Initially two experiments were performed on eight subjects to determine the effect of semantic content or meaning of a competing speech message on the identification of synthetic sentence material. The competing message was reversed and reproduced in a backward mode in an attempt to eliminate its semantic content or meaning. No important differences were found between the performance-intensity functions in the forward or backward mode. The results of 2 additional experiments with 12 subjects demonstrated the presence of a plateau or notch in the slope of the performance-intensity functions obtained in English or foreign language competing messages. This plateau effect occurred primarily when three experimental conditions were present: (1) the same speaker delivered the primary and competing messages, (2) the relative intensity levels of both the primary and competing messages were equal, (3) the temporal pattern of the competition was similar to that of the primary message. The semantic content of the competing message did not contribute to the plateau in the slope of the performance-intensity function, but the plateau was primarily due to the similarity of the temporal patterns of the primary and competing messages which occurred when the intensities of both messages were equal.

Performance-intensity (PI) functions for phonetically balanced (PB) word lists were obtained for a group of normal listeners (27 ears), and for two groups of patients with cochlear (89 ears) and retrocochlear disorders (eight ears). Listeners with normal hearing or cochlear disorders exhibited mild to moderate reductions in discrimination score as the speech level was raised above the PB maximum. In contrast, patients with retrocochlear disorders showed a pronounced rollover phenomenon, characterized by a rapid decline in performance as the speech level was raised above the maximum discrimination score.

Monaural-intelligibility functions for monosyllabic words were obtained for seven subjects with normal hearing under the following major experimental conditions: (1) speech interrupted by silence at the rates of 1, 10, and 100 interruptions per second (ips); (2) speech alternating with noise (80 dB SPL) at rates of 1, 10, and 100 ips; (3) speech in interrupted noise (80 dB SPL) at rates of 1, 10, and 100 ips. The comparison between the intelligibility functions for Conditions 1 and 2 yielded a measure of the effects of forward and backward masking during the noise-free interval. The comparison between the intelligibility functions for Conditions 2 and 3 yielded a measure of the extent to which the speech signal during the noise burst contributed to intelligibility. The results indicated that the effect of forward and backward masking on intelligibility was substantial at 10 ips and even more pronounced at 100 ips. However, at 1 ips little masking effect was observed except at very adverse signal-to-noise (S/N) ratios. The findings also demonstrated that the speech heard within the noise burst contributed to intelligibility at favorable S/N ratios when the noise was interrupted one time per second. At 100 ips, the speech signal within the noise burst made only a small contribution to the total intelligibility score, and at 10 ips the function was formed almost entirely by the speech heard during the noise-free intervals.

Monaural intelligibility functions for spondaic and monosyllabic words and identification functions for synthetic sentences were obtained for 14 experimental conditions of masking by random noise. The noise was presented continuously and interrupted or modulated (−14-dB interburst ratio) at rates of 1, 10, and 100 interruptions per second. The intensity levels of the maskers were 50 and 90 dB SPL. Results demonstrated that (1) masking effectiveness decreased for spondees and sentences as the interruption rate decreased; (2) the greatest release from masking for monosyllabic words was found during the 10-ips condition at favorable S/N ratios; however, at the more unfavorable S/N ratios, greater release from masking was observed during the 1-ips condition; (3) greater release from masking occurred for interrupted than for comparable modulated noise conditions. The possible effects of the interaction between the type of speech material and forward and backward masking during the silent interval between noise bursts is discussed.

The problems inherent in using frequency-specific stimuli in the sound field to determine threshold sensitivity are reviewed, including a discussion of some of the specific problems encountered when introducing pure tones, narrow bands of noise, and frequency-modulated (FM) tones. The results of two experiments are reported. In Experiment I, the relationship between pure tones and frequency-modulated tones is developed under earphones in an anechoic chamber, and in two sound-isolated auditory test rooms (not anechoic). Experiment I resulted in the development of a reference threshold sound pressure level for frequency-modulated signals in the sound field. In Experiment II the reference level was applied to a clinical test facility and evaluated with a group of hearing-impaired individuals. The results suggest that the sound-field reference levels accurately reflect monaural threshold under earphones, when the earphone is calibrated to the ANSI, 1969 standard, and the sound field is calibrated to the suggested standard.