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Sound absorber panels

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Sound absorber panels

Postby ricked » Wed Apr 29, 2020 8:03 am

I want to make some sound absorber panels. Am just trying to work out a design that is going to be most effective. I am finding it hard to find details on sound absorption for different products.

Is it better to have 2 2” layers of Rockwool separated by a 2” gap. Or is it better to have 1 4” or 6” layer. And how does this affect frequency response

If I’m going for an open side, how big should the holes be, assuming I’m in a 4” side, is 4 2” gaps 11” long going to be rigid enough ?

From what I’ve read if I’m doing dual 2” gaps I’d be looking at rs100, and 4-6” I’d be best going for earthwool rs60.
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Re: Sound absorber panels

Postby Sam Spoons » Wed Apr 29, 2020 10:41 am

I used Rockwool RW3 which is 60kg/m3 after much advice on here, as being the best compromise for full range absorbers. The slightly denser than usual panels are easier to use too as they are fairly rigid. I also wouldn't bother with complicated construction methods as, unless you are building to a professionally designed plan, the minor changes in absorption characteristics will be either lost or alter things in the wrong direction, and if you are
the acoustic consultant will tell you how to build the panels.

I used 2" thick panels spaced 2" off the wall/ceiling and the biggest corner 'super chunk' bass traps I could reasonably fit into my 4m square room (image below). Deeper panels will work to lower frequencies but to get good LF absorption they will need to be unfeasibly deep, at least for fitting on flat surfaces in a normal size home/project studio so it's always a compromise.

It sounds like you've done your homework though so please let us know how you get on.

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Re: Sound absorber panels

Postby Folderol » Wed Apr 29, 2020 10:56 am

I'm incredibly jealous of you people with decent size rooms available :cry:
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Re: Sound absorber panels

Postby Music Wolf » Wed Apr 29, 2020 11:20 am

ricked wrote:
Is it better to have 2 2” layers of Rockwool separated by a 2” gap. Or is it better to have 1 4” or 6” layer. And how does this affect frequency response


The physics behind this is that you are truning the Kinetic Energy of the moving air molecules into minute amounts of heat. Where you have standing waves there is no air movement at the wall, the maximum displacement is 1/4 wavelength away. The lower the frequency the longer the wavelength and so the 1/4 wave point is further from the wall.

In order to make the absorber most effective at lower frequencies you are aiming to get the mineral wool as far away from the wall as practically possible so have all your mineral wool as one slab (2 x 4" is just as effective as a single 4" slab) and space it as far from the wall as possible.

I use 4" slabs spaced 2" from the wall.
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Re: Sound absorber panels

Postby Hugh Robjohns » Wed Apr 29, 2020 11:28 am

ricked wrote:Is it better to have 2 2” layers of Rockwool separated by a 2” gap. Or is it better to have 1 4” or 6” layer. And how does this affect frequency response

In simple terms, thicker rockwool panels will be effective to a lower frequency (because lower frequencies have longer wavelengths). So a 4-inch (or even six-inch if you can afford the space) panel will be much more effective as a broadband trap than a 2-inch one... And spacing two 2-inch slabs apart won't be much more effective, really...

Far better to space the panel away from the wall as that will also make it more effective. These things work by slowing down moving air particles, and the air doesn't move next to the wall (maximum pressure, minimum velocity). The maximum air velocity is at a 1/4 wavelength away from the wall, so that's where the bulk of the absorber needs to be.

So if you're planning on allocating an overall panel depth (wall-to-front dimension) of 6-inches, I'd use four-inches of Rockwool spaced with a 2-inch gap from the wall. If you can make the wall gap a little larger, so much the better...

If I’m going for an open side, how big should the holes be...

As big as you can make them without sacrificing structural integrity! You obviously need to allow as much of the sound wave into the sides of the Rockwool as you can to maximise the absorbent surface area.

From what I’ve read if I’m doing dual 2” gaps I’d be looking at rs100, and 4-6” I’d be best going for earthwool rs60.

100kg/m3 is getting a bit too solid for a general purpose broadband absorber. I'd go with 60kg/m3 which is solid enough to be easily workable, but not so dense that sound waves struggle to find their way in.
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Re: Sound absorber panels

Postby ricked » Wed Apr 29, 2020 12:31 pm

Excellent thanks for the responses. I’ll go for rs60 / rw3 at 4” then to start with. You seem a techy bunch so:

I’m just guessing this but : My speakers sit on my desk mount, and the distance between the speaker and the wall define the frequency that is going to have maximum cancelation at the speaker. Which I assume is 1/2 wavelength of the distance of the speaker from the wall. Which is probably the most problematic frequency from the front wall.

I was going to put some treatment behind the speakers on the front wall. And as the 1/4 wavelength maximum velocity, placing the treatment half way between the speaker and the wall gives maximum effect.

I don’t see this in studio pictures online, so am I thinking about this wrong ? Or is there a standardised balance for the distance between the speaker, front wall and treatment.
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Re: Sound absorber panels

Postby tks0326 » Wed Apr 29, 2020 1:10 pm

https://www.readyacoustics.com/DIY-BASS ... E-EASY.pdf
This is a great DIY guide that I followed with great success. I used Owens Corning 703 because I knew I'd be moving them around a fair amount, but I believe it's about twice as expensive as mineral wool. Turned my small drum room from a low-mid mess to nice and tight!
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Re: Sound absorber panels

Postby Hugh Robjohns » Wed Apr 29, 2020 1:37 pm

ricked wrote:I was going to put some treatment behind the speakers on the front wall.

Good idea!

I don’t see this in studio pictures online, so am I thinking about this wrong ? Or is there a standardised balance for the distance between the speaker, front wall and treatment.

Acoustic absorbers on the front wall behind the speakers is a common enough feature. Most aren't as thick as your suggestion, but mostly for practical reasons than anything else. The closer the speaker is to the wall, the higher the null frequency and the easier it is for a normal broadband absorber to deal with it. But, if your room is big enough and you don't have a studio window in front of you, building a proper bass trap across the front wall can be a very useful thing...
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Re: Sound absorber panels

Postby Wonks » Wed Apr 29, 2020 1:46 pm

ricked wrote: My speakers sit on my desk mount, and the distance between the speaker and the wall define the frequency that is going to have maximum cancelation at the speaker. Which I assume is 1/2 wavelength of the distance of the speaker from the wall. Which is probably the most problematic frequency from the front wall.

I was going to put some treatment behind the speakers on the front wall. And as the 1/4 wavelength maximum velocity, placing the treatment half way between the speaker and the wall gives maximum effect.

Most of the rearward sound from a monitor will be bass frequencies, so you'll be looking at pretty long wavelengths, so your monitors will need to be a fair distance away from the rear walls before you run into serious problems. As the frequency rises, the more directional they will be.

Your biggest cancellation point will be with the speaker 1/4 of the wavelength away from the wall. as the sound has to travel 1/2 a wavelength there and back, when the phase of the returning wave will be 180° out from the originating wave.

So if your speakers are 30cm away from the wall, you are looking a frequency of around 300Hz for maximum cancellation from the reflection. But at 300Hz, very little sound will be coming from the rear of the monitors. With the next doubling of that frequency at 600Hz, you'll be getting almost nothing from the rear of the monitors.

So it's not something to worry about unduly. You'll probably get more of a sonic affect from the slight temporal smearing of bass frequencies due to LF waves hitting the wall and then returning a couple of ms later with very little phase change. So you can be better off moving the speakers as close to the wall as you can, and using the bass 1/4 or even 1/2 space option on the monitors (if available) to reduce the relative overall bass output level.

Your main issues will be overall bass trapping and reflections from the 'mirror' points. If you can but some treatment behind the speakers, then it won't go amiss, but there are more important places to locate it first.

What you don't want to do is put so much trapping behind the monitors and move the monitors inwards so much, that you move your listening position into the middle of the space and get the worst of the room mode cancellations.
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Re: Sound absorber panels

Postby ricked » Wed Apr 29, 2020 4:36 pm

Thanks Hugh, I was more meaning I don’t see the treatment half way between the speaker and wall.

It’s as you say normally always a few inches from the front wall.

Thanks for your comments, and the link looks really helpful.
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Re: Sound absorber panels

Postby ricked » Wed Apr 29, 2020 4:39 pm

Wonks wrote:So if your speakers are 30cm away from the wall, you are looking a frequency of around 300Hz for maximum cancellation from the reflection. But at 300Hz, very little sound will be coming from the rear of the monitors.

That makes a lot of sense and shows the problems in my thinking Cheers.
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Re: Sound absorber panels

Postby Wonks » Wed Apr 29, 2020 6:11 pm

Always better to ask the question. It made me think about it as well!
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