The Design of Speech Communication Systems

A method is presented for calculating the ability of a communication system to transmit speech intelligibly in the presence of noise. The total speech arriving at the ear of a listener is determined by adding the orthotelephonic gain of the system to the speech spectrum which would be produced by a talker at the eardrum of a listener at a distance of 1 meter. The total noise arriving at the ear is determined in terms of its spectrum level from measurements of the noise pickup of the microphone and the acoustic attenuation of the earphone cushions. The area lying between the spectrum level of the peaks of the speech and the spectrum level of the total noise arrving at the eardrum when plotted on a distorted frequency scale determines a quantity called articulation index which can be correlated with articulation scores. Methods for determining the maximum gain permissible in the system are discussed. The validity of the method is established by comparison of calculated with carefully measured articulation scores.