Cloud Effects

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Postby J.F.Oros » Fri Apr 28, 2006 8:50 pm

Zaphod wrote:Flaviu,

i think that most of the rising energy are just nulls being
wiped out of the room.

That's most probably the true...

The 40 hz peak instead, well it gets well above the average
response.  :bang

Well, it doesn't quite looks so in the treated diagram ... the peaks seem pretty aligned (+/- 3dB)...  :|

Still, I think it must be an alignment thing on the graphics... and maybe that 40 Hz resonance could not be reduced so much in magnitude because of inefficiency of treatment so low in frequency, so it appears that is higher than the other modes...
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Postby J.F.Oros » Fri Apr 28, 2006 8:56 pm

David French wrote:I have updated the before and after graphs to reflect the true levels.  I have no idea why the levels weren't accurate, but they sure weren't.  The new graphs are with volumes, gains, and software offsets untouched.

Sorry, I just saw your post David. It looks OK now  :D
(I think my guts are beggining to learn from Eric's guts  :mrgreen:)

Flav, you're probably right about the filled vs. line graphs, but I just can't resist the prettiness!

Yeah, I like them colured more too  :D  But sometimes, for the noble cause of science, we have to endure the pain  :mrgreen:
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Postby bert stoltenborg » Fri Apr 28, 2006 9:04 pm

J.F.Oros wrote:
Zaphod wrote:Flaviu,

i think that most of the rising energy are just nulls being
wiped out of the room.

That's most probably the true...

The 40 hz peak instead, well it gets well above the average
response.  :bang

Well, it doesn't quite looks so in the treated diagram ... the peaks seem pretty aligned (+/- 3dB)...  :|

Still, I think it must be an alignment thing on the graphics... and maybe that 40 Hz resonance could not be reduced so much in magnitude because of inefficiency of treatment so low in frequency, so it appears that is higher than the other modes...


Does that seem reasonable to you?
:D
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Postby J.F.Oros » Fri Apr 28, 2006 9:12 pm

bert stoltenborg wrote:Does that seem reasonable to you?

If you look at the new pictures David posted, you can see all the other peaks being reduced more than the 46-47Hz one. This one is reduced too, but not so much, probably because there is not so good absorbtion at this low frequency in the room.
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Postby Ido » Sat Apr 29, 2006 1:14 pm

Dave, clouds are 4" think, with an airgap 4-8 ", overall 8-12" thickness?  (sorry man, still not 100% clear).

measuerment questions:

1. is it sort of agreed on that for a full overall view of before/after treatment (regardless of speaker/listener positioning), especially for the modal stuff,  it's good to measure as shown,  speak in one corner, mic in farthest one?

2. (* a question from earlier which didn't get a response, I'll try again here):
is there any worth to measuring before/after implementation of treatment with just a handheld SLM that does 1/3 octave sweeps (not RTA), or with a simple SPL meter with discrete frequencies?
(as opposed to a closed circuit TEF/MLS etc.)  even just for indication/ tendencies?
how would one do so?

Ido
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Postby Terry Montlick » Sat Apr 29, 2006 1:37 pm

David French wrote:Note how absorption shifts the center frequencies of the resonances.  BTW, if anyone has resources where I could further study this phenomenon, please let me know.

I'm a bit confused, because the addition of wall absorption should shift room modal frequencies higher rather than lower. As for a resource if you are really interested, the first (and best) theoretical analysis of this is by Morse from 1939 [Insert spooky time-travel theremin music here :)]:

Philip M. Morse, "Some Aspects of the Theory of Room Acoustics," J. Acous. Soc. Am. 11, 56 (1939).

This paper derives one analytical solution to the wave equation, the thing that determines all the room modes, their damping, etc. It's a little slow-going mathematically, though not as hairy as Morse's definitive and monumental tome "Theoretical Acoustics."

One of the things that Morse does is look at the simple 1-dimensional case of two parallel walls. One wall is a poor absorber, while the other wall's absorption is changed continuously (via varying its impedance, which is taken to be purely real). What happens is that the standing wave frequencies shift upward, increasing in an S-shaped curve to 50% higher at lowest wall impedance (maximum absorption). I once confirmed this behavior with my own Finite Element Method axial mode analysis.  Morse's analysis includes the 0th room mode (nominally "DC"), which always seems to get ignored in less rigorous room mode discussions.

Note that the extremes are exactly the same as the modes in a tube closed at both ends vs. a tube open at one end only. An open tube end, just like an open window, is a perfect absorber.

Regards,
Terry
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Postby Bob » Sat Apr 29, 2006 2:37 pm

Terry Montlick
I'm a bit confused, because the addition of wall absorption should shift room modal frequencies higher rather than lower.
Now I'm a bit confused.
I thought adding absorption always made the space acoustically larger (e.g. into a membrane resonator), which would lower the center frequency, and perhaps broaden the Q.
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Postby Terry Montlick » Sat Apr 29, 2006 3:16 pm

Yes Bob. If you add a significant volume of porous absorber within the room, you get proportionally lower resonance due to the 1.4 factor of isothermal  vs. adiabatic air compression. To get the full 1/SQRT(1.4) frequency lowering, you need to completely fill the volume with porous absorber . If this absorption volume is small compared to the room volume, this effect shouldn't be significant.

To the degree that the porous absorption is confined to the wall boundaries and is of small volume, then an increase in modal frequency with wall absorption should dominate. And I'm not considering a membrane resonator, which will NOT behave as a simple real impedance.

Regards,
Terry
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Postby David French » Sat Apr 29, 2006 5:58 pm

Ido,

Ido wrote:Dave, clouds are 4" think, with an airgap 4-8 ", overall 8-12" thickness?


Yes, perfect.

Terry, thanks for chiming in here, I really appreciate it.  All I know is everytime I add absorption the center frequencies of the resonances get lower.  I did notice that almost all the resonaces of the empty room were higher than predicted... check out the table below.  I compiled that for the modes whose identities were perfectly clear.  Here's a crazy idea... could we measure the absorption coefficients of walls (or whatever) for normal incidence by comparing measured vs. predicted modal frequencies? Seems to me that their should be a direct relationship.

On a side note, could someone teach me or direct me to a place where I could learn to calculate the pressure at a given point in a room at a given modal frequency.  I know the simplest stuff, like the zero points for all the axials, but say I wanted to experimentally prove that a given resonance was a tangential.  I would need to know exactly where in the room to go for a null, and right now I have no idea how to calculate that.
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Postby bert stoltenborg » Sat Apr 29, 2006 6:19 pm

Terry Montlick wrote:Yes Bob. If you add a significant volume of porous absorber within the room, you get proportionally lower resonance due to the 1.4 factor of isothermal  vs. adiabatic air compression. To get the full 1/SQRT(1.4) frequency lowering, you need to completely fill the volume with porous absorber . If this absorption volume is small compared to the room volume, this effect shouldn't be significant.

To the degree that the porous absorption is confined to the wall boundaries and is of small volume, then an increase in modal frequency with wall absorption should dominate. And I'm not considering a membrane resonator, which will NOT behave as a simple real impedance.

Regards,
Terry


Could you please give a calculated example?
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Postby Eric Desart » Sat Apr 29, 2006 6:37 pm

David,

Just the nuls now.
If you know where to find the axials (calculate them as a fraction of a room measure) then you know the tangentials and obliques too (for a rectangular room that is).

Look here:
http://www.isvr.soton.ac.uk/SPCG/Tutori ... -rooms.htm
Look at the first colored animation that's a 2;2;0 tangential mode.

Imagine this should be an axial 2;0;0 mode (you know that one) then the vertical colors on top of one another should form bands, where the 1/4 wave bands left and right are both in under or overpressure and the center 1/2 wave band the opposite of that (e.g. left and right blue and center red or vice versa)

By making it a tangential the same happens on the traverse axis as well as shown in the animation.

Hence a tangential 1;1;0 : the nul forms a cross through the room.
This can happen on the 3 axisses. but a tangential is always enclosed by 4 walls.

With an oblique mode again the same happens but you have an additional division on the Z axis as per the same principle.
Since here the oblique is already enclosed by all 6 boundaries, you don't have 3 planes as is with the axial and tangential.

If you understand it like this it's OK otherwise you must find animations.
For axial and tangential they are easy to find, for oblique not.

Eric

Was this clear, otherwise pictures/animations are needed?
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Postby Eric Desart » Sat Apr 29, 2006 6:48 pm

Just a non-related point.

:mrgreen:  If someone disagrees at RO with someone or something, it's really not necessary to get the whole site down, you just can tell.
The site is down for about 12 hours now. The people who were in a normal conversation are blocked too now.
http://www.recording.org/modules.php?na ... forum&f=34
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Postby David French » Sat Apr 29, 2006 6:57 pm

Crystal clear. Thank you yet again Eric for another pearl of wisdom!  If you only knew how much I appreciate having a guy like you around...

RO has been working fine for me all morning.
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Postby Eric Desart » Sat Apr 29, 2006 7:16 pm

David,

Maybe something interesting to notice.
While axials will always be related to 1/4 wavelengts, Tangentials and Obliques NEVER are in neither direction.

I have somewhere some math to do reverse calcs but simple now.
Tangential are related to the pythagorean theorem in 2D and the Oblique to the pythagorean theorem in 3D.

Hence, if everything follows nicely the theory ( :twisted: ), if you find a nul traverse at 1/4 wavelength from a wall it CAN NOT be a tangential or oblique mode (and vice versa of course).

This also tells you that the NET CLASSIC / AXIOM of nuls ALWAYS at 1/4 wavelength is nonsense. They can't be for tangentials and obliques.

Eric
Last edited by Eric Desart on Sun Apr 30, 2006 7:47 am, edited 3 times in total.
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Postby Terry Montlick » Sat Apr 29, 2006 7:19 pm

bert stoltenborg wrote:
Terry Montlick wrote:Yes Bob. If you add a significant volume of porous absorber within the room, you get proportionally lower resonance due to the 1.4 factor of isothermal  vs. adiabatic air compression. To get the full 1/SQRT(1.4) frequency lowering, you need to completely fill the volume with porous absorber . If this absorption volume is small compared to the room volume, this effect shouldn't be significant.

To the degree that the porous absorption is confined to the wall boundaries and is of small volume, then an increase in modal frequency with wall absorption should dominate. And I'm not considering a membrane resonator, which will NOT behave as a simple real impedance.

Regards,
Terry


Could you please give a calculated example?

Sure, Bert.

Here are the results of FEM calculations for resonance in a 19 foot long room. One wall was given low absorptiion coefficients and the other parallel wall varied in absorption. The wall surface points (using FEM brick elements) were assigned specific absorption coefficients. So there was no modeling of absorber depth.

For the near-zero absorption coefficients, you can see the 1st axial mode peak at around 30 Hz. The 2nd axial mode is double this, at around 59 Hz. Notice the rise toward 0 Hz -- this is the 0th mode.

When the wall is made fully absorptive, the 0th mode rises to around 15 Hz, halfway to the original 1st axial mode. The 1st axial mode rised to around 45 Hz (50% higher, as Morse predicted).  I have data taken for lots of intermediate absorption values, but I need to write this up for journal publication and cannot present it here as yet. If you look up the Morse 1939 paper I mentioned, the frequency shift is shown in figure 6.

Regards,
Terry
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Postby bert stoltenborg » Sat Apr 29, 2006 7:42 pm

Thanks Terry
:D
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Postby David French » Sat Apr 29, 2006 8:08 pm

Rad, Terry.  Rad.   :8
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Postby Scott R. Foster » Sun Apr 30, 2006 12:15 am

OMG!

You physics wienies are pretending rooms are built square and plumb- LMFAO!

Bwa -ha ha ha ha ha ha...

:D
SRF
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Postby bert stoltenborg » Sun Apr 30, 2006 12:44 am

Scott R. Foster wrote:OMG!

You physics wienies are pretending rooms are built square and plumb- LMFAO!

Bwa -ha ha ha ha ha ha...

:D


Scott isn't far from as stupid as he may seem.
:mrgreen:  :mrgreen:  :mrgreen:  :mrgreen:  :mrgreen:

I got me the rebel too.
I'm gonna kick ya're ass

 :mrgreen:  :mrgreen:
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