Broadband panels distance from walls and ceiling

[tmix]

I have a "Lively" main room 32 feet by 14 feet with 9 foot ceilings.
The walls are double 5/8ths drywall - textured / painted as is the ceiling. The floor is stained and sealed concrete.

My initial plan was to hang mineral wool 2 inch thick (possibly 4 inch) 3pcf 2ft by 4 ft panels where the front of the ceiling absorbers are at the 8 foot height from the floor putting the backs of the mineral wool about 10 inches from the ceiling boundary.

The wall absorbers I had planned on having mounting rails on the wall so I could add / subtract / move panels to creat different reverb times, but my initial designs would put the backside of the 2 inch thk mineral wool about 2.5 to 3 inches off the wall.

Initially I had allways read that the more spacing behind the panels, the better (why... I cant remember..), lately I have seen several posts stating that optimally the spacing behind the mineral wool should be about the same as the mineral wool thickness to put the absorber in the most optimall null pressure area of the sound hitting the wall / ceiling boundary.

What works best?

Thanks for your input!

Tom

[jwl]

Hi Tom,

My understanding is that pulling a panel away from the wall will increase the amount of low end the panel absorbs. I don't know an equation to calculate specifics, but you can think of it this way:

A panel absorbs sound when sound passes through it, based on the velocity of the sound. If you put the panel right on the wall, then when the sound hits the wall, it will slow down, essentially stop right at the wall, before the reflection takes off again. In other words, you are putting the panel right at the spot where it slows down and stops, therefore reducing its effectiveness.

By pulling it off the wall, even by a few inches, you are ensuring that the velocity of the sound passing through is a bit higher because it won't have to slow down, stop, and then rebound from the wall.

Most people seem to agree that a 2" panel spaced 2" from the wall will work well, same with a 4" panel 4" from the wall, etc. Though I think as usual, more is better in terms of spacing, but you'll hit a point of diminishing returns pretty quickly.

That's my layman's understanding of it; perhaps one of the experts lurking here can chime in more....

[Ethan Winer]

Tom,

> lately I have seen several posts stating that optimally the spacing behind the mineral wool should be about the same as the mineral wool thickness <

Yes, and that originated in my Acoustics FAQ:

www.ethanwiner.com/acoustics.html

Find the section Optimizing the air gap in the Table of Contents for the explanation.

--Ethan

[tmix]

jwl / Nathan,
Thanks for ya'lls input. That is exactly what I was looking for. It makes sense now the differeng theories I've read, I was just assuming them to all be related to all frequencies when it really has to do with targeting specific ranges of frequencies.

Thanks again

[jwl]

Thanks for the reminder, Ethan. I recommend your FAQ as required reading for acoustics newbies. It covers all the bases to be sure. But over the past year or two, I've read so much I often forget what I pick up where.