up and relax. If, however, you find that that large a surface makes things too dead-sounding, you can cover the absorber first with thin plastic (like those 1 or 2 mil painter's drop cloths) and then put your "eye candy" fabric over that. This will brighten up the treble response without affecting frequencies in the lower, modal range... Steve

serge instrumental · Post by **serge instrumental** » Mon Oct 04, 2004 5:00 am

Thanks both for your replies.

I'm gonna buy some stuff to do it

serge instrumental · Post by **serge instrumental** » Mon Oct 04, 2004 1:05 pm

But a sealed frame is still required? or just a 10" frame (just against the wall) put there will do it?

I know that a 4" 703 is already a good basstrap(and a broadband absorber by the way).

Do-I have to put standard fibreglass (a thin layer) against the wall? Or just the bare drywall will suffice?

Thanks again for your patience

knightfly · Post by **knightfly** » Mon Oct 04, 2004 1:12 pm

Just the bare drywall will work, although filling the space between the wall and the 703 with normal fiberglass insulation will make a slight difference - there is no seal needed for this type trap; the only reason for even building a frame is to hold the absorbent material in its proper location, and for looks.

You could just build something like a child's sandbox, only instead of a solid bottom you could just use an X brace to keep it from deforming - or, you could use small triangle pieces of wood at the corners, screwed and glued to keep the corners square - this will leave the back of the box open for best absorption, and that way you won't have other acoustic problems if you happen to position the box away from the wall a few inches... Steve

Ptownkid · Post by **Ptownkid** » Tue Oct 05, 2004 9:48 am

Johns traps call for a plywood back, are saying that it is uneccessary? What kind of difference does the open back produce?

knightfly · Post by **knightfly** » Tue Oct 05, 2004 7:59 pm

A heavy, closed back is necessary for both panel traps and helmholtz (slat resonator traps - for a broadband, absorptive fiberglass or mineral wool trap, the back can (and usually should) be open. This way, you can move the trap further off the wall to improve bass absorption at lower frequencies. With a solid back, that won't work... Steve

Ptownkid · Post by **Ptownkid** » Wed Oct 06, 2004 4:23 am

So the front and rear traps could be left open?

knightfly · Post by **knightfly** » Wed Oct 06, 2004 8:35 pm

You didn't specify, so I'm assuming you mean these?

http://www.johnlsayers.com/HR/index1.htm

IF so, then yes you could leave the front and rear traps open in the back - the only problem is portability; if you intend to move them, the back will make them much more sturdy to move around; and, it won't hurt anything acoustically to have the back on them, provided they are against the wall and not several inches away from it. IF they ARE away from the wall, you'll lose the improvement that could be gotten from a greater depth of air space from front of absorbent to rear barrier, since that rear barrier would be fixed by having a back on the trap.

Hope that helped... Steve

Ptownkid · Post by **Ptownkid** » Thu Oct 07, 2004 3:08 am

Yup, all is clear.

THanks

serge instrumental · Post by **serge instrumental** » Thu Oct 07, 2004 11:55 am

MMmm quite informative

Many thanks!