To represent a sound comprehensively, we use a diagram called a sound spectrum, which expresses the level of pressure (or sound insulation) depending on the frequency. The sound insulation spectra provide full details of the acoustic performance of a building envelope system.
The weighted sound reduction index Rw, a single value to simplify the formulation of minimum requirements, is used to assess the degree of sound insulation represented by building components. The calculation, carried out according to ISO 717, uses a standard reference curve to enable an assessment relevant to the human ear. Taking into account the human perception of loudness, Rw is read off the shifted reference curve at a frequency of 500 Hz.
Assessing the sound insulation effect solely according to Rw is suitable for solid building components but not for glazing. For double-glazing, in particular, the level of sound insulation drops in the frequency range due to the cavity between the panes, resulting in the possibility of two glazing systems within identical Rw values potentially insulating against noise very differently in certain frequencies. To take this into account, spectrum adaptation terms are used (C and Ctr) to determine the true level of sound in glazing.
It is important to note that the sound reduction index values measured in this way are equivalent to laboratory measurements and are generally more favourable than those obtained in situ for the same noise source. In practice, the sound reduction is lower in situ.
However, the single number of quantities mean that glazing can be classified depending on the noise source. In other words, if one type of glazing has a better quantity than another, it will also perform better in situ when exposed to the same noise source.