Date: Sun Feb 11, 2001 3:36 pm
Subject: Re: [acoustics] Diaphragmatic Bass Absorbers
In a message dated 2/10/01 9:01:18 PM Eastern Standard Time,
> 1. Does the resonant panel needs to be shut in a air tight box for it to
> work properly?
No ... standard wall construction sucks up sound ... 5/8ths drywall hung on
studs sucks up tons of lo-mid ... no holes required .... sheets of 3/8ths
plywood hung in the middle of the room will resonate and turn sound energy
into heat .... sealed boxes on the wall are a just means of tuning the range
of effectiveness. Think of these designs as giant drums tuned to a problem
For instance peg board can be used to create a virtual array of Helmholtz
bottles and by adjusting the cavity depth and perforation percentage can be
tuned for a broad range of frequencies (an overstuffed couch with three fat
ladies siting on it does the same thing 'cept they can double as back up
Some of the designs I´ve seen only mention putting them on corners or spacing
> them from the wall x inches and nothing else.
Yes .. this is very common .... an effective old school approach to a boomy
room is a polycylindrical diffusers in every corner (a poly is a sheet of
plywood bent into a curve by mounting it in a frame by the sides) ...the
curve of the plywood reflects sound from the corner in a spread of directions
(diffusion) ... if you leave the top and bottom edges free floating it will
absorb a good deal of LF in addition to bouncing sound in a wide pattern off
the curved front.
> 2. Assuming I'm using 3/8'' plywood, will it make any difference if I paint
> the surface or is it better to use a special wood treatment product?
No ... well theoretically it would have an effect (by changing the mass of
the structure), but not enough to tell, imagine the blindfold test:
"Now listen carefully ... is the vibrating plywood sheet behind your head
painted with latex or oil based paint"?
> 3. What's the relationship between the thickness of the fiberglass and the
> amount of absortion that the panel can provide? The Master Handbook of
> Acoustics by Alton F. Everest mentions 1'' to 1 1/2'' fiberglass behind the
> panel but will I get better results using thicker fiberglass? And what
> about its spacing from the panel?
Absorber boxes with fiberglass inside have a broader Q ... they have a fatter
bell curve to the graph of the range of frequencies they absorb. Stiff
walled, non stuffed enclosures have a narrow Q (blow across a coke bottle ...
as Everest describes in the Master Handbook ... a very narrow Q). The
stuffing also damps the internal resonances and thereofre the potential to
feedback energy into the room over time (tuned enclosures can lower the rooms
energy content of a given freq., but can have the unwanted side effect of
lengthening the reverb time for that freq. as they leak back into the room)
Scott R. Foster