Don Pearce wrote:
Angelo Campanella wrote:
Above 69 Hz, the sound attenuation (insertion loss) increases rapidly
with frequency.
There are various theoretical "estimates" of such attenuation, but
the only way to really determine it to build one and try it. My
experience for such a window (I have specified and tested one such for
an audio control room window at Ball State University
Telecommnications learning center in late 1980's. I tested it as about
FSTC 54.
Angelo, have you ever tried this with the glass panes forced into a
slight curve by the frame? Stiffness against an incident acoustic wave
is increased considerably.
That is an interesting hypothesis. That stiffness will interact with
two phenomena:
1- The higher bending stiffness in one direction will raise the
coincidence frequency above the 2500 Hz resonance "achilles heel" of
window glass. The other direction will still demonstrate coincidence
transmission around 2500 Hz, I think.
2- The panel resonance of 69 Hz may or not be affected because that
resonance is by the panel alone. It is more significant that stiffness
offered by the frame will interact with a flat pane. More clamping
stiffness will *raise* the resonance frequency above 69 Hz (as will also
the curved panel surface), placing it again back in the middle audio
range, which we do NOT want to do. It is best that the glass pane be
held by limp (soft rubber) supports at its edge and be flat, I think.
Again, this is all speculation. One has to build it and try it.
Similarly, I do not use tilted glass panes for audio control windows
despite their "sexy" appearance for two reasons.
1- The effectiveness in noise isolation strongly depends on the minimum
spacing rather than the maximum spacing in that wedged space. Hence, if
one wants maximum sound attenuation, one spaces the glass surfaces as
far apart as possible.
[The absolute best sound attenuation will occur for an interior window
when you simply cut a hole of any size or shape clear through that wall.
Leave the stud cavity OPEN to the window gap. Set fiberglass in the stud
gap within the wall. Spray paint it black if you don't like to see pink
or yellow there. (The one and only application of Sonex I have ever
recommended has been as an ersatz black cover for this gap. It is in the
studio windows at Ball State Communications Complex.) DON'T cover that
gap with a flat plate of any sort, as this will trap sound in the window
gap and deteriorate the sound attenuation. (Ugly but good follows
Cut
two sheets of plate glass oversize to cover the window hole plus a 2" to
4" margin all around. Either RTV the glass to the wall all around, or
build a picture frame of wood to secure it if you don't like that much
ugly. Caulk the entire glass edge perimeter before laying that frame.
Treat the other side of the wall exactly the same way. This window will
have HIGHER sound attenuation than the surrounding wall.]
2- Titled panes always produce new reflections of ceiling and perimeter
lights and lit objects than is normally experienced. This can be very
annoying for everyday use.
Cheers,
Angelo Campanella