The fundamental principle of the sound insulation of glass and windows is the Mass Law, which demonstrates that, with each doubling of glass thickness, the corresponding sound insulation is increased by about 4 dB. This is not exact and takes account of intrinsic resonance phenomena which impair the acoustic performance at certain frequencies.

Single glazing shows two main types of resonance. One is related to its size (low frequency) and the other to its thickness (medium to high frequency). This latter resonance is very sensitive, the frequency being inversely proportional to thickness, so that, for example, 12 mm float glass resonates at I kHz whereas 6 mm float glass resonates at 2 kHz.

One means of suppressing some of the characteristic resonances is to laminate two or more glass panes together with resilient plastic interlayers, which absorb some of the incident sound energy, reducing that which passes through it. The most common laminating material for this purpose is PolyVinyl Butyral (PVB), and it reduces the loss in sound insulation at the resonant frequencies.

Other materials, including polymethyl methacrylates, are even 'softer' and so are able to reduce further the losses of sound insulation at the basic frequencies and the component glass panes are decoupled so efficiently that the associated resonant frequencies are shifted to a higher position in the noise spectrum, where they are less troublesome.

Double glazing units or secondary sashes exhibit two additional resonances. One is caused by the interaction of vibrations of the two individual panes, which may be enhanced or suppressed by the precise distance between them. The other is produced because of inter-reflections of sound trapped between the two panes, and is a high frequency phenomenon. If dissimilar glass thicknesses are used in double glazed units, there are acoustic benefits, because as one pane tends to resonate, the other provides acoustic stability. High acoustic performances for double glazed units are achieved when lamination and asymmetric construction are employed simultaneously. The addition of more panes of glass to form glazed units with more than two panes may impair the corresponding acoustic performances owing to the generation of further resonances with each extra pane.

Other gases may be used as the cavity fill in place of dry air, for their thermal benefits. For example, argon filled units show no change in acoustic performance from standard air filled units, for the same basic construction. Sulphur hexafluoride (SF6) gas, however, may be used for acoustic purposes. Its heavy molecular structure tends to enhance the middle frequency performance (about 630--2000 Hz), but it introduces a resonance at low frequencies (around 200 Hz) which limits its effective application where low frequency insulation is the dominant requirement (transportation noises).

Potentially the highest acoustic insulation may be attained by separating the two main glass components with a large air space in excess of 100 mm, creating a double window. The interactions between the panes are minimized and each acts more independently as an effective barrier. However, unless all air gaps are fully sealed by employing either fixed lights or hinged windows which have compressible seals and multi-point locking to avoid the frames twisting, the actual in-service performance of these windows is likely to be no better than that of a sealed double glazed unit. For this reason, sliding sashes are not compatible with high acoustic performance.