Acoustical changes of loudly spoken speech and their effects on speech recognition in hearing-impaired listeners

The level of speech is usually increased in conversations with unaided hearing-impaired listeners. However, the speaker may talk at conversational levels to aided hearing-impaired persons. In this case, the level of speech is electronically increased by the hearing aid. In the present study, the acoustical changes of loudly spoken speech and their effects on speech recognition were investigated in 20 patients with sensorineural hearing loss. Eight test words of the German Speech Intelligibility Test ('Freiburger Sprachtest') were recorded at original levels of 60 and 75 dB SPL by a male speaker. Both recordings were presented to hearing-impaired subjects at a playback level of 75 dB SPL. Thus, the level of the 60 dB SPL recording was increased electronically by 15 dB. For the 75 dB SPL recording, playback and recording levels were identical. The average whole-word score was 49% for the 60 dB SPL recording and 39% for the 75 dB SPL recording. This difference was statistically significant (0.002 less than P less than 0.005). The results of the speech recognition ability tests could be explained by the acoustical changes of the loudly spoken speech. In the 75 dB SPL recording, the levels of voiceless fricatives, nasals and plosives were significantly lower than in the 60 dB SPL recording. Spectrally, the fundamental frequency was increased and the configuration of the first formant was altered in the 75 dB SPL recording. The significance of the findings for clinical speech audiometry and hearing aid evaluation is discussed.