Biomedical subjects

R L Freyman

Publications and source records attributed to R L Freyman.

11 recordsLinked to original sources

PubMed1992

Potential benefits of adaptive frequency-gain characteristics for speech reception in noise.

Some current single-microphone hearing aids employ techniques for adaptively varying the frequency-gain characteristics in an attempt to improve speech reception in noise. The potential benefit of this strategy depends on the spectral spread of masking and the degree to which it can be reduced by changing the frequency-gain characteristic. In this study these benefits were examined for subjects with normal hearing under static listening conditions. In the unprocessed condition, subjects were presented with nonsense syllables in an octave-band noise centered on 0.5, 1, or 2 kHz. The frequency-gain characteristic was then modified with the goal of reducing the intensity of the frequency region containing the octave-band noise. This processing resulted in increases as large as 60 percentage points in consonant-correct scores with the low- and mid-frequency octave noise bands, and a small increase with the high-frequency noise. Masking patterns produced by the octave noises were also measured and were related to the intelligibility results via an analysis based on Articulation Theory. The Articulation Index was also used to compare the effectiveness of three adaptive rules. A simple multiband volume control is expected to provide much of the benefit of more sophisticated systems without the need for separate estimation of input speech and noise spectra.

Auditory Threshold↗

PubMed1991

Effect of consonant-vowel ratio modification on amplitude envelope cues for consonant recognition.

This investigation examined the degree to which modification of the consonant-vowel (C-V) intensity ratio affected consonant recognition under conditions in which listeners were forced to rely more heavily on waveform envelope cues than on spectral cues. The stimuli were 22 vowel-consonant-vowel utterances, which had been mixed at six different signal-to-noise ratios with white noise that had been modulated by the speech waveform envelope. The resulting waveforms preserved the gross speech envelope shape, but spectral cues were limited by the white-noise masking. In a second stimulus set, the consonant portion of each utterance was amplified by 10 dB. Sixteen subjects with normal hearing listened to the unmodified stimuli, and 16 listened to the amplified-consonant stimuli. Recognition performance was reduced in the amplified-consonant condition for some consonants, presumably because waveform envelope cues had been distorted. However, for other consonants, especially the voiced stops, consonant amplification improved recognition. Patterns of errors were altered for several consonant groups, including some that showed only small changes in recognition scores. The results indicate that when spectral cues are compromised, nonlinear amplification can alter waveform envelope cues for consonant recognition.

Adolescent↗

PubMed1991

Frequency discrimination as a function of signal frequency and level in normal-hearing and hearing-impaired listeners.

Frequency difference limens (DLFs) for pure tones were obtained over a wide range of frequencies and levels from 7 normal-hearing subjects and 16 ears of 12 listeners with sensorineural hearing losses. The normal data were fitted with a general prediction equation. Variability of the data around the DLFs estimated by the equation was quantified and used to evaluate the DLFs from the hearing-impaired listeners. The majority of DLFs from impaired listeners were poorer than one standard deviation above the estimates of the normal equation at all frequencies and sensation levels (SLs). The portion of the equation concerned with sensation level was fitted to each listener's data at each frequency. The slopes of these functions indicated that, on average, the rate of improvement of the DLF with sensation level was similar in the two groups of subjects. These results suggest that it would be reasonable to compare DLFs from normal-hearing and hearing-impaired listeners at equivalent sensation levels. The intercepts of the DLF-intensity functions represent asymptotic values obtained at high SLs. These asymptotic DLFs were abnormal in the majority of hearing-impaired subjects, with more than half the data in excess of two standard deviations above normal. However, among those subjects, the correlation between the DLF deficit and the amount of hearing loss at the test frequency was not strong (r = +.27).

Acoustics↗

PubMed1991

Dynamic processes in the precedence effect.

Three experiments were conducted to investigate the dependence of echo suppression on the auditory stimulation just prior to a test stimulus. Subjects sat in an anechoic chamber between two loudspeakers, one which presented the "lead" sound, and the other the delayed "lag" sound. In the first experiment, subjects reported whether or not they heard an echo coming from the vicinity of the lag loudspeaker during a test click pair. In seven of nine listeners, perception of the lagging sound was strongly diminished by the presence of a train of "conditioning" clicks presented just before the test click. Echo threshold increased (subjects were less sensitive to echoes) as the number of clicks in the train increased from 3 to 17. For a fixed number of clicks, the effect was essentially independent of click rate (from 1/s through 50/s) and duration of the train (from 0.5 through 8 s). A second experiment demonstrated a similar buildup of echo suppression with white noise bursts, regardless of whether the bursts in the conditioning train were repeated samples of frozen noise, or were independent samples of noise. Using an objective procedure for measuring echo threshold, the third experiment demonstrated that both lead and lag stimuli must be presented during the conditioning train in order to produce the buildup of suppression. When only the lead sound was presented during the conditioning train, the perceptibility of the lag sound during the test burst appeared to be enhanced.

Adult↗

PubMed1990

Effects of stimulus level on forward-masked psychophysical tuning curves in quiet and in noise.

Forward-masked psychophysical tuning curves were obtained from normal-hearing listeners at different probe levels in quiet and in a broadband background noise. In quiet, tuning-curve shape changed with probe level. For six listeners, tuning curves became broader with increasing probe level, primarily due to a decrease in the low-frequency slopes. For one listener, tuning curves became narrower with increasing probe level. The addition of a background noise, which was presented continuously at a level 10 dB below the noise level required to mask the probe tone, reduced the masker levels required to mask the probe tone. The reduction was greater near the tip of the tuning curve than on the tail, so that tuning curves in background noise were narrower than those obtained in quiet. Tuning curves with comparable masker levels near the tip of the tuning curve (Lmtip) were similar in shape, regardless of probe level or whether tuning curves were obtained in quiet or noise. Comparisons of tuning-curve characteristics derived by fitting tuning curves with least-squares procedures, indicated that low-frequency slopes decreased with Lmtip. As a consequence, Q10 dB values decreased with Lmtip. These results are consistent with the interpretation that tuning-curve shapes are determined by the intensities of the maskers required to mask the probe tone. The addition of a background noise restricted (partially masked) the excitation pattern of the probe so that lower masker intensities were required to "forward mask" the probe tone, and narrower tuning curves resulted from less intense markers. The results are well described by a two-process model of auditory excitation patterns.

Adult↗

PubMed1989

The importance of consonant-vowel intensity ratio in the intelligibility of voiceless consonants.

The purpose of this study was to evaluate the extent to which variations in the consonant-vowel (C-V) intensity ratio could account for variations in speech intelligibility among the productions of 10 talkers. Fifty normal-hearing individuals listened in noise to syllables consisting of voiceless consonants followed by the vowel /a/ under three conditions in which: (a) C-V ratio varied naturally as produced by the talkers, and the stimuli were calibrated according to vowel intensity; (b) C-V ratios were increased and equated via digital signal processing; and (c) C-V ratios were unmodified, but the syllables were calibrated according to consonant level rather than vowel level. Results indicated that variations in C-V ratio explained a great deal of the variation in the intelligibility of some consonants (/s, S, tS/) but not others (the voiceless stops). This difference may well be due to differences in audibility between the two groups of consonants when they are presented at similar consonant-to-noise ratios. The majority of the data suggest that the importance of C-V ratio is related to the intensity of consonants but is independent of the ratio per se between consonant and vowel levels.

Adult↗

PubMed1989

Effect of click rate and delay on breakdown of the precedence effect.

The precedence effect was tested as a function of echo-click delay and click rate after an abrupt switch in location between leading and lagging clicks. Click trains at three rates, 1/sec, 2/sec, and 4/sec, with delays ranging between 2 and 20 msec, were presented to subjects in an anechoic chamber. Duration of the click train after the switch in location was 12 sec, and echo click perceptibility was assessed throughout this period. The number of echo clicks heard was an increasing monotonic function of delay. The subjects reported a "fade-out" of echo clicks after a set number of clicks at each delay, regardless of rate. This result was interpreted as a buildup in inhibition of echoes produced by the ongoing click train. Suppression of echoes was stronger when the leading click originated from the right side than from the left side.

Attention↗

PubMed1987

Frequency discrimination of short- versus long-duration tones by normal and hearing-impaired listeners.

This investigation explored the effects of stimulus level on the frequency discrimination of long- and short-duration pure tones by 5 subjects with normal hearing and 7 with sensorineural hearing impairment. Frequency difference limens (DLs) were obtained as a function of signal intensity for 5-ms and 300-ms tones at 500, 1000, and 2000 Hz. The performance of most of the hearing-impaired subjects was poorer than normal for 300-ms tones, but not for 5-ms tones. This result was relatively independent of the stimulus sensation levels at which the data were compared. However, the current results also show an unexpected dependence of the frequency DL on the sensation level of short-duration tones. In several normal-hearing subjects, frequency discrimination performance for these short tones is poorer at moderately high levels than at low levels.

Adult↗

PubMed1987

Temporal resolution in sensorineural hearing-impaired listeners.

Temporal masking curves were obtained from 12 normal-hearing and 16 hearing-impaired listeners using 200-ms, 1000-Hz pure-tone maskers and 20-ms, 1000-Hz fixed-level probe tones. For the delay times used here (greater than 40 ms), temporal masking curves obtained from both groups can be well described by an exponential function with a single level-independent time constant for each listener. Normal-hearing listeners demonstrated time constants that ranged between 37 and 67 ms, with a mean of 50 ms. Most hearing-impaired listeners, with significant hearing loss at the probe frequency, demonstrated longer time constants (range 58-114 ms) than those obtained from normal-hearing listeners. Time constants were found to grow exponentially with hearing loss according to the function tau = 52e0.011(HL), when the slope of the growth of masking is unity. The longest individual time constant was larger than normal by a factor of 2.3 for a hearing loss of 52 dB. The steep slopes of the growth of masking functions typically observed at long delay times in hearing-impaired listeners' data appear to be a direct result of longer time constants. When iterative fitting procedures included a slope parameter, the slopes of the growth of masking from normal-hearing listeners varied around unity, while those from hearing-impaired listeners tended to be less (flatter) than normal. Predictions from the results of these fixed-probe-level experiments are consistent with the results of previous fixed-masker-level experiments, and they indicate that deficiencies in the ability to detect sequential stimuli should be considerable in hearing-impaired listeners, partially because of extended time constants, but mostly because forward masking involves a recovery process that depends upon the sensory response evoked by the masking stimulus. Large sensitivity losses reduce the sensory response to high SPL maskers so that the recovery process is slower, much like the recovery process for low-level stimuli in normal-hearing listeners.

Female↗

PubMed1986

Frequency discrimination as a function of tonal duration and excitation-pattern slopes in normal and hearing-impaired listeners.

Frequency difference limens were determined as a function of stimulus duration in five normal-hearing and seven hearing-impaired subjects. The frequency DL duration functions obtained from normal-hearing subjects were similar to those reported by Liang and Chistovich [Sov. Phys. Acoust. 6, 75-80 (1961)]. As duration increased, the DL's improved rapidly over a range of short durations, improved more gradually over a middle range of durations, and reached an asymptote around 200 ms. The functions obtained from the hearing-impaired subjects were similar to those from normal subjects over the middle and longer duration, but did not display the rapid changes at short durations. The paper examines the ability of a variation of Zwicker's excitation-pattern model of frequency discrimination to explain these duration effects. Most, although not all, of the effects can be adequately explained by the model.

Adult↗

PubMed1986

Psychometric functions for frequency discrimination from listeners with sensorineural hearing loss.

Psychometric functions for pulsed pure-tone frequency discrimination were obtained from hearing-impaired listeners at frequencies with normal hearing and at frequencies with mild or moderate hearing losses. The general form of psychometric functions at hearing-impaired frequencies was found to be the same as at normal-hearing frequencies, i.e.,d' was linear with the frequency difference between tones, in Hz. For all but one psychometric function, the addition of an intercept term to the fitting equation did not account for significantly more variance than did the slope term alone. Therefore, it was concluded that psychometric functions for frequency discrimination can be adequately described with only one parameter: the slope of the psychometric function. Deficits in discrimination at hearing-loss frequencies were manifested by more gradual slopes of psychometric functions. Procedures for normalizing psychometric functions are presented, which facilitate comparisons of normal and impaired frequency discrimination data across studies and frequencies. Comparisons of dlf's (difference limen for frequency) obtained with adaptive and fixed procedures show a bias toward larger dlf's with adaptive procedures, but only at higher frequencies. A discussion of equal-interval and equal-ratio adaptive stepping rules indicates that an equal-ratio rule may be preferable.

Auditory Threshold↗