Werner A. Deutsch

Sensitivity to interaural level and envelope time differences of two bilateral cochlear implant listeners using clinical sound processors.

Objectives: To assess the sensitivity of two bilateral cochlear implant users to interaural level and time differences (ILDs and ITDs) for various signals presented through the auxiliary inputs of clinical sound processors that discard fine timing information and only preserve envelope cues. Design: In a lateralization discrimination experiment, the just noticeable difference (JND) for ILDs and envelope ITDs was measured by means of an adaptive 2-AFC method. Different stimuli were used, including click trains at varying repetition rates, a speech fragment, and noise bursts. For one cochlear implant listener and one stimulus, the sensitivity to envelope ITDs was also determined with the method of constant stimuli. The dependency of ILD-JNDs on the interaural place difference was studied with stimulation at single electrode pairs by using sinusoidal input signals in combination with appropriate single-channel processor fittings. In a lateralization position experiment, subjects were required to use a visual pointer on a computer screen to indicate in-the-head positions for blocks of stimuli containing either ILD or ITD cues. All stimuli were loudness balanced (before applying ILD) and fed directly into the auxiliary inputs of the BTE processors (TEMPO+, Med-El Corp.). The automatic gain control and the processors’ microphones were deactivated. Results: Both cochlear implant listeners were highly sensitive to ILDs in all broadband stimuli used; JNDs approached those of normal-hearing listeners. Pitch-matched single electrode pairs showed significantly lower ILD-JNDs than pitch-mismatched electrode pairs. Envelope ITD-JNDs of cochlear implant listeners obtained with the adaptive method were substantially higher and showed a higher test-retest variability than waveform ITD-JNDs of normal-hearing control listeners and envelope ITD-JNDs of normal-hearing listeners reported in the literature for comparable signals. The envelope ITD-JNDs for the click trains were significantly lower than for the speech token or the noise bursts. The best envelope ITD-JND measured was ca. 250 &mgr;s for the click train at 100 cycles per sec. Direct measurement of the psychometric function for envelope ITD by the method of constant stimuli showed discrimination above chance level down to 150 &mgr;s. The lateralization position experiment showed that both ILDs and envelope ITDs can lead to monotonic changes in lateral percept. Conclusions: The two cochlear implant users tested showed strong effects of ILDs in various broadband stimuli with respect to JNDs as well as lateralization position. The high dependency of ILD-JNDs on the interaural pitch difference suggests the potential importance of pitch-matched assignment of electrodes in the two ears by the speech processors. Envelope ITDs appear to be more ambiguous cues than ILDs, as reflected by the higher and more variable JNDs compared with normal-hearing listeners. The envelope ITD-JNDs of cochlear implant listeners depended on the stimulus.

Time–Frequency Sparsity by Removing Perceptually Irrelevant Components Using a Simple Model of Simultaneous Masking

We present an algorithm for removing time-frequency components, found by a standard Gabor transform, of a ldquoreal-worldrdquo sound while causing no audible difference to the original sound after resynthesis. Thus, this representation is made sparser. The selection of removable components is based on a simple model of simultaneous masking in the auditory system. Important goals were the applicability to any real-world music and speech sound, integrating mutual masking effects between time-frequency components, coping with the time-frequency spread of such an operation, and computational efficiency. The proposed algorithm first determines an estimation of the masked threshold within an analysis window. The masked threshold function is then shifted in level by an amount determined experimentally, and all components falling below this function (the irrelevance threshold) are removed. This shift gives a conservative way to deal with uncertainty effects resulting from removing time-frequency components and with inaccuracies in the masking model. The removal of components is described as an adaptive Gabor multiplier. Thirty-six normal hearing subjects participated in an experiment to determine the maximum shift value for which they could not discriminate the irrelevance filtered signal from the original signal. On average across the test stimuli, 32 percent of the time-frequency components fell below the irrelevance threshold.

Characteristics of Fricatives and Sentence Duration after Cochlear Implantation

Ten postlingually deafened patients (5 male, 5 female) were examined after cochlear implantation to measure improvements in their quality of speech. Parameters such as the spectral maximum of fricatives and the duration of utterances were analysed in speech recordings taken at regular intervals after implantation. The speech samples were recorded in an audiological chamber. Parameters were analysed using STx (S-Tools Software). Frequency analyses based on the fast Fourier transform and spectral estimation methods, as well as fundamental frequency and formant extraction (cepstrum, LPC = linear prediction coding) and digital filter implementations were prepared. The results indicate a tendency towards improvement in the spectral maximum of the fricatives and affricates and a shortening of the duration of the fricative parts in affricates and of sentences in nearly all our subjects. These results showed the restored auditory feedback produced by cochlear implantation to have a favourable effect on speech production.

Improvement of Coarticulation in Broca’s Aphasia

In the clinical setting, it often is necessary for therapeutic reasons to objectivize a patient’s recovery process in a specific domain over time. This applies particularly to the articulatory abilities of the Broca’s aphasic. In such a patient one perceives quantititative and qualitative differences in performance. However, it is difficult to describe the changes in performance in an objective manner. The purpose of the study reported here is to show in a single case study of Broca’s aphasia how reliable information about improvement in language functions can be obtained without administering special tests. In this study, we analyzed and measured changes in coarticulatory speech behavior by applying direct and indirect measures and parameters not often used or described in the literature. Validity of the hypotheses and results were tested by means of statistical computations.

Internal representation of spectral information in dependence of signal type and auditory filter bandwidth

Previous studies have shown that enhancement of the spectral background (lower level spectral components) improves intelligibility as well as listening comfort of music signals in subjects with broader than normal auditory filters. This is in contrast to reported results on speech perception in noise, yielding reduced intelligibility as a result of spectral contrast reduction. A proposed model for the internal object representation by separation of ‘‘spectral layers’’ accounts for these opposing effects. ‘‘Spectral layers,’’ corresponding to different sound sources [in music: dominant/accompanying musical voices; in speech‐in‐quiet: formant peaks/modulation sidebands of the peaks (transitions); in speech‐in‐noise: speech foreground/speech background/noise components] are represented in coherent spectral amplitudes. The audibility of ‘‘spectral layers’’ is derived from individual masking properties. Pilot experiments with speech tokens and short music fragments agree relatively well with the model predicti...

NOIDESc: A novel traffic noise description scheme

A novel framework for the acoustic and psychoacoustic description of noise signals (railway and road) has been proposed in order to extend and improve traditional noise classification schemes. In addition to measures of averaged sound‐pressure level and noise exposure duration, the description scheme includes psychoacoustic parameters such as spectral centroid, spectral spread, modulation spectrum, and further dynamic aspects of the sound events corresponding in part to the MPEG‐7/4 standard. The dynamic changes between foreground and background are addressed in the model. The approach assumes the evaluation of several low‐level acoustic parameters of the noise, to be integrated into few high‐level features by means of statistical methods, such as Gaussian mixture models and cluster analysis. As the usage of noise monitoring systems is expected to increase in the near future, the cumulative collection of calibrated sound recordings will provide comprehensive regional and supraregional sound databases to serve as input for automated noise classification. [The work in progress is performed in cooperation with the division of Acoustics of the TGM Vienna and is supported by the Austrian FFG.]

A spectral layer model in speech and music perception

According to a model of the peripheral hearing system establishing a masking and overmasking paradigm, speech and music signals have been split into several spectral layers by means of spectral subtraction. Spectral components below the so‐called psychoacoustical irrelevance threshold are eliminated in a first processing step. The resulting signal, which exhibits no noticeable difference to the unprocessed one, is fed into an overmasking procedure applying flattened hearing masking functions. This action implements a kind of a spectral peak detector device. Consequently, the discrimination between spectral peaks (auditory figure) and background components, especially in reverberant environments, is improved. As a result, different audible signal parts according to different spectral layers are obtained, separating spectral peaks and weaker spectral components. In a certain range, both parts of speech and music signals are equally intelligible. In music, leading voices can be extracted and separated from t...

Accentuation of spectral pitch

Lower harmonics of steady‐state complex tones are resolved and produce individual spectral pitches when attention is focused to them. Attention to individual harmonics is enforced by sharp onset cues as simply switching partials on and off. Periodically rising and falling changes of the fundamental frequency of a complex tone (range: 1 oct, rate: 1–0.5 s) results in concurrent percepts. Initially accentuated spectral pitches of short duration are perceived followed by continuing integration of the partials into the complex tone according to the ‘‘common fate’’ rule. In comparison to the falling fundamental frequency phase higher partials accentuated during the rising phase are integrated faster. Concurrent spectral and fundamental frequency information exhibit a distinct pitch ambiguity transition immediately after the onset of the spectral pitch. The duration of the transition stage is assumed to reflect a kind of ‘‘Gestalt’’ generation time constant until the complex tone accepts its new incoming compon...

Effects of changes of the spectral masking slope on sound quality and clarity of music sounds in the normal and impaired ear

Reduced frequency selectivity as occurs in the cochlear impaired ear has been shown to be a main factor for degraded speech perception [J. M. Festen and R. Plomp, J. Acoust. Soc. Am. 76, 652–662 (1983)]. It is also assumed to have negative effects on music perception. In this paper the hypothesis is tested that the loudness relations between spectral peaks and nonmasked lower‐level components, which depend on the individual fre‐ quency selectivity, are important for (a) subjective sound quality (listening comfort) and (b) clarity, as measured in terms of detectability of altered notes in music excerpts. In order to test this hypothesis a signal processing algorithm has been utilized which enhances or suppresses the lower‐level spectral components of a complex signal according to a masking function. Processed signals have been presented to normal‐hearing and cochlear‐impaired subjects. Results revealed that subjects with reduced frequency selectivity—as measured with psychoacoustical tuning curves—tend to ...

Pitch and timbre of inharmonic tone complexes of Balinese “Gender Wayang” instruments

The tones of the scale of two Balinese “Gender Wayang” instruments have been stored on computer disk and are used as test tones in pitch detection experiments. The subjects were instructed to identify the pitch of the inharmonic instrument tones by tuning a sinusoidal test tone on the appropriate pitch height in a computer controlled up/down adaptive procedure. The experimental results support the general ambiguity of pitch perception in case of inharmonic tone complexes, though a certain preference of the subjects on the lowest partial could be observed. This perceptual ambiguity could be increased further when the first partial of the instrument tones was removed by means of narrow‐band digital filtering. Based on frequency analysis data of the natural sounds, a synthesis of the figuration “melody” of the Gender Wayang piece “Lagudelem” has been performed in which the inharmonic complex tones have been replaced by harmonic ones. The comparison of all three, the original sound probes and the filtered and...