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[OOF!]

thanks for the great response Jay.
 The good news is after building a large trap along the rear wall with 6" of 703 and a 4" gap, and filling in more of the corner/ceiling areas with 703 i've got a huge improvement in the bass response of the room.  from 100 to 300Hz the improvement was ridiculous.  I've left a 2' x 4' sealed box in the center of my 4' x 6' rear trap for a membrane or possible perforated trap to try to fix the null at 80Hz.  I have the feeling, from the calculations i've done that this dimension will not do anything for me at 80Hz since i only have 10' depth to work with.  I know if i increase the thickness of the front panel i can get closer, but what is the maximum thickness before the panel doesn't vibrate enough and becomes ineffectual?  this i haven't found in books or online.
If i can't solve the 80Hz null, I'll fill the center box with 703 and be done with it.  the room sounds so much better than before.
It was only since i switched my 1031s for S3As that i noticed the room problems.  the genelecs had so much extra on the bottom that it masked the problem.
btw the room is 16.25 feet long and the listening position is 38% from the front wall.

thanks again, all, for the advice.
David

[franman]

Some of these issues, when modally caused, will be very hard to see a difference in the measurements. I can tell you from experience.. The broadband stuff and mild dips and peaks will round off nicely, but sharp SBI or modal dips are so much routed in decay time, that you WILL hear the difference way before you can measure it with a frequency based display... The mode is still ringing but no where near as long.. The notch is still tight, but the "sloppiness" that is caused by the modal ring (or SBI issue) has been significantly damped so it 'sounds' much tighter in the middle of the room.. make sense??

[OOF!]

yes, i totally dig what you're saying.  And I'll say that all of you who commented where completely right when you stressed trapping the corners as a priority.  I already had 4 of Ethan's minitraps in each corner, but apparantly, in my room, that was enough corner-fill.  So the last thing i diod, which made a huge improvemewnt, was to fill in all the space behind the minitrap with 703, fill in the 4 inch space i'd left at the top where the corner meets the ceiling, and in 2 corners fill from floor to ceiling.  this combined with the 10" trap on the rear wall (6" of 703 and 4" air gap. with foil facing on the entire front for brightness) has made a incredible improvement.  every artist that's come in in the last week has been blown away by the sound of the room and the sound of the speakers in the room.  
So thank you for the education and for pointing me in the right direction for more information.  
david lawrence

[Greg Reierson]

I just installed PMC MB1s and have had to wrestle (both mentally and physically....) with placement. Now that I have found what I think is the best overall location, I still have a dip in the 35-45Hz range that I'd like to knock out.

I have enough space to build a large panel trap or Helmholtz resonator either above or under my rear diffusor.

Looking at this calculator,

http://www.mhsoft.nl/Helmholtzabsorber.asp

a resonator that is 80cm high by 250cm wide by 13cm deep has a resonant frequency of about 40Hz with a Q of about 3. My question is how efficient something like this would be? Or would I be better off building a number of smaller traps?

Also, how does the presence of such a large acoustic obstacle influence the modes of the room?


Any advice?

Thanks,


GR

[fader8]

OOF! wrote on Thu, 22 March 2007 10:41

i've found the info for calculating the depth etc.  but no one mentions the dimensions of the box for either a Helmholtz or membrane resonator.  what about length and width of the box?

Pursuant to jfrigo's excellent response on Helmholtz resonators, a sealed membranic, (or diaphragmatic, flexural, etc.) absorbers operation is dependent on the thicknes-density of the membrane material itself. The actual width and height is only relevant in that the more surface area you expose to the energy in the room, the more it will absorb that frequency to which it is tuned.

The danger of going too big is that this introduces a large "reflective" element at higher frequencies which could then cause other unforeseen problems. So be careful with the placement.