Aleksander Sek

Frequency discrimination as a function of frequency, measured in several ways

Frequency discrimination was measured for a wide range of center frequencies (0.25-8 kHz) using three different tasks. In the first (difference limens for frequency, DLFs) subjects were required to indicate which of two successive tone pulses was higher in frequency. In the second (difference limens for change, DLCs), two successive pairs of tone pulses were presented; one pair had the same frequency and the other pair differed in frequency. Subjects were required to indicate which pair differed in frequency. In the third (frequency-modulation difference limens, FMDLs), subjects were required to indicate which of two successive tone pulses was frequency modulated. Modulation rates were 2, 5, or 10 Hz. For frequencies up to 2 kHz, DLFs and DLCs were small (less than 0.6% of the center frequency) and were similar to one another. For frequencies of 4 kHz and above, both DLFs and DLCs increased markedly, but the increase was greater for DLFs. Thus the worsening of performance at high frequencies is greater when subjects are required to indicate the direction of a frequency change than when they just have to detect any change. FMDLs, when expressed relative to the carrier frequency, varied much less with frequency than DLFs or DLCs. At 2 kHz and below, FMDLs were larger than DLFs or DLCs. Above 4 kHz, FMDLs were smaller than DLFs or DLCs. At 2 kHz and below, FMDLs usually worsened with increasing modulation frequency. Above 4 kHz, FMDLs improved with increasing modulation frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a fast method for determining psychophysical tuning curves

Psychophysical tuning curves (PTCs) can be used to assess the frequency selectivity of the auditory system and to detect and delimit “dead regions” in the cochlea. However, the traditional method for determining PTCs takes too long for use in clinical practice. We evaluated a fast method for determining PTCs, using a band of noise that sweeps in centre frequency and a Békésy method to adjust the masker level required for threshold. The shapes of the PTCs were similar for the fast and traditional methods, for both normally hearing and hearing-impaired subjects. Rates of change of masker level of 2 dB/s or less gave the most reliable results. A relatively wide bandwidth (20 percent of the signal frequency or 320 Hz, whichever is the smaller) was needed to minimise the influence of beat detection. When the signal frequency fell within a dead region, the fast method gave PTCs with shifted tips.

Effects of carrier frequency and background noise on the detection of mixed modulation

This article is concerned with the mechanisms underlying the detection of amplitude modulation (AM), frequency modulation (FM), and mixed modulation (MM), i.e., simultaneously occurring AM and FM. In a previous study [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 92, 3119-3131 (1992)], psychometric functions were measured for the detection of AM alone and FM alone, using a 10-Hz modulation rate and a 1-kHz carrier frequency. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. The detectability of the combined AM and FM was better than would be predicted if the two types of modulation were coded completely independently. Significant effects of relative modulator phase were found when detectability was relatively high, but these effects were not correctly predicted by either of two excitation-pattern models considered. The first experiment reported here was similar to the earlier experiment, but performance was compared for carrier frequencies of 1 and 6 kHz; at the latter frequency, neural synchrony to the stimulus fine structure (phase locking) does not occur. The results at both carrier frequencies were similar to those of our earlier experiment, suggesting that the presence or absence of phase-locking information plays little role in the detection of MM. The second experiment was again similar, but bands of noise were used to mask selectively either the upper or lower side of the excitation pattern of the modulated carrier. The phase effects in this case were in the direction predicted by excitation pattern models. The overall pattern of the results could be predicted reasonably well using a multichannel excitation pattern model based on the assumption that listeners use an unweighted sum of decision variables across all suprathreshold channels with a positive signal-to-noise ratio.

Detection of combined frequency and amplitude modulation.

Zwicker [Acustica 6, 356–381 (1956)] proposed that amplitude modulation (AM) and frequency modulation (FM) are coded by a common mechanism. To test this, the detection of simultaneously occurring AM and FM is discussed. In a two‐alternative forced‐choice task, thresholds for detecting AM alone were determined. Then, thresholds for detecting FM were determined for stimuli which had a fixed amount of AM in the signal interval only. The amount of AM was always less than the threshold for detecting AM alone. The FM thresholds depended significantly on the magnitude of the coexisting AM. For low modulation rates (4, 16, and 64 Hz), the FM thresholds did not depend on the relative phase of modulation for the FM and AM. For a high modulation rate (256 Hz) strong effects of modulator phase were observed, which can be explained by assuming that detection of modulation at high frequencies is based on detection of the lower sideband in the modulated signal’s spectrum. In a second experiment, psychometric functions w...

Development of a fast method for determining sensitivity to temporal fine structure

Recent evidence suggests that sensitivity to the temporal fine structure (TFS) of sounds is adversely affected by cochlear hearing loss. This may partly explain the difficulties experienced by people with cochlear hearing loss in understanding speech when background sounds, especially fluctuating backgrounds, are present. We describe a test for assessing sensitivity to TFS. The test can be run using any PC with a sound card. The test involves discrimination of a harmonic complex tone (H), with a fundamental frequency F0, from a tone in which all harmonics are shifted upwards by the same amount in Hertz, resulting in an inharmonic tone (I). The phases of the components are selected randomly for every stimulus. Both tones have an envelope repetition rate equal to F0, but the tones differ in their TFS. To prevent discrimination based on spectral cues, all tones are passed through a fixed bandpass filter, usually centred at 11F0. A background noise is used to mask combination tones. The results show that, for normal-hearing subjects, learning effects are small, and the effect of the level of testing is also small. The test provides a simple, quick, and robust way to measure sensitivity to TFS.

Implementation of a fast method for measuring psychophysical tuning curves

Abstract Objective: To implement a fast method for measuring psychophysical tuning curves (PTCs) for use in clinical applications, such as assessment of frequency selectivity and detection of dead regions in the cochlea. Design: The method is based on that described by 17 and has been implemented in software that can be run on a PC with a good-quality sound card. In addition to the main narrowband noise masker, a lowpass noise masker can be generated to prevent detection of a distortion band corresponding to the simple difference tone. Results: The software includes a routine for measuring the absolute threshold at the signal frequency and includes methods for estimating the frequency at the tip of the PTC. A PTC can typically be determined in about three minutes. A small amount of practice (two to three runs) may be required to achieve stable results. Conclusions: The software implementation allows PTCs to be measured quickly without a requirement for specialised equipment. Sumario Objetivo: Implementar un método rápido para medir las curvas psicofisiológicas de sintonía (PTC) para su uso en aplicaciones clínicas tales como la evaluación de la selectividad frecuencial y la detección de zonas muertas de la cóclea. Diseño: El método se basa en la descripción de 17 y ha sido implementado en un software que puede correr en una PC con una buena tarjeta de sonido. Además de tener un enmascarador con ruido de banda ancha principal, se puede generar un enmascarador con ruido pasa-bajo, para prevenir la detección de una banda de distorsión, que corresponde al tono simple de diferencia. Resultados: El software incluye una rutina para medir el umbral absoluto a la frecuencia de la señal e incluye métodos para estimar la frecuencia en la punta de la PTC. Típicamente se puede determinar una PTC en unos tres minutos. Es necesaria un poco de práctica (dos o tres turnos) para lograr resultados estables. Conclusiones: la implementación del software permite medir PTC rápidamente sin requerir equipo especializado.

Sensitivity of the human auditory system to temporal fine structure at high frequencies

The frequency of sounds is coded partly by phase locking to the temporal fine structure (TFS) of the waveform evoked on the basilar membrane. On the basis of data obtained using sinusoids, it is usually assumed that in mammals, including humans, TFS information becomes unusable for frequencies above about 5000 Hz. Here, sensitivity to the TFS of complex sounds up to much higher frequencies is demonstrated. Subjects discriminated a harmonic complex tone, with a fundamental frequency F0, from a tone in which all harmonics were shifted upwards by the same amount in hertz. The phases of the components were selected randomly for every stimulus. Both tones had an envelope repetition rate equal to F0, but the tones differed in their TFS. To prevent discrimination based on spectral cues, the tones were passed through a fixed bandpass filter, centered at 14F0. A background noise was used to mask combination tones. Performance was well above chance for most subjects when F0 was 800 or 1000 Hz and all audible components were above 8000 Hz. Supplementary experiments confirmed that performance was not based on changes in the excitation pattern or on the discrimination of partially resolved components.

Polish sentence tests for measuring the intelligibility of speech in interfering noise

The aim of this study was to develop Polish sentence tests for accurate measuring of speech intelligibility in masking interfering noise. Two sets of sentence lists have been developed. The first set was composed of 25 lists and was used for sentence intelligibility scoring. The second set was composed of 22 lists and was used for word intelligibility scoring. The lists in each set have been phonemically and statistically balanced. The speech reception threshold (SRT) and slope of the psychometric function at the SRT point (S50) were determined in normal-hearing subjects. It was found that the mean SRT and mean list-specific S50list for the first set were equal to −6.1 dB and 25.5%/dB, respectively. The mean SRT and the mean list-specific S50list for the second set were:−7.5 dB and 20.8%/dB. Due to a relatively steep slope of the psychometric functions, the Polish sentence tests were shown to be accurate materials for speech intelligibility measurements against interfering noise. They are the first sentence speech-in-noise tests developed for Slavonic languages.

Comparison of the CAM2 and NAL-NL2 hearing aid fitting methods.

Objective: To compare preference judgments for sounds processed via a simulated five-channel compression hearing aid with gains and compression ratios selected according to two recently introduced fitting methods, CAMEQ2-HF (hereafter named CAM2) and NAL-NL2. Design: There were 15 participants with sloping sensorineural hearing loss. They had mild losses, typical of people who might be candidates for wide-bandwidth hearing aids. Within a given trial, the same segment of sound was presented twice in succession to one ear, once with CAM2 settings and once with NAL-NL2 settings, in random order. The participant had to indicate which one was preferred and by how much. Judgments of overall sound quality were obtained for female and male speech in quiet and for four types of music (classical, jazz, a man singing, and percussion). Judgments of speech clarity were obtained for female and male speech in speech-shaped noise, female speech in a male-talker background, and male speech in a female-talker background. Factors investigated included compression speed (slow or fast) and input sound level (50, 65, or 80 dB SPL). Results: The pattern of the results was reasonably consistent across participants, but the magnitude of the effects was small. For judgments of overall sound quality, nine participants preferred CAM2 relative to NAL-NL2, and the remainder showed no clear preference. There was a significant overall preference for CAM2. The preference for CAM2 over NAL-NL2 in overall sound quality was present for all types of stimuli, both compression speeds, and all three levels. For judgments of the clarity of speech in noise, five participants preferred CAM2 over NAL-NL2, one showed the opposite preference, and the remainder showed no clear preference. There was a significant overall preference for CAM2. The preference for CAM2 over NAL-NL2 in the context of clarity of speech in noise was present for all types of stimuli, both compression speeds, and all three levels. For judgments of the clarity of speech in a background talker, CAM2 was significantly preferred overall relative to NAL-NL2, but the effect was very small. Conclusions: For participants with mild sloping hearing loss, a simulated hearing aid unilaterally fitted using CAM2 was preferred over the same aid fitted using NAL-NL2 for overall sound quality and the clarity of speech in noise. Preferences differed only very slightly for the clarity of speech in a background talker. Further work is needed to establish whether similar preferences would be found in everyday life.

Discrimination of frequency steps linked by glides of various durations

Thresholds were measured for detecting steps in frequency linked by glides of various durations. The goals were to assess the relative importance of place and temporal information for this task, and to determine whether there is a mechanism for detecting dynamic frequency changes per se, as opposed to comparing the initial and final frequencies of the stimuli. Subjects discriminated a 500-ms sinusoid of constant frequency from a sinusoid with three parts: an initial part with constant frequency, a downward frequency glide, and a final part with constant frequency. The overall duration was 500 ms, and the glide duration was varied from 5 to 500 ms. In one special case, the portion of the stimuli when a glide might occur was replaced by a brief silent interval. The center frequency was fixed at 0.5, 1, 2, 4, or 6 kHz (condition 1), or varied randomly from one stimulus to the next over a 4-ERB range around the nominal center frequency (condition 2). The randomization impaired performance, but thresholds remained lower than the best that could be achieved by monitoring either the initial or final frequency of the stimuli. Condition 3 was like condition 2, but for each stimulus a glide in level was added at the time when a frequency glide might occur, so the initial and final levels differed; the glides in level varied randomly in extent and direction from one stimulus to the next over the range +/- 20 dB. This impaired performance, but thresholds remained lower than the best that could be achieved by monitoring changes in excitation level on only one side of the excitation pattern. Excitation-pattern models of frequency discrimination predict that thresholds should not vary across center frequency when expressed as the change in ERB number, delta E. For all conditions, delta E values increased at 6 kHz, suggesting a role for temporal information at lower frequencies. The increase was smallest for the longest glide duration, consistent with a greater relative role of place information when there was no steady state portion. Performance was better when a brief glide was present than when no glide was present, but worsened with increasing glide duration. The results were fitted well by a model based on the assumption that information from the steady parts of the stimuli (perhaps extracted mainly using temporal information) was combined with information from the glides (perhaps extracted mainly using place information).