About glass and glazing
Glazing is usually seen as problematic when it comes to acoustics, as if its optical
qualities have an effect on incident sound.
Acoustic insulation, that is the reduction of sound transmitted through glass, has little relation to the amount of sound reflected from glazing. Glass does not provide any of the resistive dissipation needed to absorb sound so it's an ill-founded myth that glass sounds
'bright'. It can't since it's only as reflective as, say, painted (sealed) plaster on brick/block, or concrete.
Hence separate treatment with drapes and or curtains really will not do anything beyond adding more selectively sound-absorbant surface the room as a whole may or may not require.
That said, large areas of glass can affect apparent bass response in difficult-to-predict ways.
Glass, though stiff, is flexible so it acts to some degree as a diaphragm absorber below the
resonant frequency of a given pane; a thinner (brighter?) sound can be the result. The effect would
depend on thickness and type(s) of glass, number of leaves, leaf separation(s), what's in the
cavity (if anything), shape of pane, mounting/installation type... in fact it's so difficult to
model we are not aware of anyone who has bothered!
To reiterate what's said on the soundproofing page:
Glass has intrinsically high density, but is
available only in relatively thin sections.
Glass has low internal damping, so a glass panel has a
fairly tightly-defined resonant signature depending on its dimensions, thickness and mounting.
Double glazing is better than a single thick pane, but far from perfect. The transmission of sound
through double glazing is driven by the coincidence effect (acoustic coupling of the leaves through
the intervening gas) and so isolation provided can be quite poor at certain frequencies.
In critical applications it's usually the case that you end up with two or more leaves of very different
thickness, spaced 100mm apart or so; in extremis the cavity is filled with sodium hexaflouride - a
dense gas - to add damping and reduce coincidence coupling.
For domestic use double glazed units are about as good as it gets,
offering 23-34dB of isolation typically. For preference, the two panes should be of differing
thicknesses and fairly widely spaced; 13-24mm is typical in commercial units and less than ideal.
However this is forced by thermal considerations; wide cavities suffer from internal convection currents increasing
heat loss.
If noise transmission through glass is a problem you could add an extra pane in a separate frame, and add sound absorbing material around the edge of the new cavity.
Laminated glass is a good option for the inner pane of windows where noise ingress is a problem, since it is very dead - the PVB interlayer damps vibration very well. It is manufactured to finished size and cannot be cut subsequently.
Be aware that laminated and thick glass gets very expensive, very quickly - and is difficult to handle.
© the twisted pair 2000
15.12.01 page added.