still having resonances in room / updated pics

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still having resonances in room / updated pics

Postby Nuuk » Sat Feb 19, 2005 11:08 pm

I have installed 3 of the 4 superchunks in my recording room. I also have an absorbtive ceiling and little chunks between ceiling and the walls. The floor is now laminate which has been glued to the existing floor (this was quite some not too nice work btw )
the acoustic has improved since i started to add absorbtion. The resonance i always had also has been dampened, but it is still there. it is pretty low, but i don't have a exact frequency at the moment. If i have someone playing my bass drum, which exites the resonance and walk around, I don't hear it in the middle of the width (red line in drawing)
i also tried to lean against the drywall walls to dampen them, put nothing improved here.
So, the question is, where could this resonance come from? i know hard to tell via internet. i also should check what frequency it definetely is. why am i not able to here this frequnecy in the middle of the length? could it be related to room modes?
maybe anyone has a hint for me, relating to my problem.
Attachments
reson.jpg
reson.jpg (16.14 KiB) Viewed 22873 times
Last edited by Nuuk on Sat Mar 12, 2005 2:04 pm, edited 1 time in total.
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Postby andrebrito » Sun Feb 20, 2005 12:20 am

One question  , you are saying the ressonance is not evident on that redline only correct ?  

That line corresponds to half of the  width of the room, 3.29 m   ->  the lowest axial mode corresponding to that distance is 52.2 Hz, so it might be that  low frequency you are dealing with perhaps ?

Also what is the height of your room ?
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Postby Howler » Sun Feb 20, 2005 9:21 am

Question: does this resonance occur at low volume? Mid volume? High volume? I'm worried about the styrofoam behind the walls, but this might simply be a mode. How thick is the plaster, and how is it compressed/attached against the styrofoam? From your drawing it seems you have no wall treatment going on. If this is so, you might be getting buildup from parallel walls and not notice it in the middle of the room.

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Postby Nuuk » Sun Feb 20, 2005 10:27 am

height is 2,17 m.. ok, now, it should be about 2,12 because of the absorbment.
the resonance is best heard at low volume level. sorry, i forgot to mention that. if i strike the bass drum very softly, i can hear the resonance pretty well. if i play it loud, it's much more difficult to hear.
I just hope this is not because of the styrofoam. the styrofoam is one, with a pretty high density - for thermal insulation. i think it's called styrodur, if somebody of you knows this product. Then there has been a grid layed on it, and the plaster was attached. i think as one would usually do it. it's probably about 1 cm thick. it just sticks to the styrofoam (3 or 4 cm), which sticks to the concrete wall.
I had a thread here, concerning this wall issues. but nobody told me, that there would be a problem. I hope this will not come out as one, because now i attached all the chunks to that wall. changing the walls will now be definetely a problem.
indeed there is no treatment on the walls, except the chunks in the corner. maybe a diffussor will now be the way to go??
the half of the length is definetely where you can't hear that resonance.

Well, and i really think i need to get into EFT. could i use it with any other mic like this one: http://www.rodemicrophones.com/specsnt5/nt5specs.htm  they have a nearly linear frequency response. or do i need to get that behringer mic? my dealer doesn't sell any behringer products anymore, so i will have to get it somewhere else.
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Postby Eric.Desart » Sun Feb 20, 2005 11:05 am

Nuuk It shows you need more absorption that reaches down to this resonance.

Andre is probably right that you're speaking about the 1st order y axis mode.
But as long as you don't provide yourself with some measurement posibilities, people can give you advice with the best of intent without avail.

What probably happens is what you see on those animations:
http://web.archive.org/web/200108092000 ... terOne.asp

You should try again and set your bass drum on the line you know don't hear it.
And then bring the bass drum to the corner and try again.

When reading between the lines I don't think that the styrofoam has anything to do with it. If the insulation is well fixed I expect any possible resonance to be higher in frequency.

But now that A LOT of modes (e.g. ALL odd y-modes axial modes, ALL [x,ODD,0] tangential modes, all [x,ODD,x] oblique modes have this nul on this center line.

If it's really 1 explicite 1st order y-axis axial mode one could think about a panel trap on the long walls.
But that one can calculate about but must tune it in practice. Then combined with this existing insulation (the gypsum cladded polystyrene) it will make a triple leaf system which will shift the theoretical resonance frequency of the added panel trap.

But if there are still other problems than altering this wall in a huge paneltrap can cause problems.

At a certain moment one can not talk a project together without one single measured data.

The safest thing is adding more corner absorption (horizontal corners). But then you need to make sure what frequency you're talking about. If it's lower than the standard chunks can handle then they must be enlarged.
What is the thickness of this ceiling absorption and the cavity behind it?  If it's only 5 cm than it probably doesn't do a thing on this mode.

Diffusers won't help you solve such a low frequent modal problem.
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Postby Nuuk » Sun Feb 20, 2005 11:46 am

Ceiling is a 3 cm thick absorbing material with 2 cm airgap behing. i discussed with scott that it won't absorb the low frequencies, but i don't have room height available for more absorbtion. so scott told me to add absorbtion in the wall - ceiling corners.
putting the bass drum in the corner (in front of the chunk) is quite interessting. the more the bass drum faces to the middle of the room, the louder the resonance is. If the bass drum faces the wall, i can't hear the resonance at all.
If i put the bass drum right in the middle of the room, it has nearly no low frequencies. except the resonanting frequency, that could be heard, due to a long decay. but not very loud.
i will record the frequency later on, so i will definetely be able to figure out what frequency it is.
will post some pics below, so you could see how everything looks at the moment.
Attachments
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reso3.jpg
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reso4.jpg
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reso1.jpg
reso1.jpg (29.01 KiB) Viewed 22769 times
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Postby Eric.Desart » Sun Feb 20, 2005 12:04 pm

Nuuk,

This is very nice work.
A thought:
This absorption in the ceiling/wall junction is very good ( :mrgreen: Scott always gives good advice ....)

But it should have been better if you used the same size as the corners in order to extend this to the lower frequencies.

What also can be good that beneath the absorption, on top of the drum you have diffusion (polies).
I think it will clear up clarity.

I think it isn't easy anymore but lowering the ceiling will make absorption more linear (even when you do it partially).
Something you can make part a cloud and use it to put indirect lightning on top.

In fact for a tracking or listening room, for me it feels a bit strange that one of the 6 surfaces is 100% absorbent over the complete surface (but that's personal)
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Postby Scott R. Foster » Sun Feb 20, 2005 1:33 pm

Eric:

Thanks for the kind words... but the truth is I always try to give simple advice, and sometimes it is good.  I am always open to valid correction, often needit , and try to remember the results. It's an iterative process.

If after all these years I am starting to get things correct [most of the time anyway] then the credit goes to the work horses of this forum.. folks like yourself, Jeff, Bert, Paul, Bob, etcetera who explain the therories behind the cause / effect processes of small room acoustics in depth , test hypothesis' in valid lab conditions and share results, document application / installation methods with real world information [pics or it didn't happen!], create better calculation tools, extensively catalog data and resources,  and other such "heavy lifting".   I have dabbled in these efforts but my results have always been outdone - frankly very little of my work in this regard stands up as there are now better tools and lists here at StudioTips for almost everything I have done in this vein - but I still take some pride in that.  I feel / hope that my limited efforts in these realms have encouraged others to do these jobs better, and share their improved results.  That's what makes StudioTips a great resource IMO, and for that I am proud of all of you StudioTips regulars... it's amazing what folks can do when they supress their egos, and all pull together on a rope.

Nuuk:

In this forum I claim nothing more than the role of gadfly and advocate of simplicity in search of the valid general rule... I am NOT an "expert".  Just an interested amatuer with the annoying habit of asking simple questions and trying to force the folks in the know to restrict themselves to simple answers that regular people can apply reliably [and cheaply if possible] to real world situations.  Once I think this group has arrived a general rule set I merely try to remember them, and report them succintly to the new folks like yourself.

FWIW I suspect your current situation is one of your absorption treatments not performing as low in the band as your circumctances would benefit from... therefore to the extent that you can thicken up any of your treatments I would expect you improve your situation.

Your installation looks great!  My only reaction to the pics is that those ceiling / wall triangles could probably stand to be a bit bigger.  Given you report good results down to the "bass drum" frequecies I suspect my gut reaction to your photo's is on track.

Could you cover your ceiling / wall wedges with an additional panel of mineral fiber to thicken em up?  Perhaps a 75mm / 100mm panel over the existing wedges in this location will get your treatment scheme deeper into the band and resolve your remaining problems [my idea here is to try and make up for the thin ceiling treatment that space considerations forced you to limit yourself to- by making things thicker on the edges].

Good Luck!
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Postby Nuuk » Sun Feb 20, 2005 1:51 pm

Thanks a lot for the comments. I'm curios how everything will look when it's painted and covered with nice looking cloth.
Adding to the wall/ceiling wedges feels like pain in the ass. At the moment i favor the thought of adding clouds to the ceiling, but i will consider both and look what space will allow. eric, what kind of indirect lighting do you think of for the clouds?

at the moment im just cleaning up. there's so my dust on mic stands etc. but will be able to do some recordings to check the frequency soon.
hmm, polies over the drums... well, i  just will have to get my low frequencies sorted and will check then where i definetely will record drums. then try out some polies.
the absorbtive ceiling also was scott's idea. it also felt strange to me at first. but i thought i could add some mdf or plywood to the ceiling in a ceckerboard pattern, if the absorbtive ceiling is too much, but it feels good at the moment. clouds could easily be attached to the frame, i think.
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Postby Nuuk » Sun Feb 20, 2005 3:32 pm

hmm, it seems my problem is related to a frequency around 43 Hz. but i will have to set up my pc to have a real liable analzyer. i will attach a mp3, so you can also hear what i'm talking about.  I had the Bass drum at around the place you could see on the previous pics and the mic near the longer wall in the middle of the length.
but how will i deal with a frequency, that is that low??  :roll:

edit: forgot attachment

http://home.arcor.de/thenuuk/frequ3.mp3
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Postby Nuuk » Tue Feb 22, 2005 2:29 pm

I did a little test on sunday. I layed a 16 cm thick rockwool layer against one of the superchunks. i could hear a reduction of the low resonance frequency, but it was still there. I will have to do this with at least 2 of the superchunks to have reasonable results.
I could add clouds about 10 cm thick without problems. probably two 1m x 2m ones. one in each half of the room.
Another idea i was thinking about is creating a square absorber along the length of the room, with size 90 cm height, and 40 cm deep. Since I wanted to have a surface to place things on (so, singers won't have to put their bevrages on the floor anymore), I could make about deeper and fill it with rockwool beneath. You can see how this will look in the pic above.
The question now is, which of the three things will have best effect on my problam? what ideas won't really help me? what do you think?
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Postby knightfly » Tue Feb 22, 2005 6:21 pm

I don't have the software on this computer to analyze your mp3, but are you sure the problem is really at that low frequency? I agree that, if it's inaudible at the centerline it seems to be axial mode related, but I also noticed that your modes have a pair of tangentials coincident with the shell resonance of a 22" bass drum AND another pair of tangentials at shell resonance of a 24" kick - ( 308 hZ and 282 hZ, respectively)

You also mentioned that turning the drum a different direction changed the result; so I'm wondering, have you tried placing more damping inside the shell to see if it's the DRUM that's resonant instead of the room?

Just a thought...  Steve
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Postby bert stoltenborg » Tue Feb 22, 2005 7:06 pm

Hey Steve,

I analyze (Cool Edit Pro) the resonance in the MP3 at 49 Hz.

Bert
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Postby Nuuk » Wed Feb 23, 2005 9:38 am

knightfly wrote:I don't have the software on this computer to analyze your mp3, but are you sure the problem is really at that low frequency? I agree that, if it's inaudible at the centerline it seems to be axial mode related, but I also noticed that your modes have a pair of tangentials coincident with the shell resonance of a 22" bass drum AND another pair of tangentials at shell resonance of a 24" kick - ( 308 hZ and 282 hZ, respectively)

You also mentioned that turning the drum a different direction changed the result; so I'm wondering, have you tried placing more damping inside the shell to see if it's the DRUM that's resonant instead of the room?

Just a thought...  Steve


Sure, if i dampen the drum, resonance of the room is also dampenend. Seem logical, because the room then is exicted less.
I also could tune the drum different, or use a other size. but i don't wanna change drums to fit in my recording room. just wanna make my room working. What happens to a bass guitar. "ähm, please don't play the E and the A in this room" Just joking  :mrgreen:  :mrgreen:
btw, i used a 22" Bass drum for this.
you could easily convert the mp3 to wav. There are a lot freeware tools out there. Any up to date audio editing or sequencing software should also be able to do that. maybe i just could upload a short wav sample.

Anyway. Really need advice on what treatment i should go for. I guess adding to the wall-ceiling wedges won't help me, because the frequency will be still too low for them. The same thing, could probably apply to the clouds - they just won't work low enough. Adding to the superchunks seems to be an option. But i'm not sure what my other idea will do (the one in the pic above). will it work low enough? is it complete nonsense?

The wav file:
http://home.arcor.de/thenuuk/freqshort.wav
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Postby Bob » Wed Feb 23, 2005 9:46 am

How about a bunch of tube traps?

49hz = 23.06'
1/4 wavelength = 5.76' = 5'  9.2"  
= 1.756 meters

Revenge of the Giant Toilet Paper Tubes

Eric's latest studiotips chat on the subject
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Postby bert stoltenborg » Wed Feb 23, 2005 11:03 am

Bob,

When using tubes you have to apply an end correction! This is really important, the bigger the diameter of the tube the larger the correction., so you won't get close to your target frequency.  :D
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Postby Bob » Wed Feb 23, 2005 11:10 am

bert stoltenborg

What's the formula for the end correction ?
Or, to put it another way, assuming my local Home Depot has 8" inside diameter tubes 6' long, please show me the math for how long I should cut them for 49 hz.

BTW, it seemed to me that making narrow band absorbers was easier with tubes than it is with membranes -- at least to hit a target frequency.
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Postby bert stoltenborg » Wed Feb 23, 2005 11:56 am

Bob,

end correction is .82*R for a normal tube. You can subtract this value from the total length needed. So a large diameter tube has to be shorter than a narrow tube.
When the tube ends in a flange your eq is .61*R. (Olson, Acoustical Engineering).

When using tubes you have to take care: I have seen several cases where a room resonance was treated with a tube resonator, but when you looked in a waterfall you saw after some time the resonance return, because the tube worked as an organ pipe returming almost more energy than it received :-) . And when you damped the pipe and the resonance disappeared the pipe didn't work anymore. This is similar to what Eric told here several times, about plate resonators if I'm not mistaken.  Every tuned device seems difficult to use!

Bert
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Postby Scott R. Foster » Wed Feb 23, 2005 12:42 pm

Bert:

Ya, but that's not a bug... its a "feature".
SRF
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Postby bert stoltenborg » Wed Feb 23, 2005 12:45 pm

:mrgreen:
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