diffusion VS absorbtion

Posted: Sun Jul 31, 2005 8:45 am

yo...

Sorry if this has been covered a million times, but I wouldn't know what to search on this topic

I am reading lots of examples of various ways to construct and install diffusers(QRD and others) and the same with absorbers(703 panels and various bass traps)

The information I am having a hard time pinpointing is when to diffuse and when to absorb. I'm looking for information on how to determine how much of each to place. I understand there may be some trial and error, but I need some starting points, maybe a diffusion/absorbtion ratio, maybe a % of surface area for each?

The room, which is currently a rough framed exterior shell, is 24 X 17 1/2 with an 8' to 12'4" gabled vaulted ceiling. I will be recording a combination of acoustic guitar, drums, voice, etc. for what will be a 'home studio.' I may partition off part of it at some point, but I'll start with one room for now.

Any starting point would be greatly appreciated.

Thanks
Keith

Posted: Sun Jul 31, 2005 8:52 am

By the way

...so far, that one room will be for recording, then transform into a CR, so I'll likely have room controls which are not permanently mounted. I understand it may not be optimum for both, but that's what I have to work with.

Cheers mates
Keith

Posted: Sun Jul 31, 2005 12:51 pm

absorption - sound that hits it is converted into heat -- and doens't reflect
transmission - sound goes right through it
specular reflection - sound bounces off it: angle of incidence = angle of reflection
diffraction - sound bends around it
diffusion - sound bounces off it in a random direction.

Everything in the room (people, walls, chairs, rigid fiberglass, carpet, posters, cupboards, equipment, etc) has to some degree each of these 5 atributes.

walls: (absorption - very little, more at low frequencies, less at high frequencies; transmission - in terms of energy very little, more at low frequencies, less at high frequencies; specular reflection - a lot, less at low frequencies, more at high frequencies; diffraction - very little; diffusion - very little)

Although walls are not very efficient absorbers per square foot, you have a lot of walls -- multiply a little (absorbtion coefficient per square foot) by a lot (of square feet) and you get a medium (amount of absorbtion).

porous absorber panel: (absorption - lots, less at low frequencies, more at high frequencies; transmission - lots; specular reflection - very little; diffraction - lots especially if its deep and has nothing around it; diffusion - very little)

membrane absorber panel: (absorption - lots, more at low frequencies, less at high frequencies; transmission - medium; specular reflection - lots at very low frequencies, lots at very high frequencies; diffraction - some especially if its deep and has nothing around it; diffusion - very little)

polycylindrical or column: (absorption - little, more at low frequencies, less at high frequencies; transmission - medium; specular reflection - little to none a all frequencies; diffraction - very little; diffusion - lots at high frequencies, little to none at low frequencies)

an empty DVD case: (absorption - little, only if closed, hardly any surface area; transmission - lots but who cares; specular reflection - lots at high frequencies; diffraction - if open then prehaps a bit; diffusion - some at high frequencies, none at low frequencies)

Skyline or QRD: (absorption - some, 1/4 wavelength effect and membrane effect; transmission - some, depends what it's made out of; specular reflection - lots at low frequencies; diffraction - a bit; diffusion - lots at mid to high frequencies, none at low frequencies)

"Diffusers are statistically diffuse, which means that when area-averaged, they are equally diffuse at all frequencies within their design range. However this does not mean that at any given point there will nnever exist hotspots at certain frequencies" ("Recording Studio Design" by Phillip Newell, pg 333)

Anything solid and irregular is a diffuser. 1.5"x3.5"x1.5" and 1.5"x3.5"x3.5" and 3.5"x3.5"x3.5" cutoffs of 2x4's, glued to the walls in a random pattern, are diffusers.

The Goal
a) you want both direct sound and sound field
b) direct sound should be 20dB louder than any reflection(s) in the first 22ms. Comb filtering and smeared stereo imaging are bad things. Diffusers maintain the energy (relative to absorbers) so may not be a good idea in small rooms at first reflection spots.
c) early reflections between 25ms to 40ms add timbral enrichment
d) late reflections over 40ms add spaciousness to the sound, which is essential for its enjoyment.
e) reflected sound field should be diffused as possible as fast as possible so as to not be anything like an echo.
f) Recommended RT60 is proportional to room volume. So small rooms need shorter RT60 than large rooms.
g) Absorbtion is inversly proportional to RT60 needs. So small rooms need more absorbtion than large rooms.

So the small room rules are
a) absorb first reflection points
b) broadband bass absorb until desired RT60
c) add diffusion to deal with soundpath time vs strength issues.

Posted: Sun Jul 31, 2005 10:49 pm

wow

Thanks, that is a definite starting point and has a lot of good information. It should get me going anyway.

Now if I can just find my decoder ring... Very Happy

Cheers
Keith

Posted: Mon Aug 01, 2005 1:02 am

Absorbtion rules (a) and (b) are easier.

How much absorbtion is needed can be found using the ITU spec which is computed for you at http://www.bobgolds.com/Mode/RoomModes.htm

So after you've done your first reflections, you can do an RT60 calc for the rest to estimate what you'll need before you build the room.
If you have the room, you can do the RT60 to estimate, and then ETF5 to measure and actually do it while taking care of other time-domain issues as well.

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