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.

Knowledge of interfering noise is necessary for the correct design of a public address system. Noise sound levels can be obtained, for example, from BS 5389-1. It is more difficult to acquire knowledge of the interfering noise spectra, and the spectrum is also important for calculating speech intelligibility. As shown in the paper, for crowd noise, it is possible to determine the spectrum by pairing the level of noise to the speech spectrum for appropriate vocal effort. The error in determining the speech transmission index for public address systems for such selected noise spectra, relative to values for measurement-acquired noise spectra, is acceptable.