Objective validation of alternative speech test sources for person-to-person speech intelligibility assessments

A sound source that mimics the acoustics characteristics of a human talker is required in objective assessments of speech intelligibility in person-to-person speech communications (PP) scenarios. The few special speech test sources available that meet the strict specifications of the relevant standards are costly and can be inaccessible to practitioners. This can prevent investigations or can lead to non-special and more affordable loudspeakers to be used as alternative sources without knowledge of their suitability. This study evaluates the validity of non-special loudspeakers in PP assessments and supplements a previous preliminary examination by consolidating and expanding early findings. Speech Transmission Index Public Address (STIPA) assessments were conducted employing three representative non-special loudspeakers and one standardized reference speech test source under laboratory conditions. Absolute errors of STIPA measurements were analysed and evaluated against two just perceivable difference (JND) threshold levels. The mean absolute errors obtained for all non-special loudspeakers for almost all scenarios were within one JND of any of the two JND levels. The striking and consistent low level of error found validated the proposition that non-special and affordable loudspeakers may be used as suitable speech sources in pilot or survey-grade speech intelligibility assessments in place of a special standardized speech source when that source is not available, or in full investigations when the relevant standard to adhere to allows the use of those alternative speech sources.