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Open-back vs. closed back panels
Old 23rd October 2013
  #1
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andrew montreal's Avatar
Open-back vs. closed back panels

Hi there,

I am returning to dealing with the acoustic treatment of my main room where I track and mix. It measures in at 17' x 14.75' x 9.5'. I was wondering... if trying to deal with certain resonances in the room creating panels of a certain thickness (or corner traps of a certain thickness), does it make a difference if the back of these structures are closed or open? I'm not talking about early reflections... I'm really dealing with fine tuning the frequency response of the room.

Thanks.

Andrew
Old 23rd October 2013
  #2
Gear Guru
Open

If the panels are mounted flush on a wall or ceiling it makes no difference to have the back open or sealed, because obviously it is sealed anyway.
An airgap can increase the LF absorption of panels, if the back is open. If it is sealed, then there is no airgap.

DD
Old 23rd October 2013
  #3
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boggy's Avatar
Quote:
Originally Posted by andrew montreal View Post
........ It measures in at 17' x 14.75' x 9.5'. I was wondering... if trying to deal with certain resonances in the room creating panels of a certain thickness (or corner traps of a certain thickness), does it make a difference if the back of these structures are closed or open? .......
Yes, there is a difference and I prefer "open back" absorbers (they are easier to model)



Old 23rd October 2013
  #4
Here for the gear
 

I have a question about this too. I recently decided to make myself some open-backed 2 ft. x 4 ft. x 4 in. panels to experiment and learn with. I made six of them, all full of rockwool 80, and made a PVC pipe frame, 4 ft. x 4 ft. by 6.5 ft. tall to mount them on -- so I could try making a booth to tame the low end of my voice just a tad.

Here's a picture of it, partially assembled, to give you an idea.

I wasn't trying to make it perfect -- just wanted something to play with so I could record voice overs without too too much boxiness. But the recording inside the structure (2 panels on top and 4 around the sides) creates a couple pretty decent spikes between 100 - 200 hz. I had to sculpt out at least 5db around 120hz to cope with the worst offender.

The room is horrible of course and this was never going to make it great, but I'm wondering if the open-backness of the panels is contributing to the problem because: I stretched the fabric really tightly around the insulation (burlap on one side and polyester on the other), not to mention adhesive spray between the stacked 2 in. panels. If I rap the panels a bit, they actually sound out a note! It's a complex, indistinct, muffled note of course, but it's there. Today I took down a panel to tap on it and try to imitate the pitch of the note with my voice. I put the panel down, kept singing the note, opened up my sine sweep test files -- and wouldn't you know it, that worst offending spike in the recorded file happened right as the sweep passed the note I was singing.

Are my firm, rock wool 80, open-backed panels singing to me? Does that ever happen? Or is it probably just the poor geometry of my booth and room?
Old 23rd October 2013
  #5
Gear Guru
NoteWorthy

Interesting question. I have often wondered why fibre treated rooms sound so clear and even harsh. The direct sound that is, speaker to ear, or voice to mic as in your booth. I suspected a resonant absorption in the 'warmth' region of say 100-200Hz. I too tried the rap test and can easily determine a note. I even measured it a few times using my Tunebot drum tuner. I have concluded for now, that these are indeed a sort of damped resonant system. However, they seem to assume different resonances depending where they are. e.g. up near an 8 foot ceiling they become 73Hz, while in corners they settle on a lower figure. Bafflingly, I have found that real drums do this too. i.e. Change pitch (and the relationship between fundamental and harmonic) depending on position in a room.

I have found that the wires suspending traps resonated very strongly, so I damp them with cloth or BluTac or such. Make sure your PVC arrangement is not ringing. Maybe fill the pipes with fibre.
DD
Old 24th October 2013
  #6
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boggy's Avatar
Quote:
Originally Posted by DanDan View Post
.......Maybe fill the pipes with fibre.
DD
Better with dry sand.
Old 24th October 2013
  #7
Gear Guru
 
Glenn Kuras's Avatar
There is different spring effect you can get from a panel depending on all kinds of things. Trying to build one though and gauge this is pretty hard.
Old 24th October 2013
  #8
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HermanV's Avatar
 

Open back all the way.

Even for fine tuning I have had great luck and results this way. I have designed limp membrane absorbers and tested sealed enclosure vs. open back and have found that it is more cost effective to "tune" rooms this way.
Old 15th November 2013
  #9
Absorption Coefficients - and Standoffs

Quote:
Originally Posted by andrew montreal View Post
Hi there,

I am returning to dealing with the acoustic treatment of my main room where I track and mix. It measures in at 17' x 14.75' x 9.5'. I was wondering... if trying to deal with certain resonances in the room creating panels of a certain thickness (or corner traps of a certain thickness), does it make a difference if the back of these structures are closed or open? I'm not talking about early reflections... I'm really dealing with fine tuning the frequency response of the room.

Thanks.

Andrew
Here's a link that I posted in a small recording studio forum elsewhere on the web - it's a pretty comprehensive list of absorption versus frequency; tests were conducted in accordance with ASTM C-423 (Random Incidence Absoprtion). What's particularly interesting is that some of the tables also show a given material's absorption coefficients for the material laid flat on the boundary versus the same material standing off of the boundary - compare the values and you'll see, especially at the low end.

Of course, as DanDan pointed out, the back of the panel has to be open if you're intending upon standing the panels off from the boundary / boundaries.

Also, if you plan on covering them with fabric, there's another ASTM test that can quantify the resistance to airflow (ASTM C-522), however, a reasonable rule of thumb is this: if yiou have a material in mind to use to cover the panels, a simple, and pretty reliable test is to bring the fabric to your lips and try to blow air through it. If the air easily passes through, then it's a good candidate to cover your absorptive material.

This is a REALLY good place to start if you're looking to optimize the absorption of a given space, and you have some data about the room in question:

http://www.bobgolds.com/AbsorptionCoefficients.htm#!

I don't know how many manufacturers list the absorption results for the materials that they sell, but C-423 is a pretty well-defined and controlled test, and many of the materials you're likely to use when 'rolling your own' panels can be found here. Seriously, this is a treasure trove of useful data.

Hope this helps...
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