This research investigates the human observer's ability to discriminate a difference in duration for noise bursts and gated sinusoids. Two observers compared two durations T and Tq-/XT in a two-alternative forced-choice procedure. The value of T ranged from 0.16 to 960 msec. For each T the value of AT for 75% discrimination was determined. The results showed that for most of the range investigated AT. was proportional to T«. Performance was not affected by a change in bandwidth from 3500 to 200 Hz. Values of AT for 75% correct did decrease, however, when the observers were given audible spectral cues from very short pulsed sinusolds. The theory best describing the results was the neural counter model proposed by Creelman [J. Acoust., Soc. Amer. 34, 582-593 (1962)-].

An experiment was conducted to determine the effect of aging on sound localization. Seven groups of 16 subjects, aged 10-81 years, were tested. Sound localization was assessed using six different arrays of four or eight loudspeakers that surrounded the subject in the horizontal plane, at a distance of 1 m. For two 4-speaker arrays, one loudspeaker was positioned in each spatial quadrant, on either side of the midline or the interaural axis, respectively. For four 8-speaker arrays, two loudspeakers were positioned in each quadrant, one close to the midline and the second separated from the first by 15 degrees, 30 degrees, 45 degrees, or 60 degrees. Three different 300-ms stimuli were localized: two one-third-octave noise bands, centered at 0.5 and 4 kHz, and broadband noise. The stimulus level (75 dB SPL) was well above hearing threshold for all subjects tested. Over the age range studied, percent-correct sound-source identification judgments decreased by 12%-15%. Performance decrements were apparent as early as the third decade of life. Broadband noise was easiest to localize (both binaural and spectral cues were available), and the 0.5-kHz noise band, the most difficult to localize (primarily interaural temporal difference cue available). Accuracy was relatively higher in front of than behind the head, and errors were largely front/back mirror image reversals. A left-sided superiority was evident until the fifth decade of life. The results support the conclusions that the processing of spectral information becomes progressively less efficient with aging, and is generally worse for sources on the right side of space.

The aims of this research were to document changes in hearing and speech intelligibility in noise that occur with ageing, noise sensitivity, and progressive bilateral sensorineural hearing loss. Five groups defined by age, clinical complaint and degree of hearing loss were tested. Each of 73 subjects participated in nine different procedures, including detection in quiet and in continuous 90 dB SPL helicopter noise, frequency and duration discrimination, consonant recognition and word identification. The effects of different types of background noise and speech-to-noise ratio were investigated. Ageing, without concomitant hearing loss, resulted in significantly greater DLs both for frequency and for duration with a 20 ms standard at both 500 Hz and 4,000 Hz. Hearing loss, unconfounded by ageing, affected masked detection and frequency discrimination at 4,000 Hz and speech intelligibility in noise. The sole finding for subjects with noise sensitivity was an upward spread of masking for detection. Across tests, the best predictors of speech processing decrements were the detection thresholds for 2,000 Hz and 4,000 Hz in quiet or in noise.

The effect of conventional ear plugs and ear muffs, and muffs with limited dichotic amplification on the ability to localize one-third octave noise bands was investigated under semi-reverberant listening conditions. Forty-eight normal-hearing subjects, half over 40 years of age, and 23 subjects with bilateral sensorineural hearing loss participated. Sound localization was assessed using an array of six loudspeakers surrounding the subject at azimuth angles 60 degrees apart. One block of 120 forced-choice speaker identification trials was presented for each of 16 listening conditions defined by ear condition (unoccluded, E-A-R plug, E-A-R muff, and Bilsom 2392 muff), stimulus frequency (500 Hz and 4000 Hz), and background (quiet and continuous 65 dB SPL-white noise). Plugs and muffs, particularly active muffs, resulted in decrements in right/left judgments based on interaural intensity but not time-of-arrival differences. High-frequency front/back discrimination was affected more by muffs than by plugs. Error patterns for the conventional and active muffs were dissimilar. Aging resulted in a decrement in unoccluded front/back discrimination. Trends for the impaired subjects were the same as those for normal subjects at 500 Hz. Many could not hear 4000 Hz with conventional protectors. Their performance was no different from normal with the active muffs.