RPG Modex in corners vs. just making a walled-corner trap

genericperson · Post by **genericperson** » Wed Feb 25, 2004 11:15 pm

http://www.rpginc.com/products/modexcorner/index.htm

RPG sort of says these are the best things since sliced bread. but i'm wondering if it's way better than simply "chopping" off the corners of your listening room with 703 cavities, which would form floor-to-celing bass trap triangles.

these things are like $230 each, 2 feet high. you would need 16 to go floor to celing in all four corners. so that's a lot of cash ($3680).

so you can see why corner-cutting looks like a more appealing idea to me!

any thoughts on this?

dymaxian · Post by **dymaxian** » Thu Feb 26, 2004 2:02 pm

Sounds like a corner-shaped panel trap to me. If you're handy at carpentry, you could build yourself 4 entire corners worth of these for a couple hundred at very most.

Good theory tho. I'm sure they work great.

Kase
www.minemusic.net

genericperson · Post by **genericperson** » Fri Feb 27, 2004 3:56 am

yeah, i sort of discovered by accident that i'm good at carpentry.

i think the cut-corner idea would be cool.

the rpg says it can be used free standing. it could have a cool use as a bass trap drink table out somewhere on the floor in an anti-node (node?) point.

dymaxian · Post by **dymaxian** » Fri Feb 27, 2004 1:15 pm

Hey Ethan, there's an idea for you...

How about building a corner-bass trap like this, 36" high... or maybe 4 of them, and putting their inside corners together, for a little coffee table in the control room?

Most of the panel traps need to be attached to walls. I'm not sure how well they'd work if they were spaced away from the walls. I think for them to be most effective the front panel needs to vibrate, but the rest of the unit needs to be anchored pretty solidly.

Neat idea, tho.

Kase
www.minemusic.net

Ethan Winer · Post by **Ethan Winer** » Fri Feb 27, 2004 4:24 pm

Kase,

> How about building a corner-bass trap like this, 36" high... or maybe 4 of them, and putting their inside corners together, for a little coffee table in the control room? <

I actually saw some photos of something like that in one of the the forums I visit, where a fellow built a corner trap into some furniture. But we have no plans to sell that kind of membrane trap again. MiniTraps are much less expensive to build and ship, and they work better too.

> Most of the panel traps need to be attached to walls. I'm not sure how well they'd work if they were spaced away from the walls. <

Wood panel traps really need to be right against the wall where the wave pressure is greatest. This is different from porous absorbers like foam, fiberglass, and MiniTraps which work on wave velocity and perform best with an air gap.

--Ethan

midigod · Post by **midigod** » Tue Oct 12, 2004 5:50 am

Hi Ethan (or anyone)-

No one's directly talked about the Modex devices for a while now, and I was wondering if you could elaborate a little on why you believe the MiniTraps are better.

I've had an analysis of my room just completed, and having corner traps was part of the recommendation, to try and get rid of some issues I'm having at 50, 63, and 120. There were some other recommendations too, which included building an extra wall 12" or 18" out from the back wall, and installing an additional drop ceiling. Their recommendation for corner trapping was the Modex product, though my room is trapezoidal, so they'd have to sit out from the corner a little bit.

What I'm trying to figure out is whether the Modex would be better than eight MiniTraps (four in each rear corner), and whether either of those would outperform stacked 703. I've seen the comparisons between the Minis and stacked 703, but I've not seen direct comparisons between those and the Modex.

I guess I'm having difficulty deciding because of the differences in design philosophy between the three ideas. If we assume that we're not talking about price, can you shed some light on the practical applicational differences of the three philosophies in the rear corner of a trapezoidal room? I'd be anxious to hear your thoughts.

Thank you!

midigod · Post by **midigod** » Tue Oct 12, 2004 5:55 am

By the way, in case it wasn't clear, I'm talking about trying to decide between a panel membrane, a solid fiber structure, and a suspended fiber structure. If we discount cost, is one really "better" than another? And if they're really for different purposes, I'm having trouble figuring out what those different purposes are.

Ethan Winer · Post by **Ethan Winer** » Wed Oct 13, 2004 12:37 am

MG,

> If we discount cost <

Cost is always a factor. If one solution costs $8000 and the other only $2000, and the $2000 solution is 90% as effective, do you still ignore the cost?

> I was wondering if you could elaborate a little on why you believe the MiniTraps are better. <

I'll start with the $8000 versus $2000 comparison.

Then I'll let you figure out how you'd mount Modex traps in the ceiling corners.

MiniTraps and Modex corners use a very different design. Modex corners are classic membrane traps - they have a center frequency where absorption is greatest, and the absorption falls off on either side. There's no question they work well, but they're very expensive and absorb only one range of frequencies. You can see the ranges available on RPG's web site.

> I've had an analysis of my room ... try and get rid of some issues I'm having at 50, 63, and 120 <

I'm sure you have "issues" at all other low frequencies too. This is where MiniTraps offer a distinct advantage, because they absorb over a much wider range. This is overlooked by an awful lot of acoustics people! Any solution that targets only the frequencies related to the room dimensions ignores the peaks and deep nulls at all other low frequencies caused by simple acoustic interference that are just as damaging.

> What I'm trying to figure out is whether the Modex would be better than eight MiniTraps (four in each rear corner) <

How many Modex traps are you comparing to eight MiniTraps? MiniTraps are four feet high, and Modex corners are only two feet high. So a fair comparison is eight MiniTraps versus 16 Modex corners.

> I've seen the comparisons between the Minis and stacked 703 <

You need to take that with a huge grain of salt.

> I've not seen direct comparisons between those and the Modex <

You can see the absorption data by frequency for MiniTraps on the RealTraps site, and then compare that with Modex corners. If cost really is a small factor, I suggest you instead consider and compare our newer MondoTraps which absorb twice as much as MiniTraps below 100 Hz.

> I'm having difficulty deciding because of the differences in design philosophy <

One huge advantage of MiniTraps is the same physical piece absorbs over a wide range. With tuned membrane traps you need a mix of types and sizes to trap all the frequencies that need trapping. For example, if you build the panel traps shown in my 1995 EM article, you'll build both low-bass and high-bass types. Each absorbs over a range of about one octave. If you build ten traps you really have only five, because half of them help over one range and the other half help over the other range. With MiniTraps or MondoTraps each trap handles the entire range, so some number of MiniTraps absorbs twice as much as the same number of octave-wide membrane traps.

--Ethan

midigod · Post by **midigod** » Wed Oct 13, 2004 2:06 am

>Cost is always a factor. If one solution costs $8000 and the other only $2000, and the $2000 solution is 90% as effective, do you still ignore the cost?

No, I'm just trying to focus on the application differences. I'll concentrate on cost once I know the operational advantages of each.

>There's no question they work well, but they're very expensive and absorb only one range of frequencies.

Thank you for this education. That fact alone makes me not want to purchase them for my application.

>So a fair comparison is eight MiniTraps versus 16 Modex corners.

Agreed.

>> I've seen the comparisons between the Minis and stacked 703 <

>You need to take that with a huge grain of salt.

I hear you. Is this because you doubt the empirical validity of the test data shown here,
or because the units are different enough physically that they're not that comparable?

>I suggest you instead consider and compare our newer MondoTraps which absorb twice as much as MiniTraps below 100 Hz.

I'll do that, and that brings up an interesting question. If we know that my room requires a bunch of LF absorption, do you see an advantage to building a new wall 18" thick versus putting up some Mondos or even Minis? (We're talking here only about a wall comparison - the corners are a different item and thus I'm not talking about corners in this question.) I guess I'd like to not have to bring the dimension down by a foot and a half, but my brain has trouble comprehending that a thinner solution will work as well.

In case I haven't given enough information,
here's a link to their recommendations for my specific room. I give that link only in case I've not given you enough info. Not because I'm trying to get a free analysis from you!

You've really helped a ton. Thank you so much, Ethan. And by the way, I already have a studio pack of 8 minis and 3 micros in the room, now mostly placed for first reflections, with some front corner placement too.

Ethan Winer · Post by **Ethan Winer** » Wed Oct 13, 2004 5:29 am

MG,

> Is this because you doubt the empirical validity of the test data <

The ASTM demands a minimum of 64 square feet of surface area for devices being tested. This is to ensure that the change in the test chamber's reverb time is sufficient for the difference to be above the noise floor. That test used one fourth the minimum required for valid results. I've pointed this out to the guys who promoted those tests more than once, but they seemed uninterested in messy technicalities such as obtaining accurate data. Perhaps you could ask them to show you a copy of the official lab report. I'd like to see that myself, so if you get it please email a copy to me too!

> If we know that my room requires a bunch of LF absorption, do you see an advantage to building a new wall 18" thick versus putting up some Mondos or even Minis? <

Build a new wall made of what?

> I guess I'd like to not have to bring the dimension down by a foot and a half <

Do you mean a new solid wall to give the room better modal distribution? Both are important, but I'll take larger and poorer dimensions with plenty of bass trapping over smaller yet perfect dimensions with little or no bass trapping.

> I give that link only in case I've not given you enough info. Not because I'm trying to get a free analysis from you! <

Not a problem! I'm sure they did a competent job, though I question the use of 1/3 octave analysis as the only low frequency measurement.

> I already have a studio pack of 8 minis and 3 micros <

That's a very large space, so I'm sure you'll benefit from additional bass trapping in all of the corners, including the wall-ceiling corners. The wall-floor corners can be used as well if you have an appropriate place. Adding more MiniTraps will certainly help a lot. Or you could instead add some MondoTraps which will help even more and to a lower frequency.

--Ethan

midigod · Post by **midigod** » Thu Oct 14, 2004 3:11 am

>Build a new wall made of what?

It's an absorptive wall, with a 1" fabric track system on open wood framing, filled loosely with fiberglass. The wall is not intended to alter room dimensions, only act as a large absorber.

>but I'll take larger and poorer dimensions with plenty of bass trapping over smaller yet perfect dimensions with little or no bass trapping.

They're trying to get the larger room with a ton of trapping, though I'm pondering doing the trapping with panels rather than the absorptive wall.

>Adding more MiniTraps will certainly help a lot. Or you could instead add some MondoTraps which will help even more and to a lower frequency.

For the sake of argument and comparison, let's say I have 120 square feet of combination Mini and Mondo trapping, installed in corners, with some installed on a wall and spaced away from the wall. How would that compare with 120 square feet of the absorptive wall they speak of, which probably would not extend quite all the way to the ceiling? Hmm... there are a few variables in there that require extra calculation.... So how about this: I'm curious as to how a 2'X4' Mondotrap with 4" of proprietary technology performs versus a constructed wall of the identical 2'X4' size, but 18" thick of loose glass fiber.

-Craig

Ethan Winer · Post by **Ethan Winer** » Thu Oct 14, 2004 3:58 am

Craig,

> The wall is not intended to alter room dimensions, only act as a large absorber. <

Gotcha. That's an admirable goal, and 18 inches of fiberglass is always a good thing to have on the rear wall. Assuming you can comfortably give up that much space.

> For the sake of argument and comparison <

The only way I know to compare these apples and oranges - MondoTraps in the corners and on the wall, versus 18 inches of fluffy fiberglass flat on the wall - is to try both ways and measure each. I know the MondoTraps will do a great job, and I'm sure 18 inches will help too. But determining for sure which will help more, by how much, and at what frequencies, is beyond my abilities.

--Ethan

midigod · Post by **midigod** » Thu Oct 14, 2004 5:43 am

>But determining for sure which will help more, by how much, and at what frequencies, is beyond my abilities.

Thank you for being honest!

Building the wall will be the most hassle overall, and I really don't want to give up a foot and a half of front-to-back space. But installing Mondos and Minis will be more expensive than the construction of the wall. I think I've already discounted the use of Modex in this application, so I guess I'd better mull it over some more.

Thanks very much for your help!

-Craig

Eric_Desart · Post by **Eric_Desart** » Fri Oct 15, 2004 5:43 am

Note:

I wrongly quoted my whole message rather than editing it.
I'm sorry.

So it was mainly double.
Therefore deleted all this text.
Sorry for this wrong action.
Correct message below.

Eric_Desart · Post by **Eric_Desart** » Fri Oct 15, 2004 7:37 am

Ethan Winer wrote:> I've seen the comparisons between the Minis and stacked 703 <

You need to take that with a huge grain of salt.

Ethan Winer wrote:MG,

> Is this because you doubt the empirical validity of the test data <

The ASTM demands a minimum of 64 square feet of surface area for devices being tested. This is to ensure that the change in the test chamber's reverb time is sufficient for the difference to be above the noise floor. That test used one fourth the minimum required for valid results. I've pointed this out to the guys who promoted those tests more than once, but they seemed uninterested in messy technicalities such as obtaining accurate data.

Those test are executed by RAL (Riverbank Acoustical Lab). A lab of that statue doesn't amuse itself with non-valid testing. Those measurements were made to investigate something. As such it was in NOBODIES interest to perform invalid or inaccurate testing.
Auralex payed for those tests. They include DIY solutions competing their own comercial products. It is certainly not in the interest of Auralex, often identified with foam, to generate measurement data showing that simple glassfiber DIY solutions, within reach of anyone +/- match their own top range bass trap.
RealTraps is the only one questioning (over and over) the validity of those tests. Can this be related with the fact that MiniTraps are the worst of the tested samples, shown in the related graphs?

Ethan I can't help it that you can't read your copy of ASTM, which only requires this 64 m2 for compatibility for type A mounting or standard adjacent sample tests as ceilings etc, were one LOOKS FOR ABSORPTION COEFFICIENTS which has nothing to do with the validity in function of measurement accuracy of the tests themselves.

This must be the tenth time that this is discussed.
You do not understand the statistics controlling the validity of those tests.

ASTM describes exactly
Extracts: Literally text from ASTM C 423:

9. Test Specimen
9.1 Floor, Wall, or Ceiling Specimens for Absorption Coefficient:
9.1.1 The specimen shall be a rectangular patch assembled from one or more pieces.
An area of 6.69 m2 (72 ft2) is customary and recommended, in a shape 2.44 by 2.74 m (8 by 9 ft). An area less than 5.57 m2(60 ft2) shall not be used, and extreme aspect ratios, such as long narrow strips, shall be
avoided.

9.2 Specimens that are Offıce Screens:
9.2.1 Size—For test purposes, an office screen shall have an overall area, measured on one side and including the frame, of not less than 2.32 m2 (25 ft2).
For the purpose of determining the sound absorption coefficient, a, the total area of the screen is the area of the two sides. It does not include the area of the edges, that is, the product of the perimeter of the screen and its thickness.
Should the screens submitted for test be too small, two or more should be fitted together to make, in effect, a single screen.
To prevent extreme aspect ratios, the ratio of the screen or combined-screen height (including frame) to width (including frame) used to calculate the total area shall be no greater than 2:1 and no less than 1:2.

9.3 Specimens that are Detached Objects—The absorption of objects, such as space absorbers, theater chairs, or ceiling baffles, is dependent on the number tested together and their distance from each other and from the room boundaries.
Complete information shall be given in the report.

This is the COMPLETE text of point 9.3. I didn't exclude any part. There just isn't a thing more related to quantity.
ASTM distinguishes between standard tests and discrete object tests (as ISO does).

Note that point 9.1 refers to absorption coefficient while ALL your measurements MUST be approached as discrete objects as per the same standard, and which means that they ONLY should be expressed as Sabines, valid for a very well defined setup.
RealTraps measurements are covered by point 9.3 (discrete objects). Even your MiniTraps and the other ones against the wall are NOT covered by point 9.1

You DO NOT GET absorption coefficients from the IBM lab but sabins AS SHOULD BE for ALL published RealTraps measurements. You apply yourself a poetic license transforming them in absorption coefficients. I'm 1000% sure that the IBM reports do not and never will present those values as absorption coefficients.
Your suggestion on your site that people can use those (own made) absorption coefficients to compare with other materials is plain FALSE.

Point 9.2 refers to other smaller quantities (office screens)

Point 9.3 refers to detached, discrete objects AND DO NOT REFER TO ANY QUANTITY, only to the exact description of the measurement etc.

But those literally quotes already prove that you use this 64 m2 solely to confuse the physical valid comparison measurements executed by RAL.
Those RAL measurements are perfectly VALID comparative measurements.

Your conclusion that ASTM specifies 64 sft in function of measurement quality or measurement validity is a strict personal interpretation, not supported by the standard itself, nor by ISO 354 for that matter.

Your measurements, neither the against the wall nor the corner absorption measurements fulfill whatever standard application. Corner absorption NEVER does apply to a standard, it even contradicts the intent of measuring in a diffuse field the standard is designed for.
The confusion you systematically spread about those RAL measurements more describes you than those comparisons. Either you can't read the standard, or don't understand it or wittingly misinform people.

Not m2 or sft are important but added absorption, which are by definition controlled by the statistical boundaries calculated per frequency (you know: those strange formulas looking like Chinese to you).

I calculated ANY individual frequencies from those RAL measurements and told that I will put graphs on my site proving the validity.
I also checked them with the related ISO standard where those measurements also fulfill all required specifications in function of measurement validity related to quantity, WHERE I REPEAT: THE ADDED ABSORPTION VERSUS THE EMPTY ROOM IS DEFINING AS IS FOR ASTM not m2.

BTW if you're interested some of the low frequent values of your MicroTraps DO NOT FULFILL the requirements for valid testing.
So don't ask for measurements to others while releasing NOTHING, NEVER, anything yourself.

I'm really should like to see the NEW MiniTraps test you did end august/begin September 2004. I wonder why you don't publish the most recent IBM MiniTrap test you explicite executed to find the measurement values back of the First MiniTraps Test.
You did integrate the MondoTraps, why not the New MiniTraps IBM test? I know you measured them the same day about.

But I know that telling all this for the tenth time now won't make any difference whatsoever for you. You were never interested in what really is.
So you will repeat the same story over and over.