Sentence intelligibility in noise for listeners with normal hearing and hearing impairment: Influence of measurement procedure and masking parameters

Speech intelligibility measurements strongly depend on several procedural parameters. In order to obtain comparable results from different test procedures, these parameters must be investigated as to which should be standardized and which could be set freely. This study investigates the influence of noise level. noise type, and presentation mode on speech reception thresholds (SRTs), and intelligibility function slopes in noise for normal-hearing and hearing-impaired subjects. The noise presentation level had no significant influence on either SRTs or slope values, provided that the presentation level exceeded hearing threshold. Two stationary, speech-shaped noises produced identical results. Speech-simulating fluctuating noise yielded about 14 dB lower SRTs for normal-hearing subjects and about 10 dB lower SRTs for 20% of the hearing-impaired subjects. Of the hearing-impaired subjects, 30% did not benefit from the modulations and showed similar SRTs as for stationary noise. Using continuous noise yielded lower SRTs compared to gated noise. However, the difference between the results in continuous and gated noise was not significant for the hearing-impaired subjects. A presentation level of 65 dB SPL (normal-hearing subjects) or 80 dB SPL (hearing-impaired subjects) and an interfering noise with a spectrum similar to the mean long-term average speech spectrum (LTASS) is suggested for comparable adaptive measurement procedures. A fluctuating, speech-shaped noise is recommended to differentiate between subjects.