Acoustic Properties of Glass
The sound attenuation of any material depends on its mass, stiffness and damping characteristics. With a single glass pane the only effective way to increase its performance is to increase the thickness, because stiffness and damping cannot be changed. The sound transmission loss for a single glass pane, measured over a range of frequencies, varies depending on glass thickness.
Thicker glass tends to provide greater sound reduction even though it may actually transmit more sound at specific frequencies. Every glass pane thickness has a weak frequency value; that is, a frequency for which that glass is less ‘noise absorbent’ than for the others. That value is known as critical frequency. See the graphic below :
Sound reduction (in dB) measured at different frequency bands for glass panes of different thickness. Source: Detailing for Acoustics, Lord and Templeton.
A 4 mm-thick glass is rather transparent (poor attenuation measured in dB) for high frequencies at the range of 3500 Hz; 6 mm-thick glass is poor for frequencies around 2000 Hz; and 10 mm-thick glass performs bad at 1300 Hz. The higher the mass the less of a problem critical frequency appears to be: 25 mm-thick glass has no weak point as it can be noted from the graph above.
An insulating glass unit built with two panes of the same thickness experiences the issue of critical frequency: it is said that the two panes vibrate (resonate) together at that frequency, thus reducing the glass overall acoustic performance.
For this reason we recommend using different thickness in a double glass unit. A 6-12-4 mm glass will absorb more sound at high frequencies of 2000 Hz (claxon noise) than a 6-12-6 mm glass, in spite of having less mass. On the other hand, at lower frequencies between 125 and 250 Hz (traffic noise) this is not the case: a 6-12-6 mm glass reduces sound more effectively than a 6-12-4 mm glass. At low frequencies sound attenuation is directly proportional to mass.
Laminated vs. monolithic glass
A laminated glass will attenuate sound transmission more than a monolithic glass of the same mass. See the graph below :
Sound reduction of monolithic (solid) glass compared to laminated glass with the same mass. Source : Detailing for Acoustics, Lord and Templeton.
A laminated glass of 2+2 mm reduces sound at high frequencies considerably more than a monolithic glass 4 mm-thick (that’s 8 to 10 dB of additional attenuation). Why? because the critical frequency effect disappears due to the sound damping provided by polyvinyl butyral (the soft interlayer used to permanently bond the glass panes together dissipates energy by vibration). The same applies to the 3+3 mm laminated against the monolithic 6 mm. In contrast, at low frequencies (traffic noise) the effect of butyral is less pronounced, although it is still positive (about 2 dB increase).
Air cavity effect ( Acoustic Properties of Glass )
Surprise : A standard double glazed unit does not reduce sound transmission much more than a monolithic glass. What matters is the thickness of the air space between glass panes, but only for really wide cavities.
Effect of air space width on the acoustic performance of double glazing. Source: Detailing for Acoustics, Lord and Templeton.
The acoustic attenuation of a 6-12-6 mm glass is generally superior to that of a monolithic 6mm-thick glass, but only by 2 or 3 dB, and still there may be low frequency bands where the DGU performs worse. Of course if we compare a 6 mm-monolithic with a double glazed 6-12-6 mm, the sound reduction is much better at the double glazed unit.
What really matters is the width of the air space, not the small one found at double glazing but the one of a double skin. The ideal cavity width to boost sound attenuation is 200 mm. For widths less than (or greater than) 200 mm the effect is less noticeable (although a wide air space will always perform better than a narrow one). A double glazing with 10 mm air space performs almost like a 20 mm airspace.
The three hand-sketched graphs shown here above are all taken from the first edition of a great book called “Detailing for acoustics“, written by Peter Lord and Duncan Templeton. There are three editions by now and I highly recommend buying one if you are an architect interested in acoustic issues applied to buildings.