Slat Absorber Question - John?

[knightfly]

Hey John, you mentioned over on the RO thread I invited you to, that slot absorbers don't work much below 100 hZ?

Can you explain why, if the cavity behind the absorber is deep enough, and would act as a bass trap going lower than that, why that wouldn't also be true of a slot absorber?

Also, when calculating modes for the booth in question, would you use the dimension to the FACE of the absorber, or the dimension of the ENTIRE space? I have yet to discover the answer to this one in print...

Sorry to bother you about something that's probably in one of my acoustics books, but they're at work and I'm NOT (damn, I may just have to "bite the bullet" and buy a second copy of some of those books)

Thanks for your help on the other board, as well as for any insight you may have on this question... Steve

[John Sayers]

Oh - they will work under 100hz it's just that their size is a limiting factor. I designed some low frequency slats for Guruland's drum room where the slots were made from 6" x 2" with the 6" being the slot depth and the 2" being the slat width. With a 2'6" depth and a fine gap you can get down to 30hz. So what I achieved was a broadband low frequency absorber with a upper frequency diffuser.

cheers
JOhn

[knightfly]

Thanks John, you had me worried there for a bit -

Now, about the other half of my question - "Also, when calculating modes for the booth in question, would you use the dimension to the FACE of the absorber, or the dimension of the ENTIRE space? I have yet to discover the answer to this one in print..." - Which is the correct way? Thanks again... Steve

[John Sayers]

to the face of the absorber as it's the new reflective surface.

cheers
john

[knightfly]

Thanks again John, I was finally able to peacefully coexist with my acoustics books for a few minutes last nite and reached the same conclusion.

This is getting scary - this is at least the second time I actually learned something about acoustics WITHOUT raising at least six OTHER questions I didn't know I had... Steve