Speech Transmission Index (STI) is a well-established method to evaluate the performance of a transmission channel for speech communication. The STI assessment technique is regulated by the IEC 60268-16 standard and its usage is widely adopted both in standard bodies and in tender specifications. STI is a successful implementation of an evaluation method that, despite inherent limitations, correlates very well with perception. In engineering practice it is often necessary to compare STI evaluations with limit values. This also requires specifying boundary conditions of the evaluation. In practical applications where STI fails to reach the target values, specifically in cases where distributed sound systems are used, it has been found that there is a polarization of the effects affecting the STI value. This leads to a particular sensitivity of the STI to boundary conditions and in fact, makes it impractical to reach targets in several scenarios. In these cases the specification of the STI limit targets is not sufficient to design effective sound systems and should be supported by a detailed and clear specification of the boundary conditions such as, but not limited to, the background noise.
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