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Formulas for acoustic absorption/diffusion

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Formulas for acoustic absorption/diffusion

Postby minima » Tue Aug 11, 2020 7:58 pm

Hi,
I'm interested in building my own acoustic absorbers/diffusers and there are lots of Youtube videos describing the process.

However, one thing that doesn't appear to be talked about is...how much do I need for the dimensions of my room? After much research into the subject, I'm not much nearer the truth...

I have heard the general rule of 'about a third of the room' but this is obviously no more than a generalisation and doesn't take into account the absorption/diffusion coefficients of the materials used. It may be a fair ballpark figure for commercially available panels perhaps, but without knowing their exact material makeup or specs (not always given) I wouldn't know exactly how much to get even if I was buying them. I have found a table of absorption coefficients online for commercially available materials but only measured between 125Hz & 4kHz which obviously isn't wide enough for studio applications. How much will it differ outside of these parameters? Maybe not so much going up the scale but going down in frequency, I imagine the figure changes significantly. Particularly important to know when making bass traps. And exactly where do they all go? I know about the wall and ceiling mirror points for absorbers & corners for bass traps but how big should they be/what area should they cover at these particular points?

I am familiar with the RT60 measurement rule and the Sabine Equation and these are fine for analyzing the behaviour of a room...but precise info on how to actually resolve any resulting issues appears to be something you have to piece together.

For instance a) how would I make an RT60 measurement? (would someting like REW Wizard do it with a suitable measurement mic? I don't see it menitoned in the product specs). b) how would I know how much of what materials would resolve it without applying too much/too little? ie how much absorption do I need without going too far or not enough? (Some say you can't go too far but I know that's false because firstly I've experienced a dead control room! Secondly pro studio control rooms are never completely dead in my experience and I've been to quite a few of the big London studios) There is obviously a correlation between the size and placement of panels too ie do I make a few big ones or many smaller ones? How do I measure exactly where they are needed or will be most effective and how much to use? (There must be formulas surely?!) c) (related to (b)) apparently the ideal RT60 time is different depending on the room size, so how do I know what the ideal figure for my room is?

And diffusers... how do I know how much diffusion I wll need & how is that measured? The diffusion characteristics of materials appears to be very hard to come by.

I realise this is where the pros come in and I emailed a couple for quotes last year, but they never replied, presumably my home studio is too small-fry for them to bother with! So I'm trying to accumulate whatever info I can on my own because I want to undertake this with a clear idea of my objectives and not based on guesswork & luck which apparently seems to be the norm! Thank you :)
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Re: Formulas for acoustic absorption/diffusion

Postby Sam Spoons » Wed Aug 12, 2020 12:02 pm

The 'can't have too much absorption' comment does apply when it comes to bass trapping in a small room, you will never get a 5 x 4 m room to behave like a properly designed control room at bass frequencies. As for the rest, I strongly suspect that unless you have very sophisticated measuring gear and the expert knowledge to interpret it's results those measurements will lead you astray and you would be much better to do the job by ear.

I have a 4 x 4 x 2 m room with 7 off 1200 x 600 mm panels and three fairly large bass traps. It works pretty well for mixing/listening but I'm about to add another half dozen panels to improve certain areas of the room for tracking. That will result in approximately ⅓ of the wall and ceiling areas above waist hight being treated. Below that there is plenty of random diffusion from all sorts of gear/instruments etc (though I might add some absorbers on the lower part of the walls behind the drum kit).

I built my absorbers using 60kg/m3 Rockwool RW3 as I was advised by a couple of knowledgable people in here that it was a good compromise.
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Re: Formulas for acoustic absorption/diffusion

Postby blinddrew » Wed Aug 12, 2020 12:35 pm

I would suggest investing in a copy of the Master Handbook of Acoustics https://www.amazon.co.uk/Master-Handboo ... 158&sr=8-1
And having a read through chapter 12 in particular.

I read it cover to cover before starting work on my place. As Sam says, in a small room like mine a lot of the detail wasn't directly relevant, but there is a huge amount of information there if you can use it.
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Re: Formulas for acoustic absorption/diffusion

Postby Hugh Robjohns » Wed Aug 12, 2020 12:52 pm

This really isn't something that you can do on paper with a calculator, largely because the physical details of the room construction will affect the acoustic properties of the materials used and, as you've already identified, obtained appropriate absorption coefficients for all the materials at all relevant frequencies is next to impossible anyway!

So instead, we rely on experience, in-room measurements, and a target 'reverberation' time with respect to frequency. REW Wizard can do all you need, but its results can be difficult to interpret accurately and can, in some cases, be quite misleading.

In practice, for a domestic project studio, a more suck-it-and-see approach actually works remarkably well. Start off with the essential treatments, then appraise the sound of the room, and tweak further if/as required.

So make up conventional broadband absorber panels for the mirror points, install as much effective bass trapping as is practical, and have a listen...
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Re: Formulas for acoustic absorption/diffusion

Postby minima » Sun Aug 16, 2020 1:54 pm

Hi, thanks all for your comments.
Sam Spoons - I'm still not convinced about there being no such thing as too much absorption as I have experienced this for myself in a friend's studio, roughly the same size as mine (5 x 3 x 2.4m) and I know I definitely don't want too apply that much! But thank you for the insights into how you went about things in your own studio, that's handy knowledge, and your point about the interpretation of the results being specialist knowledge in itself does in fact make perfect sense.

blinddrew - I will definitely check out that book, I hadn't come across that in my research, excellent tip. thank you!

Hugh - I'm sure there are formulas that the pros have access too (even if different materials have different properties in different rooms, I'm sure someone somewhere has calculations for what causes the differences and how they can be predicted!) but yes, I see now that perhaps I'm being too fastidious in my research. & need to step back & take a more pragmatic approach. I may soon be moving to a larger two-room studio (if so, I'll have a live room for the first time ever, which is exciting!) and I just want to get it right!

Thank you all for your help, very grateful! All the best to you
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Re: Formulas for acoustic absorption/diffusion

Postby Sam Spoons » Sun Aug 16, 2020 2:08 pm

minima wrote:Hi, thanks all for your comments.
Sam Spoons - I'm still not convinced about there being no such thing as too much absorption

What I said was "The 'can't have too much absorption' comment does apply when it comes to bass trapping in a small room, you will never get a 5 x 4 m room to behave like a properly designed control room at bass frequencies."

I went on to say "I'm about to add another half dozen panels to improve certain areas of the room for tracking. That will result in approximately ⅓ of the wall and ceiling areas above waist hight being treated."

The problem with too much absorption is not so much that there is too much but that it only works over a relatively limited range of frequencies. Absorbing only the higher frequencies will result in a bad sounding room and as most 'broadband absorbers' only absorb frequencies from the low mid range up simply covering the whole room in them would be a bad idea.

In a truly anechoic control room you would hear only the direct sound from the speakers which in many ways would be ideal but such spaces are difficult to build and most people find them uncomfortable to work in.
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Re: Formulas for acoustic absorption/diffusion

Postby Hugh Robjohns » Sun Aug 16, 2020 4:04 pm

minima wrote:Hugh - I'm sure there are formulas that the pros have access too...

You already know the formula. And you can research the absorption coefficients for different materials at different frequencies. But the calculations will still only get you into the ball park. The actual room will be close to the calculated value but different because of the unpredictable effects of the actual construction and materials variation, as I mentioned.

In-room measurement is the better approach if you want precision. Although its arguable as to whether that's necessary for an amateur project studio installation.

On another point, Sam was saying you can't have too much bass trapping in a small room which is generally true. The more bass trapping you have, the more even the bass response will be. And by the time you have 'too much' you wont be able to get into the room any more! :lol:

You can, though, very easily end up with far too much mid and high frequency absorption, which is a very common mistake and results in a boomy, boxy sounding room which is unpleasant to work in.

There are several good books on the practicalities of building a studio. I like the Rod Gervais one, but the Newell ones are good too, if for a higher budget level.

https://www.amazon.co.uk/Home-Recording-Studio-Build-Like/dp/143545717X

https://www.amazon.co.uk/Recording-Studio-Engineering-Society-Presents/dp/1138936073/ref=sr_1_1?dchild=1&keywords=Studio+acoustics+Newell&qid=1597591124&s=books&sr=1-1
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Re: Formulas for acoustic absorption/diffusion

Postby minima » Tue Aug 18, 2020 11:08 am

Thank you all for your comments and assistance, much appreciated :)
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