Natural Science Forum / Physics / Acoustics / April 2005

There are double glazing configurations (200mm air gap + absorption) and heavy
triple laminates up in this region. Limit is probably flanking details.

The real problem is not achieving the performance, but doing it without
over-design.  "An engineer is someone who can do for a penny what any fool
can do for twopence."*   From published data, you can get fairly near to
this performance with quite modest specifications:

(1) I have a test report for an office partition system with 10 mm and 8 mm
laminated glass with 60 mm separation, giving Rw = 47 dB. It doesn't say if
it's acoustic laminate but I guess it would be.  (2) Pilkington data for
secondary glazing gives Rw = 49 dB for 10 mm and 6 mm glass with 200 mm air
gap, not laminated glass, and I think this probably includes significant
flanking transmission.  With thicker acoustic laminated glass and large air
spaces, and sound absorbent material around the sides of the cavity, there
seems no reason why these figures should not be considerably exceeded.
Acoustic laminated glass is available up to about 17 mm thick.

I have certainly achieved much higher performance for broadcast and
recording studio walls incorporating large observation windows.  However I
have not found much data and have to resort to experience, informed
guesswork, and a bit of over-design.  Pilkington reckon there is decreasing
return on cavity width after about 150 mm but it depends how much low
frequency insulation you want.

I have achieved the low 50's (field test) in a studio with two separate
panes mounted in separate window frames on the two separate drywall
walls (Double Stud, double Drywalls). 1/4" glass panes suffice. That
arrangement had about a foot of air space. The frames are solidly
attached to the respective drywall walls but have no connection
whatsoever to each other. I call the gap between studs a "black hole",
as it serves to totally absorb any sound that comes its way. To satisfy
architects who want to cover the black hole with something, I suggest
using 1" foam absorber, which is available in colors and is transparent
to sound.

    Angelo Campanella