Loudness summation across frequency under masking and in sensorineural hearing loss.

The loudness summation across frequency was measured in unmasked and masked normally hearing subects, using noise bands centered at 1 kHz. Homolateral masking reduced the loudness summation of supracritical noise bands to a degree roughly proportional to the degree of threshold shift produced by the masking. Between thresholds and moderate sensation levels, the loudness summation increased at an invariable rate in the unmasked and the masked condition, reflected in an invariable supracritical growth of loudness with increasing stimulus bandwidth at identical sensation levels. At high intensities, the loudness summation decreased to roughly similar values in both conditions. Similar results were obtained by pooling the data from patients with sensorineural hearing losses and comparable threshold elevation. The critical bands appeared to be identical in unmasked and masked normally hearing subjects and in patients with sensorineural hearing losses. Contralateral, 35 dB effective masking produced a slight but consistent central masking effect; compared to the unmasked condition, the loudness summation of supracritical noise bands was reduced between threshold and medium stimulus levels, while it was increased at high levels.