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Author Topic: Absorption vs diffusion vs diffraction  (Read 5624 times)

Sonovo

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Absorption vs diffusion vs diffraction
« on: December 28, 2006, 05:46:13 am »

Hi Fran, Ethan, Guys,

I'm currently going through my mastering room to see where improvements can be made, especially as I recently went from a console-less setup to a console in massive laminated wood. Gotto love ergonomics Cool

Due to the new furniture, I notice a slight flutter echo as well as some 'ringing' noise in the  mids at high levels. I'd also like to better tame the low end of the room as well. It's not bad, but it can always be better, right? Very Happy

Roughly speaking, the room is 570cm wide by 750cm long (actually there is a row of cupboards forming a kind of room divider here, they reach almost to the ceiling - about 2 meterss high, the room continues behind them for another 4 meters approximately) with 250cm ceiling height.

There are absorbtion panels hung 30cm from ceiling to catch primary reflections, and absorbers placed at primary reflection points on both side and back walls, as well as a heavy curtain to (I hope) attentuate HF reflections from cupboard wall behind listening position).

My monitors are dipoles, which may or may not influence the best way to go about treating the room.

I expect at minimum I'll want to put some sort of absorbent or diffusive panels above the listening position (and console) to kill the reflections there, as well as some sort of bass trapping in the room corners behind the monitors. Sound reasonable?

Anyway, while reading up on things at RealTraps, Auralex, RPG, and other similar sites, it seems that each company advovates treatment based on their main product (not suprisingly). I.e. RealTraps seems to emphasise absorption (esp. at bass and low mid frequencies), RPG focuses on diffusion and diffraction, and Auralex would like to sell you foam. Lots of foam.... None of them seem to advocate all three (RPG perhaps coming the closest with their trinity approach: imaging, spatial and bass managment tools). However they seem to focus on specific bass frequencies when using membrane absorbers, while Ethan seems to advocate more broadband designs.

From the collected experience in acoustical design here, is there any reason to favor one approach over another? Perhaps a combination of bass trapping and diffusion provides the best results in smallish rooms? Ethan's site seems to demonstrate pretty clearly what happens, yet most any high end facility you find on the net is usually using RPG and/or Aralex (albeit in much larger and purpose-built rooms).


Cheers,
Thor

p.s. Yes, I know I should get a professional in to assess the room. I'd also like to learn more about it myself.
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jimmyjazz

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Re: Absorption vs diffusion vs diffraction
« Reply #1 on: December 28, 2006, 02:07:21 pm »

Room size definitely plays a role in the tools one is "allowed" to use in creating the best acoustic environment possible.  Small rooms exhibit (at least) two disadvantages relative to large rooms:

1.  the frequency at which the modal response becomes relatively "smooth" is well into the range of musical tones, as opposed to being below the lowest bass frequencies of interest

2.  the time delays associated with reflected sounds are "short", which creates problems with imaging as well as frequency response


Both phenomena force the designer to use what is likely more absorption than might otherwise be considered desirable when one is only trying to control excess reverberation.
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J.F.Oros

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Re: Absorption vs diffusion vs diffraction
« Reply #2 on: December 28, 2006, 02:42:13 pm »

Thor Legvold wrote on Thu, 28 December 2006 12:46

Anyway, while reading up on things at RealTraps, Auralex, RPG, and other similar sites, it seems that each company advovates treatment based on their main product (not suprisingly). I.e. RealTraps seems to emphasise absorption (esp. at bass and low mid frequencies), RPG focuses on diffusion and diffraction, and Auralex would like to sell you foam. Lots of foam.... None of them seem to advocate all three (RPG perhaps coming the closest with their trinity approach: imaging, spatial and bass managment tools). However they seem to focus on specific bass frequencies when using membrane absorbers, while Ethan seems to advocate more broadband designs.

I find it right that a company making acoustical products to try to advocate the use of their products. That doesn't necessary mean that their solution is bad because of that. There are many solutions for a problem, and they give you one of them. But of course there are products and products, and a company can have some good products, and some not so good. There is less chances that one company can produce and give you the best products&solutions for all your problems.

And here comes the advantage of an independent consultant, not tied with any products company, that can give you a solution not influenced by commercial interests, combining the best solutions/products on the market (at least in his experience).

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Ethan Winer

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Re: Absorption vs diffusion vs diffraction
« Reply #3 on: December 28, 2006, 04:11:08 pm »

Thor,

> My monitors are dipoles, which may or may not influence the best way to go about treating the room. <

Yes, it does matter. With dipoles you want absorption on the front wall to avoid those reflections coming back at you delayed and causing comb filtering.

> I expect at minimum I'll want to put some sort of absorbent or diffusive panels above the listening position (and console) to kill the reflections there, as well as some sort of bass trapping in the room corners behind the monitors. Sound reasonable? <

Above you absorption is usually better than diffusion, especially since you do not have a high ceiling.

> RealTraps seems to emphasise absorption (esp. at bass and low mid frequencies) <

Yes, though you may have missed that we have a new diffusor product that is also a bass trap. I've done a lot of playing around with this new diffusor lately, and my opinion is still that absorption makes more sense than diffusion when the treated surfaces are fairly close. For example, diffusion could be useful at the first reflection points in a large room, but not when they're only three feet away.

> Ethan seems to advocate more broadband designs. <

Yes, and this is because small rooms have peaks and nulls at all low frequencies, not just those related to the room dimensions.

--Ethan

Sonovo

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Re: Absorption vs diffusion vs diffraction
« Reply #4 on: December 28, 2006, 08:48:27 pm »

Ok,

so something to attenuate the back wave on the front wall, as well as an absorbent solution on the ceiling above the listening position appears to be a basic starting point for my current situation. Smile

It does appear that larger purpose built rooms need less absorption and more diffusion, and smaller rooms benefit more from absorption rather than diffusion.

I do understand that each company wants to sell their own product, but was wondering at the lack of mention of other types of products to provide a complete solution. Obviously each room is different (as is budget!) and will have different requirements. It's good that there's so much to choose from!

Thanks again for the replies!

Cheers,
Thor
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franman

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Re: Absorption vs diffusion vs diffraction
« Reply #5 on: December 31, 2006, 02:53:22 pm »

I'm with Ethan on the bass trapping issue (as always).. smaller rooms have more "issues" in the musical range and (much) larger rooms have fundamental modes that are in the lowest octave (or lower)... The modal build up in large rooms happens as you start to enter the usable bass range for most music. This is one of the advantages of the larger room... Of course to have a well controlled bass response in larger rooms does require attention to these lower frequencies more than in smaller rooms...

So, in a small room, stick with as much broadband bass trapping as you can handle, control the first reflections (as you mentioned), and then play with diffusion to taste.

I also tend to agree that in smaller rooms, diffusion is not the best treatment on close surfaces. The scattering affect of diffusion can help create a more enveloping listening experience for home theaters, etc (thus you often see diffusors on ceilings over listening areas in home theaters), but this can tend to cloud imaging and make it difficult to discern short directional cues in the recording for mixing and mastering environments. Stick with reasonable thick absorption at the critical first reflection points in smaller rooms as far as I'm concerned.

Definitely have to consider the rear firing issue on dipole main speakers. I really don't care for this design, but I udnerstand there are a lot of high end loudspeakers that use this concept. I would put some absorption behind the speakers anyway (even with direct firing loudspeakers) as the diffractive reflections off the front wall will cause imaging issues in most cases....

my 10 cents...
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Sonovo

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Re: Absorption vs diffusion vs diffraction
« Reply #6 on: January 02, 2007, 03:23:40 pm »

Hi Fran,

thanks very much for your input and for clarifying and expanding on Ethan's points.

I'll be looking into absorption as my first line of correction, and diffusion only as necessary. I definately see your point as to the effects diffusion can have in too small a room, or too close to the listening position.

As far as dipoles, I really like panels, and have yet to hear a box speaker I really like. I guess I just have weird taste Very Happy. I will definately have another look at the front wall with regard to the back wave from the speakers. The existing treatment (front wall, ceiling and side walls) was installed about 3 years ago, and with all the changes I've made during the last year it's really about time I go through the room and adjust/add to what's already there.

Cheers,
Thor

franman wrote on Sun, 31 December 2006 16:53



So, in a small room, stick with as much broadband bass trapping as you can handle, control the first reflections (as you mentioned), and then play with diffusion to taste.

I also tend to agree that in smaller rooms, diffusion is not the best treatment on close surfaces. Stick with reasonable thick absorption at the critical first reflection points in smaller rooms as far as I'm concerned.

Definitely have to consider the rear firing issue on dipole main speakers. I really don't care for this design, but I udnerstand there are a lot of high end loudspeakers that use this concept. I would put some absorption behind the speakers anyway (even with direct firing loudspeakers) as the diffractive reflections off the front wall will cause imaging issues in most cases....

my 10 cents...

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Sonovo

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Re: Absorption vs diffusion vs diffraction
« Reply #7 on: January 12, 2007, 05:25:59 am »

Fran, Ethan, Guys,

thanks again for the tips. I've made some slight adjustments to the speaker placement (which helped), and perhaps most importantly I found an acoustician who is flying in this weekend to have a go through the room and help me get it as good as possible.

No excuse for the real thing Razz.

I'll let you all know what we end up with and what the measurements show when I get some time next week.

Cheers,
Thor
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franman

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Re: Absorption vs diffusion vs diffraction
« Reply #8 on: January 12, 2007, 10:53:24 pm »

who have you hired Thor??
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Sonovo

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Re: Absorption vs diffusion vs diffraction
« Reply #9 on: January 13, 2007, 04:44:59 pm »

Funny enough, it ended up being a guy that your guy recommended.

I spoke with Lars, and he knows a guy here that he says is good. I called him and made an appointment, and am interested to see how things go...

Of course, after the measurement and recoemmended treatment is done, I'll still need to order and install what's needed to improve the room.

Cheers,
Thor


franman wrote on Sat, 13 January 2007 00:53

who have you hired Thor??

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Sonovo

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Re: Absorption vs diffusion vs diffraction
« Reply #10 on: January 14, 2007, 04:13:41 pm »

<drumroll.......>




Ta da!


Well, the acoustician came and went, measuerd everything up down and upside down, in corners and nooks and crannies and at the listening position (and several alternate listening positions), raised, lowered and skewed, moved around the speakers, furniture, etc....

We had a great time, talked shop, music, tried what we could, and found that the setup I have (speaker/listening position) is pretty much the best I can get, and quite good. So my ears aren't completely off base, which was nice to have confirmed  Smile .

That said, obviously there are always improvements to be made, and he recommended several things I can do in order to bring the room up several notches. The room itself wasn't too bad, and the things he discovered mirrored my own measurements. Of course, he was able to give specific advice as to how to deal with each of the issues, which is well worth the price of admission.

So tomorrow I'm planning on buying a new fuse (first thing we did was blow a tweeter fuse), mounds of fibreglass isolation (got to fill up the front corners behind the loudspeakers), hooks and eyes (my existing treatment for primary reflections was correct, but poorly placed/located), a few rockwool panels as well as a few other things. That should pretty much do what can be done in the room. Oh, and he'll send me the plans for a Heimholz resonator tuned to one specific frequency that was causing a few problems.

So that sums up the current state of affairs, I'll let the forum know how things sound when I get everything implemented (and hopefully will make a new set of measurements).

Thanks again for all the great advice and help. Very Happy

Cheers,
Thor

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franman

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Re: Absorption vs diffusion vs diffraction
« Reply #11 on: January 16, 2007, 11:28:50 pm »

Thanks for keeping us posted.. was it Stig Erik you worked with?? He knows what he's doing..
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Sonovo

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Re: Absorption vs diffusion vs diffraction
« Reply #12 on: January 22, 2007, 03:58:34 pm »

Yes,

it was Stig Erik.

All in all we had a great time, and had pretty similar views on what we heard. I was glad to be able to pick his brain as to how to best deal with the issues at hand. Reflection absorbers have been re-mounted and I've bought a bunch of rolls of insolation, now all I need it time to install everything!

Cheers,
Thor
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