A coincidence dip is when the glass vibrates or resonates at the same frequency as the source of the noise. Essentially it is a frequency range in which glass has a greater transparency to sound waves.
With float glass, the point at which the coincidence dip occurs varies slightly with its thickness, but generally occurs with high frequencies such as those associated with aircraft noise. It should only be used therefore where lower frequency noises, such as those associated with traffic, need to be reduced.
With laminated glass, the PVB interlayer allows greater sound reduction by reducing the coincidence dip making it better at reducing higher frequency noise such as that associated with voices and aircraft.
Standard double glazed units do not provide good noise control, requiring very wide air spaces, of up to 200mm, to be effective. A solution is using different thickness glasses as the two panes of the unit, 2 panes of laminated glass or a combination of both depending on the level of noise reduction required.