Noise spectra for calculation the speech transmission index in public address systems

To precision calculate the speech transmission index (STI), it is necessary to know the level and spectrum of the interfering noise. Simplified design methods used in public address systems are limited to obtaining an appropriate signal-to-noise ratio based on A weighted sound levels. For this reason, among others, guidelines for designers usually contain information on typical interfering noise levels and do not provide information on the noise spectrum. As shown in the paper, the effect of the noise spectrum on STI can be very high, and when such spectrum cannot be obtained by measurement or computation, standardised spectra appropriate for a given type of noise can be used for STI calculations.