Analyses of acoustic problems involve measuring the level and frequency of all the noise components and determining their modification by the glazing at all frequencies, in order to derive a residual transmitted spectrum. This may then be judged against notional targets or criteria. These procedures are time consuming and may involve expertise.

As expedient alternatives, single figure indices have been developed which give quick, approximate indications of the acoustic efficiency of materials, including glazing. They should be used with caution and should not be regarded as complete substitutes for full analyses.

In considering the definition of these indices it should be noted that the range of frequencies encompassed varies nationally. Typically the British Standard range covers 100-31 50 Hz (16 values) in third octaves and 125-4000 Hz (6 values) in full octaves; the American standards cover 125-4000 Hz in both third octave (16 values) and full octave (6 values) analyses.

MEAN SOUND INSULATION r
values of the measured sound insulation of the glazing. Because of the slight difference between the British Standard and American Standard frequency ranges (namely a shift upwards by a third octave in American data compared with British) there may be a small discrepancy in their comparison. Typically, the American Rm is I dB higher than the corresponding British Rm derived from the same data.

r INDEX (ISO 717,BS 5821)
To take account of the ear's response, this more complicated index has evolved. It involves plotting the sound insulation curve on a series of reference curves, whose shape approximates to that of the 'A weighting' or ear's response curve. By finding the reference curve which is closest to that under investigation, the Rw index is derived according to prescribed rules, the main one of which is that the total shortfall (where the actual sound insulation is less than the reference curve chosen) must not exceed 32 dB. The Rw index is characterized by the sound insulation of this matching reference curve at 500 Hz (Figure 22.9).

STC VALUE
The sound transmission class index originated in the USA and formed the basis of the Rw index calculation procedure. The main difference is in the frequency range considered, so that typically STC values are about 1 dB greater than Rw indices derived from the same data.

rtra INDEX
The Rm, Rw and STC indices are derived from the basic sound insulation performance of the glazing across the frequency bands. In order to obtain better correspondence with people's reaction to the transmitted noise, it is also necessary to take account of the dominant components of the incident noise. The RTRA index is calculated by adopting a spectrum shape of typical road traffic noise, and then modifying it, frequency by frequency, according to the sound insulation data of the glazing. The residual aggregate level, in dBA, is the RTRA index. Unlike the indices above, it may be used directly in simple calculations to yield approximate levels of interior noise in dBA:

It has also been found that simple correction factors may be applied to RTRA to derive realistic estimates of corresponding attenuation to railway and aircraft noises. This is currently under consideration as part of a European (CEN) standard.