Dau, Torsten

Thompson, Eric Robert

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 16, 2017Characterizing binaural sensitivity to dynamic interaural level differencesThompson, Eric Robert; Dau, TorstenPublished in:Fortschritte der Akustik - DAGA 2006Publication date:2006Link back to DTU OrbitCitation (APA):Thompson, E. R., & Dau, T. (2006). Characterizing binaural sensitivity to dynamic interaural level differences. InFortschritte der Akustik - DAGA 2006Characterizing binaural sensitivity to dynamic interaural level differencesEric R. Thompson1, Torsten Dau11 Tech. Univ. of Denmark, DK-2800 Lyngby, Denmark, Email: [et, tda]@oersted.dtu.dkIntroductionThe binaural system is usually described as being slug-gish and there are many reports in the literature of timeconstants of binaural temporal windows, ranging fromabout 10 ms to 250 ms, depending on the experimentalmethod. A recent physiological study [1] reported someneurons that respond to fluctuations in interaural corre-lation and encoded rates to at least an order of magnitudehigher than the detectable rates reported in psychoacous-tic studies using similar stimuli. In the current study, twopsychoacoustic experiments were performed in order todetermine if the binaural system is able to make use ofthe available temporal information or if a simple lowpassfilter is sufficient to describe the performance of humanlisteners in binaural detection tasks with dynamic inter-aural cues.MethodsHigh frequency stimuli, centered at 5 kHz, were used forboth experiments so that only the envelope of the signaland not its fine structure should be available as inputto binaural processors. The test subjects were presentedwith three binaural sound intervals over headphones ina sound attenuating booth. The signals had a durationof 500 or 1000 ms, depending on the signal modulationfrequency. In the first experiment, a binaural extensionof experiments from [2] and [3], all three intervals hada sinusoidal amplitude modulation (SAM) imposed onpure-tone or interaurally uncorrelated narrowband noise(3, 30, or 300 Hz wide) carriers. In two of the intervals(reference condition), the SAM was diotic, while in a ran-domly selected signal interval, the SAM had an interauralphase shift of pi, as shown in Equation 1.xL = (1 +m sin(2pifst+ φ))c1(t)xR = (1 +m sin(2pifst+ φ+ pi))c2(t) (1)The test subject had to identify the interval with theantiphasic SAM in a 3-alternative, forced-choice experi-ment. Since all three intervals were similarly amplitudemodulated, detection could only be performed throughthe combination of modulation phase information in thebinaural system. The modulation depth m was var-ied adaptively using a 2-down, 1-up paradigm until thethreshold was determined as the median of six reversalswith a step size of 1 dB. The data shown in the results arethe mean and standard deviation of four normal-hearingtest subjects, including the authors, with four repetitionseach.In the masked discrimination experiment, a pure-tonecarrier was amplitude modulated with an interaurallyuncorrelated narrowband noise maskers and a sinusoidalsignal as shown in Equation 2. Again, the reference in-tervals were modulated with the same sinusoidal signal,only without an interaural phase difference.xL = (1 + n1(t))(1 +m sin(2pifst+ φ))c(t)xR = (1 + n2(t))(1 +m sin(2pifst+ φ+ pi))c(t) (2)The bandwidth of the masker was fixed for each sig-nal modulation frequency at a half-octave centered atthe signal frequency. The power of the masker was alsofixed at -10 dB, and the modulation depth required fordiscrimination with a fixed signal modulation frequencywas measured for a range of masker center frequencies.This experiment is a binaural extension of a monauralexperiment from [4].ResultsThe measured thresholds required for discriminating be-tween interaurally antiphasic and homophasic SAM whenimposed on pure-tone and narrowband noise carriers areshown in Figure 1. The thresholds with a pure-tone car-rier (×) show a lesser sensitivity (i.e. greater modulationdepth required for detection) than corresponding monau-ral thresholds. The thresholds also increase with increas-ing modulation frequency at a rate of about 1.5 dB/oct.These thresholds are similar to those reported in [3].2 4 8 16 32 128−35−30−25−20−15−10−50Signal Modulation frequency (Hz)Modulation depth, 20log10m (dB)Spi (pure tone)NuSpi (3 Hz)NuSpi (30 Hz)NuSpi (300 Hz)Figure 1: Modulation depths required for discrimination ofinteraurally antiphasic AM from homophasic AM imposed onpure-tone (×) and 3 Hz- (▽), 30 Hz- (3) and 300 Hz-wide (2)interaurally uncorrelated noise bands.When using uncorrelated narrowband noise carriers, theinherent interaural fluctuations of the carriers themselvesmake it more difficult to detect imposed AM, which canbe seen in the overall increase of the thresholds for thethree noise carriers in Figure 1. The amount of maskingresulting from those inherent carrier fluctuations can becalculated by looking at the difference in threshold whenusing the noise carriers and the pure-tone carrier. Thisdifference is plotted in Figure 2. The masking curve witha 3 Hz-wide carrier starts high for low signal modulationfrequencies and decreases rapidly to an asymptotic value,while that for the 30 Hz-wide carrier starts at about thesame level of masking and stays high until above 32 Hz,and the masking for the 300 Hz-wide carrier starts at alower level for the 2 Hz signal modulation and remainsfairly flat over the entire measured frequency range. Theshapes of these curves are similar to the correspondingmasking curves measured monaurally in [2], but are gen-erally flatter and have a higher DC-offset at about 8 dBof masking. This could indicate similar bandpass tun-ing in the binaural system as proposed for the monauralsystem in [2] only with broader tuning.2 4 8 16 32 12802468101214161820Signal Modulation frequency (Hz)Binaural Masking (dB)3 Hz30 Hz300 HzFigure 2: Masking level caused by the intrinsic interauralfluctuations of the 3 Hz- (▽), 30 Hz- (3) and 300 Hz-wide (2)interaurally uncorrelated noise bands. Calculated as the dif-ference between the thresholds measured with noise carriersand that measured with the pure-tone carrier.The measured masked discrimination thresholds whenusing a narrowband noise AM masker are shown in Fig-ure 3. These threshold curves show a bandpass tuningcentered around the signal frequency with a slow high-pass roll-off and a faster lowpass roll-off. The bandwidthof this tuning is about twice as large as the correspondingtuning reported in [4] for the monaural system.2 4 8 16 32 64 128−22−20−18−16−14−12−10−8−6−4−20Masker Center Frequency (Hz)Signal Modulation Depth (dB)fsig=4 Hzfsig=8 Hzfsig=32 HzFigure 3: Modulation depths required for discrimination ofinteraurally antiphasic AM from homophasic AM imposed ona pure-tone carrier in unmasked (dashed line) and masked(solid lines) conditions. Measurements made with a fixedsignal AM frequency of 4 (3), 8 () and 32 Hz (©) anduncorrelated narrowband noise masker with fixed power andbandwidth for a range of masker center frequencies.Model SimulationThe same masked detection experiment was performedusing the binaural model from [5] as an artificial listener.This model uses a sliding integrator window, which actsas a lowpass filter, to account for binaural sluggishness.The predicted masking levels (i.e. difference of maskedand unmasked discrimination thresholds), normalized forsignal center frequency, are shown in Figure 4. Thesepredictions show a very slight increase in masking at thesignal frequency, but then show an increase in maskingfor higher masker center frequencies, as opposed to theroll-off seen in Figure 3. This confirms that a lowpassfilter model can not predict the tuning seen in the exper-imental results and suggests further development for thismodel using a bandpass filterbank similar to the monau-ral filterbank from [2].−4 −3 −2 −1 0 1 2 3 4051015Masker Frequency re Signal Frequency (oct.)Masking Level (dB)fsig=4 Hzfsig=8 Hzfsig=32 HzFigure 4: Model simulation of the same experiments shownin Figure 3 using the model from [5]. Data are plotted asmasking level (i.e. masked threshold minus unmasked thresh-old) vs. masker frequency relative to signal frequency in oc-taves.ConclusionsThere is frequency tuning in the binaural processing offluctuations in interaural level differences which can notbe accounted for by a simple lowpass filter. This indicatesthat the temporal resolution that has been measured inneurons that respond to binaural input may be availableto higher processing centers to use to segregate soundsources. The Breebaart model [5] will be extended toaccount for this data.References[1] Joris et al. Auditory midbrain and nerve responses tosinusoidal variations in interaural correlation. J. Neu-rosci. 26 (2006), 279-289[2] Dau et al. Modeling auditory processing of amplitudemodulation. I. Detection and masking with narrow-band carriers. J. Acoust. Soc. Am. 102 (1997), 2892-2905[3] Stellmack et al. Monaural and interaural temporalmodulation transfer function measured with 5-kHzcarriers. J. Acoust. Soc. Am. 118 (2005), 2507-2518[4] Ewert and Dau. Characterizing frequency selectiv-ity for envelope fluctuations. J. Acoust. Soc. Am. 108(2000) 1181-1196[5] Breebaart et al. Binaural processing model based oncontralateral inhibition. I. Model structure. J. Acoust.Soc. Am. 110 (2001), 1074-1088

Characterizing binaural sensitivity to dynamic interaural level differences

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Characterizing binaural sensitivity to dynamic interaural level differences

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