Simulation of the effect of recruitment on loudness relationships in speech.

Persons suffering from perceptive deafness commonly find it difficult to understand amplified speech, and their understanding of speech is easily destroyed by competing speech or noise. One source of perceptive distortion is recruitment, which exaggerates loudness differences among the acoustical elements of speech. A transposition of these distorted loudness relationships from the deaf-subject span of hearing to the normal span is illustrated graphically, and achieved in practice with an electronic processor. A recording of processed speech, simulating for normal listeners the loudness relationships perceived by deaf subjects with recruitment, accompanies this paper. Recruitment-compensation processing for hearing aids is also simulated. The recruitment simulation is validated by an experiment with four unilaterally deaf subjects, who compared processed speech in the normal car with unprocessed speech in the impaired ear. The simulation suggests that (1) recruitment is a sufficient cause for loss of intelligibility in the deaf, whether or not there are other causes; (2) compensation for this recruitment is a necessary, although possibly insufficient, condition for restoring that intelligibility; (3) the benefit of using both compression and post-compression equalization in a hearing aid designed to compensate recruitment is likely to be considerably greater than the arithmetic sum of the separate, limited benefits of each process; and (4) the combined processing, by restoring redundant speech-recognition cues to the subjects's perception, can increase the resistance of intclligibility to acoustical interference.