How I built my bass traps...

[PaulP]

Originally Posted by dykstraster@gmai

I'm in a similar boat. Just trying to hammer out the details. In the above thread you'll find a link to Primacoustic's page. Scroll down, there's photo of them installing the vinyl in a trap. If you don't mind, if you find a good mounting method, would you mind posting it in that thread?

Here's the link : Primacoustic - Acoustical Solutions.

The wooden strips appear to be screwed to the sides. I wonder what effect
different tensions would cause, surely it must affect the resonant frequency.
Or just how limp is 'limp' ?

Looks like a great model for a corner absorber.

Paul P

[SaSi_SiDi]

The primacoustic design is based around a 3" thick absorber panel mounted 5" away from the wall for a total thickness of 8" (as far as I understood the explanation and diagram).

The limb mass is suspended in the back of the absorber panel allowed to freely vibrate. That means that it should have a clearance from any surface (either the absorber panel or the back of the cavity) of 1" or so.

Now, as we all know, the wave velocity is higher as we move away from rigid boundaries, so the sheet is best suspended as close to the absorber panel as possible. At least that's how I understand it.

Regarding mounting, I am considering to use two pieces of 30x40mm hardwood planks and sandwitch the bitumen sheet between them. Then mount the sandwitch to the wall.

I am thinking of two scenarios. First scenario is for a corner trap and in such a case the sheet needs to hang at an angle to the wall-wall corner. My design for the corner traps calls for a 30 degree angle, so the bitumen sheet needs to hang at this angle behind the trap. Obviously I need to find a way to mount the wood planks at the two ends (angle cut to mate with the wall).

I am not sure if it's feasible to hang a single 250cm (8ft) long bitumen sheet. It has to do with the weight. The 10meter long roll is a bitch to lift and carry but manageable for me. I haven't weighed it but I am guessing 35~40kg (70~80lbs), so I am looking at a 7~12kg weight for the sheet plus wood support. It's easy to lift 10kg but not that easy to make precision mating with a 10kg inflexible sheet 250cm long. I may have to split it into parts.

I am also thinking that if I make a single sheet 250cm long, then the weight of the sheet itself will make the top part of it tension to the point that it defeats the point of "freely vibrate". I also need to think about ageing and the weight might elongate the sheet and deform it - perhaps to the point of failure. I have never worked with bitumen sheets before freely hanging. They are usually secured and glued in place.

So, the alternative is perhaps to combine the sheet in the back of each corner trap module. I am planing to make the floor to ceiling trap in parts, each module being 2ft high. Perhaps I need to re-design the top part of the frame so that the bitumen sheet can hang from there in the back of the trap.

[PaulP]

Originally Posted by SaSi_SiDi

The primacoustic design is based around a 3" thick absorber panel mounted 5" away from the wall for a total thickness of 8" (as far as I understood the explanation and diagram).

The limb mass is suspended in the back of the absorber panel allowed to freely vibrate. That means that it should have a clearance from any surface (either the absorber panel or the back of the cavity) of 1" or so.

In the Primacoustic corner trap the limp mass isn't exactly hung, as in hanging
from a horizontal support at the top, but tied all along both sides to strips of
wood that are screwed to the sides of the enclosure. So it's not just hanging
free. Depending on how tight the fit is, the limp mass will be more or less
taut.

I can't find how the limp mass is attached in the other absorbers but I don't
see why it wouldn't be the same.

Paul P

[SaSi_SiDi]

It is likely that the free hanging limb mass will have an unsuitably low resonance frequency and that my mounting it at both edges (with some tension) tunes the sheet into a usable frequency range. Something like tuning the strings on a string instrument.

More things to think about. Need to find a way of experimenting and measuring. Perhaps by hanging some weight from each sheet.

Perhaps a long, 8ft, sheet of bitumen isn't a bad idea. The topmost part, tensioned by the weight of the material below will have a higher resonance frequency while the lower portion will tune lower in the spectrum. Question is what the range will be.

Would it be a reasonable measurement to hang 3 to 5 sheets, each 8ft long at a distance of 1" from the wall and measure with REW before and after each sheet is mounted?

[nosebleedaudio]

Just built 4 custom bass traps for a client..
Fit very nice in the corners.

[SaSi_SiDi]

Nice looking. Is the light coming through the trap what it appears to be? Light coming through pidgeonhole windows in the back of the trap? If so, then the trap must be pretty thin. A 4" thick panel would not allow light to be seen through.

Can you describe the trap?

[stoneagejim]

sweet traps!

im off down to diy store this weekend! just moved into a new flat and i need to tune this horrible BOOOMY room!

again.... very nice traps

jim

[mikethebike]

Hi All

I have been lurking around this great thread seeing how folks have been getting on with their projects - I am just about to do some corner experiments myself and was loking for the right Rockwool to use. For the UK folks, I came accross this place that has some info on the products and seems to be the cheapest going.

rockwool flexi | universal slabs | rockfibre insulation | various densities | rwa45 | rw3 | rw4 | rw5 | rw6

The RW3 or RW4 (60 Kg/m3 and 80 kg/m3) seems to be the right stuff to use and comes in 100mm (4 inch slabs) - Any comments as to which is best?

Regards

Mike

[Finnish]

My story so far!

Bass trapping!

[Finnish]

Some updates to situation:

More bass traps

[mikethebike]

Hi all -

a question about the correct stuff to use in the construction of these absorbers.

In the UK we have stuff called Rockwool which is made from "volcanic ash" - .i.e. it is neither fibreglass or mineral wool (or is it?). It comes in 60/80 and 100 Kg/m3 densities. Which would be the most appropriate to use for these absorbers?

Thanks

Mike

[PaulP]

Originally Posted by mikethebike

In the UK we have stuff called Rockwool which is made from "volcanic ash" - .i.e. it is neither fibreglass or mineral wool (or is it?). It comes in 60/80 and 100 Kg/m3 densities. Which would be the most appropriate to use for these absorbers?

In this thread alone there are 79 references to rockwool. A search of the
forum turned up 500 hits.

Search is your friend.

Paul P

[dykstraster@gmai]

Paul, did you REALLY count?

In terms of the question, use the denser stuff for 2-6" thick traps, less dense for 6"-12", and go with standard builder's grade fibre batts for anything over a 1' thick.

[mikethebike]

ooooops

[Finnish]

All bass traps (and material) on the wall, I'll do some measurements with Room EQ Wizards tomorrow if I have to time.

Traps on the wall

[Finnish]

Ok, here's the latest info

My first room measurement... Aargh!