A review of the MTF concept in room acoustics and its use for estimating speech intelligibility in auditoria

Abstract

The concept of the Modulation Transfer Function (MTF) can successfully be applied to evaluate the quality of speech transmission from a talker to a listener in an auditorium. Typically, depending on the auditorium acoustics, the intensity modulations contained in the original sound are to some extent reduced when measured at a listener’s location, especially for higher modulation frequencies. The implementation of such an acoustical MTF analysis with a sinusoidally modulated test signal is described in detail. The performance of a sound transmission system as revealed by the MTF can be expressed in one single index (the Speech Transmission Index, STI), which relates well to the performance as determined by intelligibility tests with talkers and listeners. A review is given of a series of studies on various aspects of the chain of relations between auditorium acoustics, MTF, STI, and speech intelligibility, illustrating the use of this approach for estimating speech intelligibility, either from MTF calculations at the design stage of an auditorium or from MTF measurements in actual situations.