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Bass traps

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Bass traps

Postby kinglouis » Fri Oct 20, 2017 7:53 am

Hi,

I am just finishing building the new studio and am aware that whilst I have lots of movable acoustic panels, I haven’t quite understood the best way of filling the corners for bass traps. The room is 6.6m x 3m approximately, with the desk in the middle of the narrower side.

I don’t want to take up too much space. I’ve mixed fairly well without bass trapping for a decade but am aware that it would be nothing but a good thing to tackle it if possible!

Is it okay to just go from floor to ceiling with dense rock wool slabs cut into corners approximately a foot deep? I know I can’t stop all the bass frequencies without coming 10 feet into the room but any improvement is welcome.

Any advice or thoughts welcome.

Many thanks
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Re: Bass traps

Postby Sam Spoons » Fri Oct 20, 2017 9:36 am

That's more or less what I'm going to do but I'm sure more knowledgable folk will be along shortly, :D
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Re: Bass traps

Postby Hugh Robjohns » Fri Oct 20, 2017 9:49 am

kinglouis wrote:Is it okay to just go from floor to ceiling with dense rock wool slabs cut into corners approximately a foot deep?

Yes, of course. Bigger would be better, but you know that already and you have to come up with a pragmatic compromise in a domestic home studio.

Don't forget that you can also install corner traps along the tops of the walls to take advantage of the wall-ceiling corners -- and even the wall/floor corners (although that's not usually very practical).

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Re: Bass traps

Postby kinglouis » Fri Oct 20, 2017 10:29 am

Thanks chaps - the bit I don’t fully understand is (having read various articles about it from SOS) is the insulation going to make any impact on the bass frequencies which I think are much longer and more tricky to control, or do you need to start getting involved with floating high density fabrics in front of the trap or leaving a space behind the insulation in the corner... sheesh!
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Re: Bass traps

Postby Hugh Robjohns » Fri Oct 20, 2017 12:19 pm

kinglouis wrote:Thanks chaps - the bit I don’t fully understand is...is the insulation going to make any impact on the bass frequencies which I think are much longer and more tricky to control, or do you need to start getting involved with floating high density fabrics in front of the trap or leaving a space behind the insulation in the corner... sheesh!

All of the above.

Mineral wool and/or foam absorb sound energy by forcing the moving air particles to fight their way through a dense labyrinth, and as the air particles rub along the foam cells and wool fibres there is friction and sound energy is converted into heat.

But this concept only works if the air particles are moving in the first place... and the one place they don't (can't) move is against a boundary like wall, because there's no where for them to go. So the air pressure is at a maximum close to the wall but particle velocity is zero.

The maximum particle velocity occurs a quarter-wavelength from the boundary, so that's the best place to put the absorbers -- spaced away from the wall because there's nothing useful happening right against the wall. As a useful rule of thumb, if the absorber is spaced away from the wall by the same distance as it's own thickness, that will optimise its efficiency and maximise its low frequency effectiveness.

However, when we're talking bass traps, a quarter-wavelength at 50Hz is 170cm or 5.5 feet, and few project studios can afford to install 5 feet thick absorbers around all the walls. So some compromises and alternative strategies are usually required.

For starters, while you can't practically build an absorber 5 feet thick, you can make one five feet high -- or more -- and that's what you're doing with the corner traps. Sound waves travelling obliquely will run along the surface of the corner trap and lose energy, which is why floor-to-ceiling corner traps are useful. As before, most designs feature a gap behind simply because its wasteful and ineffective to put absorbing material there -- better to use it further away from the wall where the air particles are still moving.

And there are several alternative ways of absorbing LF energy, such as limp-mass membranes (your "floating high density fabrics"), or tuned cavity resonators and various other schemes. These are harder and more expensive to implement for the DIY-er, but not impossible, and again there are plenty of designs to use on the web.

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Re: Bass traps

Postby Martin Walker » Fri Oct 20, 2017 2:25 pm

Hugh Robjohns wrote:Don't forget that you can also install corner traps along the tops of the walls to take advantage of the wall-ceiling corners -- and even the wall/floor corners (although that's not usually very practical).

One wall/floor corner I can heartily recommend is mid front wall - in my small studio I had space behind my desk to drop down a large bass trap ( 4 feet wide by 2 feet high by 1 foot deep) that remains largely invisible.

This made a significant improvement to a front/back room mode.


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Re: Bass traps

Postby kinglouis » Fri Oct 20, 2017 5:21 pm

Thank you both, I really appreciate the info. I will try stacking dense rock wool from floor to ceiling in all corners leaving a 2” in from the corner with a mass loaded vinyl-type material freely hanging in front. And will let you know how it goes.

Many thanks,

Alex
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Re: Bass traps

Postby Martin Walker » Mon Oct 23, 2017 7:26 pm

Good luck Alex, and may the floors be with you! ;)


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Re: Bass traps

Postby kinglouis » Tue Oct 24, 2017 12:47 pm

Acoustic mineral wool panels and mass loaded vinyl all arrived - one last question before assembly...

Most designs show the limp mass on the outside, the first thing the sound hits. Wouldn’t that result in higher frequencies being reflected off it before it has hit the mineral wool?

Would it not make more sense to have the heavy vinyl on the inside of the panels - free/limp floating of course. To my mind that would allow the panels to deal with the higher frequencies before the lower end hits the limp mass.

I claim no knowledge of the physics of it and would be very happy to be told why my analysis doesn’t add up!

Many thanks,

Alex
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Re: Bass traps

Postby Hugh Robjohns » Tue Oct 24, 2017 1:51 pm

kinglouis wrote:Most designs show the limp mass on the outside, the first thing the sound hits. Wouldn’t that result in higher frequencies being reflected off it before it has hit the mineral wool?

Yes... but then this is supposed to be a bass trap, not a broadband absorber. There is always a danger of having too much HF absorption in a room, so putting the mass-loaded vinyl at the front mitigates against that with the inherently large frontal surface area of the bass trap.

Would it not make more sense to have the heavy vinyl on the inside of the panels - free/limp floating of course. To my mind that would allow the panels to deal with the higher frequencies before the lower end hits the limp mass.

It is certainly an option, and PW and I did exactly that many years ago in a studio SOS where we built a vocal booth, and there was limited space for absorption, so the bass trap had to double as a broadband trap too. it's not a problem, as long as you take its HF absorption into account when you're assessing what other treatment to place around the room.

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Re: Bass traps

Postby kinglouis » Tue Oct 24, 2017 2:08 pm

That makes total sense Hugh - after all I’ve never particularly paid much attention to corners in any of my previous studio spaces so there’s not much sense in suddenly getting precious about the hf responses from them when, as you say, the point of the exercise is to try to control bass!

Thanks for the steer, really helpful.

Alex
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Re: Bass traps

Postby Hugh Robjohns » Tue Oct 24, 2017 2:28 pm

It took me a while to track it down, but here's the Studio SOS where we built a large panel limp-mass bass trap. Might provide some useful ideas:

https://www.soundonsound.com/techniques ... ocal-booth

And here's another with smaller broadband traps using limp-mass again in the corner traps.

https://www.soundonsound.com/techniques ... -treatment

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Re: Bass traps

Postby awjoe » Tue Oct 24, 2017 3:19 pm

kinglouis wrote:Thank you both, I really appreciate the info. I will try stacking dense rock wool from floor to ceiling in all corners leaving a 2” in from the corner with a mass loaded vinyl-type material freely hanging in front. And will let you know how it goes.

Many thanks,

Alex

I'm probably late to the party with this, but I know a guy who cut his material into triangular sections, stacking each piece on top of each other in the corner, floor to ceiling. You could leave some space between the fiberglass and the wall. That way, the deepest depth of material was right in the center.
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Re: Bass traps

Postby kinglouis » Tue Oct 24, 2017 9:23 pm

Cheers awjoe - had thought of that sort of super-chunk approach and it would have been a good one, especially as the acoustic rock-wool I was using was considerably more floppy than the stuff I normally use.

I ended up cutting bevels into full lengths, floor to ceiling, held in by wire. About 35cm wide, leaving a decent gap in the corner but a good 4” of Rockwool deep. I cut the vinyl into 40cm wide strips and hung it in front of the Rockwool and then stapled some nice fabric in front of the vinyl. No idea if it’s doing what it’s supposed to but it looks pretty good and hopefully isn’t too far off the mark.
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Re: Bass traps

Postby Hzx » Sun Jul 21, 2019 3:29 am

Anyone have thoughts on whether it is better to seal the air gap behind the MLV, or hang the MLV so it flaps. We're looking at making corner traps, and have (mild- yellow on the Bob Gold analysis) modal warnings at 25.7 Hz and 34 Hz.
It seems like sealing the gaps tunes the trap???? Not sure how to calculate this for corner units though. Any ideas welcome!
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Re: Bass traps

Postby Hugh Robjohns » Sun Jul 21, 2019 9:46 am

You're right, sealing would make it a tuned box. In the (few) installations I've done with MLV I've left it hanging free, and I've always been happy with the results.
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