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Author Topic: membrane bass traps thickness  (Read 13437 times)

Thomas Jouanjean

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Re: membrane bass traps thickness
« Reply #30 on: February 05, 2009, 03:46:24 pm »

Bruno Gouveia wrote on Thu, 05 February 2009 14:31

 
My explanation is that as bass frequencies are more difficult to propagate through air they get trapped by the air itself and so the panels eventually absorb the energy as they vibrate, whereas in the small room we get to ear the vibration.


I'm not sure I understand you there... LF propagate very good in air, and carry a lot more energy. What you describe here is normal absorption by membrane like-panels, which take LFs 'out' of the room. Re-emission from those should fill the room with particular frequencies and up to a point (and not necessarily the ones emitted in the first place). Although I agree distance plays a role in how much you hear it (normal behaviour though).

Again, I'm not sure I fully understood your point... (Portugese -> English -> French and back can carry confusions)

EDIT: beautiful hall

EDIT 2: don't you have Helmholtz-like behaviour too in this room? If the panels are loose, it means there is a cavity...
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Thomas Jouanjean
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Bruno Gouveia

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Re: membrane bass traps thickness
« Reply #31 on: February 05, 2009, 04:27:43 pm »

Thomas Jouanjean wrote on Thu, 05 February 2009 20:46



I'm not sure I understand you there... LF propagate very good in air, and carry a lot more energy. What you describe here is normal absorption by membrane like-panels, which take LFs 'out' of the room. Re-emission from those should fill the room with particular frequencies and up to a point (and not necessarily the ones emitted in the first place). Although I agree distance plays a role in how much you hear it (normal behaviour though).



Like I said I'm not sure if it applies, but considering loudspeaker reproduction of low frequencies if the speaker isn't big enough it won't be able to develop LF waves that one can hear, by other words, there will be negligible radiation. Check this book   http://www.google.com/books?id=qc-mjjSFAlAC&pg=PP1&d   q=john+watkinson+sound+reproduction&ei=vViLSeTdCZb0ygS22 KG6BQ#PPA96,M1 page 96.

The point is, if the reproducing device is small and the frequency is low air itself will act like a spring and won't let the LF leave the premises. But I'm still struggling to grasp it better...

andrebrito

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Re: membrane bass traps thickness
« Reply #32 on: February 05, 2009, 05:14:46 pm »

No, not cavities behind wood panels in concert halls ! Laughing That is a basic mistake that reduces bass in concert halls for music purposes.

LF are not absorbed by air... air is the most important absorber in such places.

I know Casa da Musica well, was there a few times! There are some ressonators (the organs!), some diffusion as well.
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andrebrito

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Re: membrane bass traps thickness
« Reply #33 on: February 05, 2009, 05:22:18 pm »

Yes the loudspeaker will not have enough inercia at low frequencies but not only that, since you have to consider the box the loudspeaker is in
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franman

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Re: membrane bass traps thickness
« Reply #34 on: February 08, 2009, 03:30:28 pm »

Hey Thomas and Bruno... I don't consider myself an expert by any means in large room acoustics, but I know the physics is basically the same. There are typically different goals. I'm a little confused too:

We use Membrane Traps, Helmholtz Resonators and other Broadband LF trapping to INCREASE the amount of percieved bass in a (small) room like a control room. In other words, the more we trap, the better and tighter and more detailed the Bass sounds. It is a popular misconception in control room treatment, that bass traps will "remove" bass from the room...

BUT, this seems to be what both are you are implying unless I'm not understanding correctly>>> that too many loose panels acting as membranes will take bass response away from the room.... Am I getting this right?? sorry for my confusion.... Maybe it's totally different in large performance spaces, but I know how larger recording rooms behave and we still strive to add sufficient LF trapping in these rooms to ensure the bass is tight and the decay is comparable to the rest of the spectrum... as you know it's easy to absorb too much High and Mid frequency and end up with a boomy sounding room.

Thoughts>????
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Bruno Gouveia

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Re: membrane bass traps thickness
« Reply #35 on: February 08, 2009, 04:06:52 pm »

franman wrote on Sun, 08 February 2009 20:30

Hey Thomas and Bruno... I don't consider myself an expert by any means in large room acoustics, but I know the physics is basically the same. There are typically different goals. I'm a little confused too:

We use Membrane Traps, Helmholtz Resonators and other Broadband LF trapping to INCREASE the amount of percieved bass in a (small) room like a control room. In other words, the more we trap, the better and tighter and more detailed the Bass sounds. It is a popular misconception in control room treatment, that bass traps will "remove" bass from the room...

BUT, this seems to be what both are you are implying unless I'm not understanding correctly>>> that too many loose panels acting as membranes will take bass response away from the room.... Am I getting this right?? sorry for my confusion.... Maybe it's totally different in large performance spaces, but I know how larger recording rooms behave and we still strive to add sufficient LF trapping in these rooms to ensure the bass is tight and the decay is comparable to the rest of the spectrum... as you know it's easy to absorb too much High and Mid frequency and end up with a boomy sounding room.

Thoughts>????


I'am confused too. I mean, I still have to study this subject further. The fact is, two big rooms I know of Casa da Musica and a church full of loose wood here in Porto have a lack of bass energy. For sure the wood is coupling with the sound as one can access easily with an accelerometer or a piezo pickup. What it does to the sound in the room, is what's puzzling me...

I also can add that I already spend some time in a small room 7x5x3 and is also full of loose pine wood everywhere and the ceiling has wood conglomerate panels. The acoustics are bass heavy in this case, or they are lacking treble. But overall it's a pleasant acoustic, already done good recordings there.

andrebrito

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Re: membrane bass traps thickness
« Reply #36 on: February 08, 2009, 06:49:52 pm »

Lack of bass does not mean the bass energy is less than the middle and high frequency energy in the room, it just means that for the goals of the room and the perception of the listener something is missing.

Also bass energy in a large room is basically the octaves of 125-250 Hz where in a small room bass energy is often seen as something below 100 Hz or below the Schroeder frequency.

Keep in mind that auditorium is first rather small and secondly not sure how the seat dip effect behaves in such a room.

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Thomas Jouanjean

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Re: membrane bass traps thickness
« Reply #37 on: February 09, 2009, 03:10:37 am »

franman wrote on Sun, 08 February 2009 14:30

We use Membrane Traps, Helmholtz Resonators and other Broadband LF trapping to INCREASE the amount of percieved bass in a (small) room like a control room. In other words, the more we trap, the better and tighter and more detailed the Bass sounds. It is a popular misconception in control room treatment, that bass traps will "remove" bass from the room...


When trapping or using resonators, you are taking (LF) energy 'out' of the room, into heat. And because you do so, you even out the freq response of the room, by reducing and sometimes basically stopping any interactions between direct field energy and reflected/re-emitted energy. I don't think it is wrong to say that one takes bass out of the room - but I agree it should not be mistaken for another concept, that bass is not existing anymore in the room.

franman wrote on Sun, 08 February 2009 14:30

BUT, this seems to be what both are you are implying unless I'm not understanding correctly>>> that too many loose panels acting as membranes will take bass response away from the room.... Am I getting this right?? sorry for my confusion.... Maybe it's totally different in large performance spaces, but I know how larger recording rooms behave and we still strive to add sufficient LF trapping in these rooms to ensure the bass is tight and the decay is comparable to the rest of the spectrum... as you know it's easy to absorb too much High and Mid frequency and end up with a boomy sounding room.

Thoughts>????


What can happen in such a room, is that the decorative loose panels (which for most I suspect have the same overall size) are acting as resonators with a certain bandwidth.

Also, in these types of room, for the vast majority of the public, the ratio of (direct energy) / (reverberated energy) will be in clear favor of the reverberated field, which therefore implies the most energy perceived by the ear is from the reverberated field.

If those panels are such that they resonate and kill, say, the 100Hz / 150Hz band (because of differences of incidence), for those freq you are only left with direct energy, and none in the reverberated field. Which means that overall, this will be perceived by the public as a lack of bass, because it creates an imbalance in the reverberated field - which accounts for most of the perceived energy. Decay very is messed up freq wise.

Chances are these re-emit in the LMF band too, therefore increasing perceived energy with regards to bass, increasing the imbalance. (Those modern rooms I have visited have little to no MF treatment as well. Treating HF is no use, air does that job pretty well.)

EDIT: having a very live, but very balanced room reverb is important for those halls. They can't be deadened - or they sound dull and life less. This is an important difference with studios.
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Thomas Jouanjean
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Bruno Gouveia

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Re: membrane bass traps thickness
« Reply #38 on: February 09, 2009, 07:13:31 am »

Thomas Jouanjean wrote on Mon, 09 February 2009 08:10



Chances are these re-emit in the LMF band too, therefore increasing perceived energy with regards to bass, increasing the imbalance. (Those modern rooms I have visited have little to no MF treatment as well. Treating HF is no use, air does that job pretty well.)




Hey, this makes a lot of sense and can explain what's happening!! Very Happy

andrebrito

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Re: membrane bass traps thickness
« Reply #39 on: February 09, 2009, 07:21:11 am »

Yes that's what I'm talking about...air does the job if the room is large enough. But that Concert Hall is not that large. And even the MF treatment has to be carefully placed in it.

Old concert halls have lots of ornamentation in their architecture
and provide good acoustics by diffusing the sound in the room. But new concert halls don't have that and then we need to have artificial ways to provide the necessary diffusion to the room.

In fact for musical purposes, diffusion plays a much major role than absorption since the audience is indeed the major absorbent in the hall.

In Casa da Musica the chairs were designed to match human absorption so the sound does not sound that different when it is empty or at 50 % of its capacity, or people just in one side of the available seats
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Thomas Jouanjean

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Re: membrane bass traps thickness
« Reply #40 on: February 09, 2009, 11:52:53 am »

For the fun of it, a small video of a scattering in one of the "demo" rooms in ODEON...

Shows the whole shebang...

HERE
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Thomas Jouanjean
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franman

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Re: membrane bass traps thickness
« Reply #41 on: February 09, 2009, 08:20:43 pm »

Link no work??  Confused
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Thomas Jouanjean

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Re: membrane bass traps thickness
« Reply #42 on: February 10, 2009, 02:30:44 am »

Try again...
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Thomas Jouanjean
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andrebrito

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Re: membrane bass traps thickness
« Reply #43 on: February 10, 2009, 07:08:26 pm »

That is a nice picture !

I have worked with ODEON, CATT, EASE and other ray tracing image-hybrid software. ODEON is one of my favorite but $$%#$#@$@# expensive
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