panel trap experiment outline

[z60611]

Ethan Winer:

I think it's safe to say, at LF, resonance is more powerful than mass.
Going by mass law, at 63hz the TL should be about 25dB, yet it's 13.9dB.
Or to put it another way, at 63hz, Brian added mass (the bass trap), and the TL went down.

[rod gervais]

Bob,

> only 1/3 of the wall had the bass trap on it. Covering 100% of the wall with the bass trap would probably lower the TL at those frequencies. <

Thinking about this further, it seems just as likely to improve isolation even more. Certainly isolation will be even better at the higher range, and possibly better at lower frequencies too due simply to the additional mass. If the trap side of the wall were covered completely it would be heavier, and the slight reduction in loss at 63 Hz could be pushed down in frequency to where it may not even matter anymore.

Ethan,

I would expect just the opposite result at LF - although I think I might agree with the high frequency thoughts.

I would expect a wall completely covered with the LF Bass trap (so as to create a complete 3rd leaf) will most likely increase the LF loss in TL that we see taking place in the test.

The added mass of a 1/4" panel is so little - and the trap vibrates so freely (much more so than it would if it were added simply to the existing face of a sheetrocked wall) that the added mass would not help to overcome the effect of lowered TL we see in the report above.

Basically - as I remember Eric's statements regarding this - Eric has been correct in his statements.

I don't recall ever "hearing" him say the what the amount of LF loss in TL there would be - just that there would be an effect on a wall with the trap placed in direct contact with the wall.

Ethan - you have also been correct in your statements that the effect (if there was an effect) would be minimal enough as to not really be a concern.

Brian my friend, thanks again for providing us with test data that at least puts this in perspective.

Sincerely,

Rod

[Brian Ravnaas]

You're welcome, Rod, and i'm sorry i have been too busy to really follow up on my own data.

At a future time, this will be tested on a higher performance wall for another good reference point as Eric suggested.

But it will be a little while until that one gets done, but i hope to come back to post tonight.

[Ethan Winer]

Rod,

> The added mass of a 1/4" panel is so little - and the trap vibrates so freely <

That all makes sense. I asked originally because I really didn't know for sure!

> you have also been correct in your statements that the effect (if there was an effect) would be minimal enough as to not really be a concern. <

And that's what really matters. Though again, I considered it an assumption, not a statement, because I didn't know. I only knew what made sense with my limited understanding.

> Brian my friend, thanks again for providing us with test data that at least puts this in perspective. <

Brian deserves a huge round of applause for doing this. Think about it. I asked a tough question of a guy who barely knows me, and not only did he give me his considered opinion, he went to the enormous trouble of actually building a bunch of bass traps onto a wall in a lab that charges real money. That is unbelievably generous!



--Ethan

[bpape]

Absolutely. That was quite a bit of work to go through and the results certainly do put things into perspective.

Thanks Brian!

Bryan

[Eric_Desart]

Brian,

Thanks a great lot, I know what such measurements mean.


Bryan,

What perspective? I see lots of question marks. I like the way Bob feels and searches. Bob my respect for your wondering mind.
What is the effect on the absorption of all this? In how far did structural resonance defined those results? etc ......

I've just not the energy now to go into this for now.

Eric

[Brian Ravnaas]

With respect to this test:


The single wood stud wall is a complex mechanical system. The change to a bit of a dip at 63hz should not be taken to imply that the panel traps were resonating at 63hz.

Per the plan of the group, the traps were 2' wide, and they were installed on a wall with 2' stud spacing. They were, more or less, plugged right into the mechanical resonant system of the original wall. The result of that is beyond mass-air-mass calculations and beyond my capacity to calculate.



The effect of the traps on the TL of a different type of wall (resilient channel perhaps, or staggered studs) wouldn't likely be the same. In that case you are adding alot of mass to one of the leaves of the wall, which could have a positive effect, and you are adding some resonances which will have some type of effect, though i'm not sure exactly what.



It is possible to less or more predict the effect of such traps against a single panel, like a brick wall, but on double leaf walls the effect gets too complicated for me to guesstimate ahead of time.


It is reasonable to say that the 63hz dip does not imply that the traps are maximally absorbent at 63hz. I can see that absorption data, however informal, might be a good thing to include on the next test.

[Eric_Desart]

Brian Ravnaas,

I DO like and respect you, and am VERY grateful that you show some of the question marks which there certainly still are.

You're a real scientist, and I don't mean this cynically but with respect.
But you see how incredibly easy people rather look for slogan-like answers then seeing these question marks.

Which does NOT diminish ANYTHING whatsoever on the extreme value of these test for which I agree in my gratitude to you with the others. That was in is an extremely generous offer and thing to do, not only costing money but a lot of energy as well.

Many thanks.

Eric

[bpape]

Eric.

What I was speaking of was the tradeoff between the potential for decreased TL due to the 3rd leaf being somewhat offset by the additional mass so that the net result was a minimal TL issue.

Bryan

[Eric_Desart]

Bryan,

As long as you don't know what happens here, it's very risky to conclude whatever further than this are very interesting results for this setup and combination. And that's it.

With a double leaf system the mass spring is mainly defined by the stiffest spring.
This means that when using RC or RISC that with a small cavity the cavity depth will become defining no matter how soft or stiff this spring.
When enlarging the cavity therewith softening the airspring at a certain moment the stiffness of the RC or RISC becomes defining for the MSM.

Here we have a wall were the original MSM is most likely defined by the structural resonance.
Basically this are very interesting measurements. The next question is: What were we measuring in fact?

How is the phase relationship between those 3 leafs?
Is it posible that you don't find the tuning frequency of the trap itself in this TL measurement, or does this trap hardly works anymore?
Is the frequency of the trap shifted that much or not?

Hence you see conclusive answers where I don't see them (yet).
I hope more will become clear in the future.
For now I see interesting questions, only assumptions of answers.
Are we speaking about a triple leaf system measurement or a construction on those walls with 2x4" beams?

For now, I can only emphasize to make such a trap tunable, at least untill knowing how this works and which parameters are involved.

Eric

[Brian Ravnaas]

For now, I can only emphasize to make such a trap tunable, at least untill knowing how this works and which parameters are involved.

Eric

Without a doubt this is compulsory.

The traps over concrete will behave in a reasonably predictable way. Over stud walls, they will interact with things mechanically and you can anticipate an unpredictable situation.

In retrospect its unfortunate we didn't take absorption for the test above, i will do so on the next test (absorption of the base wall, then the base wall with the traps).

Don't expect the net result to be the same for every construction.

[Brian Ravnaas]

Brian Ravnaas,

I DO like and respect you, and am VERY grateful that you show some of the question marks which there certainly still are.

You're a real scientist, and I don't mean this cynically but with respect.
But you see how incredibly easy people rather look for slogan-like answers then seeing these question marks.

Which does NOT diminish ANYTHING whatsoever on the extreme value of these test for which I agree in my gratitude to you with the others. That was in is an extremely generous offer and thing to do, not only costing money but a lot of energy as well.

Many thanks.

Eric

Nothing you said above is anything i would take as cynical at all, Eric.

Testing, and presenting the data, brings with it both good and bad. All the tests we have run trying to compare the effect of damping in an apples to apples way have brought both compliments and unfavorable scrutiny. Some folks think that's great, others are very demanding for even more tests and even accusatory. I guess the lesson is that you can't please everybody.

Tests like these aren't likely to please everybody either. With all the different philosophies and so forth that exist in the world, any given test is going to upset somebody for everybody that it satisfies. MLV tests are one example. Its widely used, and in 3 or 4 seperate tests the net results are basically the same - the gains are about proportional to the mass change. And some people find value in reporting that, others accuse you of running the tests wrong (it wasn't installed limply enough, it was installed too limply, you have to use both 1/2" and 5/8" drywall to make it work, you have to use thinner insulation blah blah blah).

Indeed, i for one would be happier if that product did have some unique effect, as the results would be more satisfactory to the general public. But you can only present what is tested and leave it at that.

Similar for another damping material that comes with big marketing claims. We tested it and it yielded only very moderate gains relative to the same wall with nothing. This concerned me in the same way - if the product at least helped a reasonable bit, the results would be more palatable in public presentation. So we re-tested with special documentation and horrendous expense, same results. I would be more comfortable with it all if the product performed better than it does. lol. sometimes a guy can't win. But that's how it goes.

Long ago we promised some triple leaf tests in a thread at studiotips. We ran those tests, but ultimately I decided to leave them on my computer as the results really weren't that unfavorable to the triple leaves. And as triple leaf construction is certainly undesirable, and as I didn't want to be the focus of an intense argument, i simply left the data lie.

And about triple leaves... that very dramatic double-stud comparison that moves from a quad leaf to a triple leaf to a double leaf is both a very rare case that maximally outlines the negative potential and would not be nearly as dramatic in terms of dB of performance lost if taken from a Tennekes or Theater perspective rather than an Rw or STC perspective.


I do appreciate the good sound reason that you present. A little knowledge can be dangerous, and (as i always jokingly say) test data is awesome, its conclusions that are sometimes dangerous.

We'll run the toher test on a decoupled wall and present it as well, and from there all one can do is assess the situation as best it can be assessed. I don't anticipate a substantial performance loss in that test for the simple reason that you're adding quite a bit of weight to the wall, which should work to lower MSM. BUT I'VE BEEN WRONG BEFORE.

As always, its my hope that ultimately tests i/we run help people.

[bpape]

I agree Eric. My only point was toward this particular test (which is a common situation - assuming that's why it was chosen) and the fact that in this particular case, the somewhat predicted reduction in TL didn't really happen - and at certain places just the opposite.

Obviously as you change the wall structure, the interaction with these types of absorbers will change. The question then becomes will the TL issue yield similar results.

Bryan

[knightfly]

"And about triple leaves... that very dramatic double-stud comparison that moves from a quad leaf to a triple leaf to a double leaf is both a very rare case that maximally outlines the negative potential and would not be nearly as dramatic in terms of dB of performance lost if taken from a Tennekes or Theater perspective rather than an Rw or STC perspective. "

Brian, been to busy of late to have much time for anything but a couple hours of sleep, but have a question - from the above quote, are you saying that the "famous" 2-3-4 leaf comparison then has more effect in vocal ranges (500 hZ center) than in music ranges? This would seem to contradict most people's perception of wall construction (not that that couldn't happen)

Second question - in your wall tests (such as published on the GG site) were there any tests run of DOUBLE framed walls, such as the supposed "63 dB STC" wall in the above example? (I'm referring to 2 layers drywall on studs with insulation, space, studs/insulation, 2 layers drywall)

All I see on your site is single frame construction, some with sound board, some with RC, etc.

Absolutely no disrespect intended, quite the opposite - I'm just curious if there were any tests done of walls such as people HERE might construct given the chance to build from scratch (or tear out and start over)

I know this is a bit of a hijack from the panel trap/3-leaf discussion, sorry guys... Steve

[AVare]

... the fact that in this particular case, the somewhat predicted reduction in TL didn't really happen - and at certain places just the opposite....

??? TL dropped at the resonant frequency range (predicted) and TL increased above that range (predicted). Please clarify.

Andre