Biomedical subjects

A R Leeuw

Publications and source records attributed to A R Leeuw.

7 recordsLinked to original sources

PubMed1999

Importance of temporal-envelope cues in consonant recognition.

The role of different modulation frequencies in the speech envelope were studied by means of the manipulation of vowel-consonant-vowel (VCV) syllables. The envelope of the signal was extracted from the speech and the fine-structure was replaced by speech-shaped noise. The temporal envelopes in every critical band of the speech signal were notch filtered in order to assess the relative importance of different modulation frequency regions between 0 and 20 Hz. For this purpose notch filters around three center frequencies (8, 12, and 16 Hz) with three different notch widths (4-, 8-, and 12-Hz wide) were used. These stimuli were used in a consonant-recognition task in which ten normal-hearing subjects participated, and their results were analyzed in terms of recognition scores. More qualitative information was obtained with a multidimensional scaling method (INDSCAL) and sequential information analysis (SINFA). Consonant recognition is very robust for the removal of certain modulation frequency areas. Only when a wide notch around 8 Hz is applied does the speech signal become heavily degraded. As expected, the voicing information is lost, while there are different effects on plosiveness and nasality. Even the smallest filtering has a substantial effect on the transfer of the plosiveness feature, while on the other hand, filtering out only the low-modulation frequencies has a substantial effect on the transfer of nasality cues.

Adolescent↗

PubMed1998

The relation between otoacoustic emissions and the broadening of the auditory filter for higher levels.

The active behaviour of outer hair cells (OHCs) is often used to explain two phenomena, namely otoacoustic emissions (OAEs) and the level dependence of auditory filters. Correlations between these two phenomena may contribute to the evidence of these hypotheses. In this study auditory filters were calculated from probe thresholds in notched-noise maskers over a range of at least 25 dB. Further. transient evoked otoacoustic emissions (TEOAEs) were measured at several stimulation levels. Ten normal-hearing and nine hearing-impaired subjects were tested. A linear increase of the width of the auditory filter with 2.2 dB/Hz was found up to a specific saturation level. The group of selected hearing-impaired subjects with mild hearing loss showed no wider than normal auditory filters. As expected, the increase of the width of the auditory filter correlated positively with the level of TEOAEs for click intensities of about 80 dB peak SPL. However, for subjects with TEOAEs wider auditory filters at a masker level of 65 dB/Hz were found for subjects with larger TEOAEs. This result cannot be explained by a model by which the cochlea shows an active behaviour for lower stimulation levels, influencing both the TEOAE levels and the filter skirts, and a passive behaviour for higher stimulation levels.

Acoustic Stimulation↗

PubMed1994

Frequency-resolution measurements with notched noises for clinical purposes.

Knowledge of the frequency resolving power of the pathological ear can be helpful for good hearing-aid prescription. In particular, the slope of the low-frequency edge of the auditory filter will determine the extent of upward spread of masking. The most precise method for measuring this filter is the "notched-noise" procedure developed in the early 1980s. However, at this moment, the notched-noise protocol is too time consuming for clinical use. In this paper we developed a protocol that is applicable in the clinic. In the first part of the study we investigated the minimum number of threshold measurements necessary for a reliable estimation of the filter parameters. For this purpose we analyzed 99 filters originally obtained with 13 threshold measurements. The influence of reducing the number of notch widths used to obtain filter shapes on the variability of filter parameters was investigated. Subsets of seven and five notch widths gave about the same standard deviations of the differences between the parameters obtained with the subset and the parameters obtained with the full set of 13 thresholds, while subsets of four and three notch widths gave significantly higher variability. However, because small deviations of one threshold determination can give rise to a large change in filter parameters, especially for filters with flatter skirts, it is strongly recommended to reduce the variability by measuring thresholds twice (test and retest). In the second part of the study we investigated a faster method for measuring thresholds. The forced-choice paradigm normally used in notched-noise procedures was replaced by a Békésy paradigm. The Békésy paradigm did not significantly increase intra-individual standard deviations, but did reduce the measuring time by more than 50%. In conclusion, the new procedures reduce the measurement time needed to obtain reliable estimates of auditory-filter parameters by a factor of about 5. The new protocol lasts about 15 min for each filter measurement, which appears to be acceptable for clinical use, at least for difficult-to-fit hearing aid users.

Acoustic Stimulation↗

PubMed1991

Advantages of directional hearing aid microphones related to room acoustics.

In this study, two types of hearing aids were used. Both aids had the same frequency characteristics for frontal sound, but one employed an omnidirectional microphone and the other a directional microphone. The frequency characteristics of both hearing aids were measured for five azimuths on KEMAR and in situ in 12 normal-hearing subjects. For these subjects we also determined the speech reception threshold (SRT) with background noise in two rooms with different reverberation times. The direction of the speech stimuli was always frontal; the direction of the noise was varied. Additionally, directional hearing was measured with short noise bursts from eight loudspeakers surrounding the subject. In the less reverberant room, sounds coming from behind were less amplified by the hearing aid with the directional microphone than by the one with the omnidirectional microphone. In this room the monaural SRT values were largely determined by the level of the background noise. For the directional hearing aids there was an extra binaural advantage which depended on the direction of the background noise. Only for low-frequency noise bursts was directional hearing better with directional hearing aids. In the more reverberant room, no distinct differences between the frequency characteristics of the two hearing aid types were measured. However, a systematic difference between monaural SRT values measured through the two hearing aids was found. This difference was independent of noise azimuth. In conclusion, hearing aid(s) with a directional microphone showed no disadvantages and clear advantages under specific conditions.

Acoustics↗

PubMed1990

Speech reception in reverberation related to temporal resolution.

In this study, the influence of temporal properties of the auditory system on speech recognition, either in noise or in both noise and reverberation, has been investigated. Temporal resolution parameters consisted of both the temporal resolution factor (TRF) and the minimum detectable gap (MDG). TRFs were measured for probe-tone frequencies of 500, 1000, and 3000 Hz; MDGs were measured for octave-band noises centered at 500, 1000, and 3000 Hz and for a wideband noise. TRF and MDG proved to be rather independent parameters of temporal resolution. For both temporal-resolution parameters, there was considerable overlap between the performance of the normal-hearing and hearing-impaired listeners. A correlation analysis shows that MDG for wideband noise is related to speech recognition in reverberation. However, there is a mutual dependence of speech recognition and MDG on the pure-tone audiogram.

Adolescent↗

PubMed1990

The temporal resolution factor with and without background noise.

In this paper we discuss the measurement of the temporal resolution factor (TRF). The major merits of this measurement are that it is simple enough to be used clinically and that the results are, within certain limits, independent of presentation level. Addition of a continuous background noise has been suggested to enhance the discriminative power of TRF between normal-hearing listeners and listeners with poor temporal resolution. However, we showed through a simple model that this background noise may introduce a strong dependence on presentation level, which must be taken into consideration. To test our model we developed a new measurement protocol. This protocol was applied in measurements of 5 normal-hearing and 9 sensorineurally hearing-impaired listeners. Their results could reasonably well be fitted by the model. The model predicts large measurement errors for TRF values determined with background noise. We found the standard deviations of TRF values, calculated from test and retest measurements, to be smaller in conditions without a continuous background noise. In addition, our results did not show the predicted enhancement in discriminative power between normal-hearing and hearing-impaired listeners. Therefore, the primary conclusion is that the determination of the TRF is not improved by adding a background noise.

Adaptation, Physiological↗

PubMed1987

Speech understanding and directional hearing for hearing-impaired subjects with in-the-ear and behind-the-ear hearing aids.

With respect to acoustical properties, in-the-ear (ITE) aids should give better understanding and directional hearing than behind-the-ear (BTE) aids. Also hearing-impaired subjects often prefer ITEs. A study was performed to assess objectively the improvement in speech understanding and directional hearing afforded by ITE aids versus BTEs. In 28 hearing-impaired subjects, who visited our Centre for a check-up of their ITEs, the following parameters were measured: (a) thresholds for third-octave bandpass noises between 0.25 and 4 kHz, (b) speech reception thresholds for short Dutch sentences in quiet and with background noise, (c) directional hearing. All three experiments were done binaurally with the subjects wearing their ITEs, BTEs, and no hearing aid. With the 2-cc coupler, the gain used by the subject was measured. The SRT values for the ITE were significantly lower than those for BTE. More gain at 2 and 4 kHz in the ITE proved to be a responsible factor for this improvement. Directional hearing was not improved by wearing an ITE. Large interindividual differences were found between functional gain and the 2-cc coupler measurements. The mean functional gain at 4 kHz for an ITE is higher than the gain measured in a 2-cc coupler. For a BTE, the functional gain at 2 and 4 kHz is lower than the 2-cc coupler gain.

Adult↗