Diotic loudness summation in normal and impaired hearing

Diotic loudness summation at 500 and 2000 Hz was measured in 10 normal-hearing and 10 cochlear-impaired listeners. Diotic stimuli were matched in loudness to monaural "standards" of 70, 80, and 90 dB SPL. Diotic loudness summation averaged about 9 dB at 500 Hz for both groups. At 2000 Hz, the hearing-impaired listeners showed reduced diotic loudness summation at the 70-and 80-dB levels, but showed normal diotic loudness summation (about 9 dB) at the 90-dB level. The results indicate that diotic loudness summation is normal in cochlear-impaired ears, provided that the stimuli are presented sufficiently above threshold.