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Old 03-04-2008, 04:29 PM   #14
Mind Riot
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Ahhh, here we go! I'm new to Reaper, but not new to room acoustics and setup. Now I can contribute something to the forum!

Quote:
Originally Posted by pipelineaudio View Post
I guess Ill kick this off...

703 vs 705. There is a claim that one is more/less effective than another.

What exactly does "more effective" mean. Sounds rather nebulous. One may be more effective for laying on when floating down the colorado river while the other might fly further given an identical toss. Perhaps some quantifiable terms would be better

Under WHAT circumstances would I want to use 703 and what circumstances 705?
'Effectiveness' of each material is related to frequency. 705 is denser than 703, which means it will be more effective at absorbing the stronger low frequencies (higher density of fibers=more obstruction of airflow).

Higher frequencies don't need as much density or mass to absorb, so the differences in materials don't matter at that point. In fact, once you get above 10 pounds per cubic foot density you can start reflecting some highs and mids instead of absorbing them. 703 is 3 lbs per cubic foot, 705 is 5 lbs per cubic foot.

So 705 will perform better as a bass trap due to it's higher density, but 703 will perform just as well for mids and highs.

How much difference there is in performance has been documented on Ethan's site (if memory serves), but many people have used 703 all around their rooms with good results. Making sure your panels are the right thickness and are positioned correctly is key; a 703 panel positioned correctly will outperform a 705 positioned wrong.

There are benefits for bass trapping in using 705, each person will have to decide what to use based on their budget. But there are alternatives to using Owens Corning stuff, any equivalent density rigid fiberglass or rock or mineral wool will perform similarly. I treated my room mostly with 8 lb per cubic foot density mineral wool from McMaster Carr. About $8 per 2'x4'x2" sheet, and they'll ship it anywhere in the states.

Quote:
Originally Posted by cAPSLOCK View Post
But there are many more aspects of using this treatment that matter quite a bit to its performance.

1. FRK or not? (back or no back)
2. In corner/On plane.
3. Space from wall
4. Total thickness.

cAPS
1. FRK or not?

There are tradeoffs to either. Having an FRK backing facing into the room can increase the effectiveness of the material at lower frequencies. This happens because as the air vibrates at the bass frequencies the paper or foil backing vibrates in sympathy and is then damped by the fiberglass. It absorbs a bit more energy this way as opposed to plain.

The tradeoff is that some mid and high frequencies will bounce off because they aren't strong enough to cause the backing to vibrate or to penetrate the paper or foil. This can work great for treating a practice space or live recording room where you want to control the bass but not make the room too dead. But for a critical listening room for mixing you want the room to be controlled and clean, not contributing any of it's own reverberation. So I wouldn't recommend using faced panels for a small mixing room.

2. In corner/on plane, 3. Space from wall

I hope I'm understanding this right, that you're referring to placing panels straddling the room corners versus placing them flush with the wall surface.

Placing strictly absorptive panels straddling room corners gives significantly better absorptive performance than on walls for a few reasons.

1) Bass frequencies build up in corners where surfaces meet. Simple addition, sound reflects off surfaces and bounces around, two or more surfaces meeting means more sound builds up there. So placing absorption at those points will do more for controlling sound, particularly critical bass frequencies, than placing them elsewhere. Wall/wall junctions, wall/ceiling junctions, wall/floor junctions, are all good places for bass traps. But if you can manage putting panels at wall/wall/ceiling junctions or wall/wall/floor junctions where three surfaces meet it's even better.

2) Placing absorptive panels at corners also gives you a large air space behind the panel while minimizing the loss of space in the room. A two foot wide panel placed straddling a corner will leave a large triangular air space behind it, which will help absorption.

Sound will pass through the panel and some of it will be absorbed, but the sound that isn't will bounce off the walls behind the panel and be absorbed again on the return trip. And the bigger the air space behind the panel, the lower the frequencies it will absorb (to a point), because the longer bass waves will have a bit of room to form and be absorbed a second time when they hit the panel.

Think of it like this: you bounce a ball off a wall. If you put a panel flush against the wall, the ball will just bounce off of it. It might not bounce as far because the panel is there.

But if you move the panel away from the wall and throw the ball behind the panel, it will bounce back and forth between them.

Now sound waves go through the panel and not behind it, but you get the idea. Once they're through, if they have room, they'll bounce off the wall and be absorbed again.

Lower frequencies have longer wavelengths, so the bigger the airspace the more effective the absorption at lower frequencies. Think of a bigger ball that needs more room.

Bass frequency wavelengths can be several feet long, but obviously putting a panel several feet off a wall is not very practical. But it's not really necessary because there is a lot of energy that can be absorbed at a quarter or even an eighth of the total wavelength.

But that's all talking about bass trapping. For mids and highs, which have shorter wavelengths that are easier to absorb, placing panels on walls will work fine, although spacing them off the wall by a couple inches or so will help them absorb to lower frequencies. More bass absorption is always a good thing.

4. Total thickness

For bass trapping in corners, four inches is effective but six is even better. Most people do four, and if they're positioned right they'll work nicely.

For wall mounted first reflection panels and general mid and high end absorption two inches works fine although spacing them from the wall a couple inches will help them work better to lower frequencies.


I really dove into all this stuff when I treated my room a few years back, so if there's anything I can do to help please just ask. I don't know everything by any means, but I'll be glad to share anything I do know. I actually co-moderated Ethan Winer's Acoustics forum for a few months a couple years ago at his request, and I learned everything I know about room treatment and acoustics from him.

I hope this helps!
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