Speech intelligibility criteria for PA system qualification in large aircraft

Abstract

The military aircraft test and evaluation community uses the Modified Rhyme Test (MRT) test for evaluating speech intelligibility of a communication system in an aircraft. An MRT test is conducted for the flight member/mission areas specified in the MIL/ANSI/ISO specifications. These standards specify that the MRT shall be conducted in a noise field that simulates the actual aircraft noise at the member’s position. Since the method is based on the perception of words by listeners, there are no limitations in respect of the characteristics of the sound system or those of the environment. Modern communication systems, however, incorporate several new electronic designs, e.g., automatic gain control, analog-to-digital (ADC) conversions which can introduce non-linearity in the system. This paper will present results of investigations conducted in a large military aircraft where such features in conjunction with high background noise levels and moving speaker were considered and evaluated. Investigations were focused on ‘‘relative’’ measurements between different system configurations. The MRT was found to be the most reliable method for final verification of PA systems if performed as specified in the ANSI S3.2-1989. Use of an alternative analytical method to measure speech intelligibility as a replacement for MRT will also be discussed.