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Biomedical subjects

T Letowski

Publications and source records attributed to T Letowski.

16 recordsLinked to original sources
PubMed2001

Binaural helmet: improving speech recognition in noise with spatialized sound.

This study assessed the effects of spatialized sound presentation on a listener's ability to monitor target (T) messages in the presence of competing (C) messages and high-level (110 dB[A]) background noise (BGN). In a simulated military environment, 8 participants wore two-channel, active noise reduction (ANR) equipped helmets and listened to combinations of T and C messages (89 dB[A] at the ear). T messages were presented synchronously with 0, 1, 2, and 3 C messages in four listening modes: (a) BGN + diotic, (b) BGN + dichotic, (c) BGN + spatial audio, and (d) quiet + spatial audio. Best overall performance occurred in the spatialized modes (c and d) and poorest in the diotic mode (a). As expected, speech recognition was better in quiet than in BGN when multiple C messages were present. Findings indicate that message spatialization in acoustic space improves auditory performance during times of heavy message competition, even in high-level noise. The proposed technology has numerous applications, such as multichannel communications in tactical operations centers, monitoring of complex security systems, and air traffic control.

Adult↗
PubMed2000

Evaluation of acoustic beacon characteristics for navigation tasks.

The goal of the present study was to investigate the human factors issues related to acoustic beacons used for auditory navigation. Specific issues addressed were: (1) the effect of various beacon characteristics on human accuracy in turning toward the direction of the acoustic beacon; (2) the difference between real and virtual environments on human accuracy in turning toward the acoustic beacon; and (3) the perceived sound quality of various acoustic beacons. Three experiments were conducted in which acoustic beacons were presented in a background of 80 dBA pink noise. Results of the localization tasks revealed that (a) presentation mode (continuous versus pulsed beacon sound) did not affect the overall localization accuracy or number of front-back confusion errors; and (b) the type of acoustic beacon affected the size of localization error. Results of the sound quality assessment indicated that listeners had definite preferences regarding the type of sound being used as a beacon, with (a) non-speech beacons preferred over speech beacons, (b) a beacon repetition rate of 1.1 rps preferred over either the 0.7 or 2.5 rps rates, and (c) a continuous operation of a beacon preferred over a pulsed operation. Finally, sound quality ratings and localization errors were highly negatively correlated. This finding demonstrates the usefulness and practical values of sound quality judgements for audio display design and evaluation.

Acoustics↗
PubMed1999

Comparison of air-conduction and bone-conduction hearing thresholds for pure tones and octave-band filtered sound effects.

The purpose of this study was to measure air-conduction (AC) and bone-conduction (BC) hearing thresholds with pure-tone and filtered sound effect stimuli using standard audiometric equipment. A group of 20 young, normal-hearing listeners participated in the study. Pure-tone stimuli were 250, 500, 1000, 2000, and 4000 Hz. Sound effect stimuli were 12 natural sounds that were spectrally limited to an octave bandwidth centered at either 250, 500, 1000, 2000, or 4000 Hz. The AC and BC detection thresholds were measured using a clinical audiometer (Madsen Orbiter 922) with a B-71 bone oscillator and TDH-50 earphones. Results indicated that detection thresholds for the pure-tone and corresponding octave-band sound effect stimuli were within 3 to 4 dB of each other for both AC and BC testing. The findings support the notion that octave-filtered sound effects are a viable alternative to pure-tone stimuli for use in audiology clinics.

Adolescent↗
PubMed1998

Spatial audiometry: detection of spondaic words in noise.

It is important to select appropriate stimuli and test conditions for developing standardized spatial audiometric tests. In three experiments, binaural detection thresholds (BDTs) for a target signal, located at either 0, 45, 90, 135, 180, 225, 270, or 315 degrees azimuth, were measured in the presence of a masker positioned at one of these eight locations. Target signals included spondaic words from the CID W-1 list. The masker was speech spectrum noise (SSN) or multitalker noise (MTN) presented at a constant level (65 dBA). Bekesy tracking was used to measure BDTs in listeners with normal hearing. Results indicate that BDTs are significantly influenced by the (a) angular separation between the target and noise source and (b) choice of spondaic words used as target stimuli. BDTs for various spondaic words differed as much as 13 dB for a given angular separation. BDTs measured in SSN and MTN for otherwise identical test conditions differed less than 3 dB. A single spondaic word appears to be appropriate for spatial audiometric tests of detection. Nonsignificant differences between masked BDTs obtained for SSN and MTN noises indicate that for spatial detection, the masking effects of these noises are comparable. These results indicate that the development of a clinical test of spatial detection should include the use of SSN and a single spondaic word, with detection being measured for a set of four or five signal and noise source configurations.

Adolescent↗
PubMed1997

Effects of age, speech rate, and type of test on temporal auditory processing.

Cognitive slowing that accompanies aging may be reflected in temporal aspects of auditory processing. The purpose of this study was to investigate the effects of age, type of test, and rate of speech on temporal auditory processing. Listeners were divided into three groups: young (25- to 35-year-olds), middle aged (45- to 55-year-olds), and older (65- to 75-year-olds). A method of time compression known as Synchronized Overlap Add (SOLA) was used to increase the rate of speech. This method provides a high-quality speech signal and limits the distortions that may confound the temporal effects on time-compressed tests of speech intelligibility. Listeners performed four speech understanding tasks: sentence repetition, sentence intelligibility rating, connected discourse intelligibility rating, and connected discourse comprehension question and answers at three time compression rates (60%, 70%, and 80%). Although the older group performed more poorly on all tests, only the connected discourse intelligibility rating test was sensitive to age differences among all three groups. This difference did not appear to increase with rate increases but was present only at the 70% compression rate. In addition, variability was especially high in the oldest group of participants.

Adult↗
PubMed1996

Comprehension of time-compressed speech: effects of age and speech complexity.

The periodic sampling method of time compression is characterized by periodic removal of segments of speech according to predetermined compression rate (CR) and discard interval length (DIL). Fifteen middle-aged (42-54 years old) and 15 older (60-69 years old) adults participated in the study that assessed the combined effects of CR and DIL on the comprehension of time-compressed speech by aging adults. Three CRs (30, 45, and 60%), seven DILs (35 through 155 msec), and two types of speech materials were used. The subject's task was to report associations among the items mentioned in a time-compressed passage. In all cases, performance of the subjects deteriorated with increasing CR and DIL. The older adults were affected more by CR and DIL values than middle-aged adults. The difference in sentence complexity between the two speech materials affected both groups equally. In general, the results of the study indicated that (a) time-compressed speech differentiates between speech comprehension by middle-aged and older adults and (b) the effects of CR and DIL became more independent with increasing age of the listener and increasing complexity of the speech material. Reported results support the concept that time-compressed speech may be an effective signal in clinical assessment of adults whose auditory complaints are not explained by their peripheral hearing losses.

Adult↗
PubMed1995

Most comfortable loudness shift as a measure of speech attenuation by hearing protectors.

The feasibility of using most comfortable loudness (MCL) as a real-world measure of speech attenuation introduced by hearing protection devices (HPDs) was studied. The authors compared three insert HPDs and an earmuff under three test conditions: (a) monaural earphone listening, (b) binaural earphone listening, and (c) sound field listening. The earmuff was used only in the sound-field condition. In addition, three sets of fitting instructions were utilized. Twelve normal hearing subjects participated. Results indicate that MCL shift can be used as a simple measure of speech attenuation provided by HPDs. The observed MCL shifts were affected by the fitting instruction but not by the listening condition. Findings support the notion that MCL-based tests of speech attenuation by HPDs can be conducted reliably both in sound field and under earphones and may be a simple and valid tool for assessing changes in speech audibility due to the wearing of HPDs.

Adult↗
PubMed1995

Understanding of time-compressed speech by older adults: effect of discard interval.

Fifteen subjects, aged 60 to 74 years, participated in a study to assess the role of discard interval (DI) length in the understanding of time-compressed connected speech by older adults. The subjects' task was to report associations among the items mentioned in a passage. Results indicate that performance deteriorated gradually until a critical DI of 75 msec was reached, beyond which performance dropped sharply. Results should help in the better understanding of speech perception by older listeners and may be useful in development of a standardized test for evaluating speech understanding in older individuals whose auditory complaints are often not explained by their peripheral hearing loss.

Age Factors↗
PubMed1993

Detection of warble tones in wideband noise with and without hearing protection devices.

Noise attenuation by linear hearing protection devices (HPDs) is considered to be independent of the environmental noise level. Linearity of HPDs is one of the basic assumptions underlying the Real Ear Attenuation at the Threshold (REAT) method of HPD testing, which utilizes quiet conditions. However, non-linear growth of masking at high intensity levels may result in the effects of HPDs on detection of speech and warning signals being different from those at high noise levels. The purpose of this study was to determine if the non-linear growth of masking can affect detection of warble signals by persons using HPDs in wideband noise presented at the 100 dB(A) level. Audiometric thresholds of hearing of 10 listeners were measured under four experimental conditions: (1) noise off, HPDs off (condition C1); (2) noise off, HPDs on (condition C2); (3) noise on, HPDs on (condition C3); and (4) noise on, HPDs off (condition C4). The results of this study indicate that detection of signals by persons using HPDs in noise < 100 dB(A) can be well predicted from noise attenuation characteristics of HPDs measured in quiet without the need for a correction factor accommodating non-linear growth of masking. This finding has implications for predicting the audibility of warning signals in noise by persons wearing HPDs.

Adult↗
PubMed1993

Acoustical properties of speech produced in noise presented through supra-aural earphones.

This study determined the acoustical properties of speech known as Lombard Speech produced in background noise. Tape recordings were made for ten normally hearing adults (5 women, 5 men) reading connected speech (131 word passage "My Grandfather") at their most comfortable level in quiet and in wideband, traffic, and multitalker noise delivered through earphones at 70 and 90 dB SPL. Spectral analysis of the recordings revealed that, compared with speech in quiet, Lombard speech was characterized by: (1) an increase in overall SPL; (2) smaller vocal pitch shifts for female than male subjects; (3) shifts in spectral distributions of speech energy; and (4) the same spectral slope above 630 Hz regardless of subject gender, noise level, or noise type. Overall, the results of this study do not provide support for the theory that acoustical properties of Lombard speech are identical with loud speech produced in quiet.

Acoustic Stimulation↗
PubMed1992

Relationships between speech recognition threshold, average hearing level, and speech importance noise detection threshold.

The aim of this study was to determine (a) if the speech importance noise detection threshold (SINDT) could be used as an accurate estimator of speech recognition threshold (SRT), and (b) whether the SINDT correlates well with various measures of the pure-tone threshold average (PTA). Pure-tone threshold, SRT, and SINDT tests were administered to 94 adults with various degrees of hearing loss. The results demonstrated that SRT can be accurately predicted from SINDT and that these two are highly correlated (r = 0.97). Since a SINDT measurement takes less time than SRT determination, the SINDT test may be recommended as an alternative for SRT in some applications.

Adolescent↗
PubMed1992

High-frequency threshold measurements using insert earphones.

Several recent studies have reported large intersubject variability of high-frequency thresholds measured with circumaural earphones. In the present study, high-frequency thresholds of 10 subjects were measured with circumaural (Sennheiser HD-250) and insert (Etymotic ER-1) earphones at 10, 12, 14, and 16 kHz. Overall results show significantly smaller variability of the threshold data obtained with insert earphones than with circumaural earphones. The above data indicate that insert earphones may be more suitable for high-frequency testing than circumaural earphones.

Acoustic Stimulation↗
PubMed1986

Induction loop listening system designed for a classroom.

The principles of designing an induction loop listening system are discussed. Step-by-step procedures for building an induction loop are presented. The loop described was installed in a medium-size classroom and listening tests were performed comparing the loop and loudspeakers. Two groups of hearing impaired listeners were used. One group of listeners wore hearing aids while the other group did not. Result indicated that for both groups, speech perception was enhanced with the loop system.

Correction of Hearing Impairment↗
PubMed1985

Vibrotactile performance by normal and hearing-impaired subjects using two commercially available vibrators.

The aim of this study was to measure vibrotactile sensitivity and stress pattern recognition of untrained normal-hearing and hearing-impaired subjects. Vibrotactile thresholds were measured as a function of frequency with two vibrators at three body placements. Stress pattern recognition was tested with two vibrators at two body placements. Vibrotactile threshold results indicate that wrist placements were less sensitive than fingertip placement and the hearing-impaired subjects were equally or less sensitive than the normal-hearing subjects. Vibrator plunger size had the greatest effect on threshold in the most sensitive frequency range of the skin. Stress pattern recognition results indicate that subject performance is a function of body placement but not a function of plunger size. Moreover, stress pattern recognition curves obtained in this study had lower plateaus for the hearing-impaired subjects than for the normal-hearing subjects. Implications for vibrotactile training are discussed.

Adult↗
PubMed1980

An influence of binaural hearing aids on positioning of sound images.

An influence of binaural hearing aids on positioning of sound images produced by signals from two loudspeakers was investigated with normally hearing and hearing-impaired subjects. The image can be positioned along a horizontal line connecting the centers of the sources by adjusting the relative sound pressure levels of signals. When the listener is a symmetrical receiver, the image appears on the midline (0 degrees azimuth) for equal levels or away from the midline for unequal levels. Any asymmetry of the receiver causes a shift of the image away from the midline. In the study, the level differences, delta Ls, necessary to position sound images at 0 and +/- 12 degrees azimuths were measured. The baseline data were collected for 10 normally hearing subjects in unaided and aided conditions. In the aided conditions binaural aids were balanced (equal gains) or unbalanced (10 dB disparity in gains). The normally hearing subjects positioned the image on the midline with a group mean delta L of 0 dB in both unaided and balanced aided conditions. To position the image at +12 or -12 degrees azimuth they needed mean delta Ls that could be predicted from a theoretical equation. With unbalanced aids, a group mean delta L of +3 or -3 dB was needed for the midline image. The delta Ls for the side images also were shifted. For 12 hearing-impaired subjects with bilateral lossses, the midline delta Ls in unaided condition were within the normal range (+/- 2 dB). For two monaural hearing aid users with long experience, the midline delta Ls were greater than normal. The delta Ls for the side images were equal to or greatr than delta Ls for normally hearing subjects. In aided balanced and/or unbalanced conditions, some hearing-impaired subjects needed delta Ls for the midline image outside the normal range, and some could not position the image at the side azimuths. It was concluded that the binaural hearing aids can be a source of receiver asymmetry.

Adult↗