Uncertainties in speech transmission index measurements

Abstract

A detailed study has been carried out into the uncertainties associated with Speech Transmission Index (STI) measurements. The uncertainties and measurement errors are shown to be either systematic or random in nature. Systematic errors were found to include limitations of the technique itself as well as uncertainties related to measurement hardware and software implementations. Systematic errors were found to be caused by a range of issues including measurement microphone properties, test signal generation and replay errors, talker loudspeaker directivity and frequency response variations, and measurement system algorithms. Some forms of digital signal processing are also shown to affect the measured result and were found to be highly dependent upon the nature of the test signal and its processing. Measurement uncertainties due to random errors were found to include out of band, high sound pressure level, low frequency modulations or overloading of the microphone preamplifier stages, the pseudo random nature of the STI test signal itself, and the sensitivity of some test signals, such as maximal length sequences, to short term environmental and acoustic changes. Changes and fluctuations in the background noise level during measurements were also found to be a significant cause of error and uncertainly.