The effects of different frequency responses on sound quality judgments and speech intelligibility

Four speech programs and two music programs were reproduced using five different frequency responses: one flat and the others combinations of reductions at lower frequencies and/or increases at higher frequencies. Twelve hearing impaired (HI) and 8 normal hearing (NH) subjects listened monaurally to the reproductions at comfortable listening level and judged the sound quality on seven perceptual scales and a scale for total impression. Speech intelligibility was measured for phonetically balanced (PB) word lists and for sentences in noise. Significant differences among the reproductions appeared in practically all scales. The most preferred system was characterized by a flat response at lower frequencies and a 6 dB/octave increase thereafter. There were certain differences between the NH and HI listeners in the judgments of the other systems. The intelligibility of PB word lists did not differ among the systems, and the S/N threshold for the sentences in noise only distinguished the flat response as worse than all others for the HI listeners. There was little correspondence between intelligibility measures and sound quality measures. The latter provided more information and distinctions among systems.