4.1 This classification provides a single number rating for transmission loss or noise reduction data that have been measured or calculated. This rating is based on the difference between the overall A-weighted sound level of the sound spectrum given in Table 1 and the overall A-weighted sound level of the spectrum that results from arithmetically subtracting the transmission loss or noise reduction data from this spectrum. The spectrum shape is an average of three spectra from transportation sources (aircraft takeoff, road, and diesel locomotive passby). A study showed that this classification correlated well with the A-weighted and loudness reductions (see ISO 532) calculated for each of the individual spectra used in developing the rating for the one-third octave band range of 50 to 5000 Hz. The calculated numeric value of the rating is based on the sound transmission loss or noise reduction values for a particular specimen and depends only on that data and the shape of the reference source spectrum used in the calculation. The values shown in Table 1 have an arbitrary reference level. Single-number ratings should always be used with caution. Specimens having the same rating can result in different indoor spectra depending on the variation of their transmission loss with frequency. Also, if the actual spectrum of the outdoor sound is different from that assumed in Table 1, the overall A-weighted outdoor-indoor noise reduction may be different from the OINIC. The strong low-frequency content of the spectrum in Table 1 means that specimen achieving a high rating must have strong low-frequency transmission loss. Use of this classification with the spectrum in Table 1 in situations where the source does not have a spectrum similar to Table 1 could result in requirements for more low-frequency transmission loss than is necessary for the application. Examples where this can occur are stage 3 jet aircraft, high-speed freeways with sound dominated by tire noise, and train passbys with sound dominated by horns.6
4.2 This classification requires sound transmission loss (TL), apparent outdoor-indoor transmission loss (AOITL(θ)), or outdoor-indoor noise reduction (OINR(θ)) data in one-third octave bands from 80 to 4000 Hz. Due to accuracy limitations given in Test Method E90 and Guide E966, measurements below the 100 Hz one-third octave band are not usually reported. Studies have shown that data in the 80 Hz one-third octave band are necessary to obtain acceptable correlations for transportation sound sources. For the purposes of this classification, measurements in the 80 Hz one-third octave band are deemed to be of acceptable accuracy.
4.3 Users of this classification should recognize that low frequency measurements of sound transmission loss may be affected by the test specimen size or the specimen edge restraints, or both, particularly for small modular specimens such as doors or windows. Consequently, the outdoor-indoor transmission class (OITC) may also be affected by these factors, resulting in some uncertainty of the field performance of assemblies bearing a rating number using this classification, but to what extent is unknown.