experiment II. For three of five source positions the sensitivity to the ’spectral morph-ing’ procedure is reduced in the reverberant condition. However, for higher values of the morphing factor α, the detection rate is above the threshold. Therefore, although the sensitivity of subjects to individual information is reduced, the reduction is not sufficient for using dummy head HRTFs without a change in the spatial perception.
The investigations of the HRTFs presented in Chapter 3 show that the spectral differ-ence between subjects are smaller than the differdiffer-ences between a subject and the dummy head. Therefore, it can be assumed that the reduction in sensitivity to the spectral cues is sufficient for using individualized HRTFs. This can be investigated by using non-individualized HRTFs for the spectral morphing’ procedure rather than dummy head HRTFs. If the detection performance is below the threshold even for higher values of α this would indicate that in reverberant conditions individual spectral information is not needed. However, this has to be investigated.
The sensitivity to ITD variations is also reduced in reverberant environments. From the investigation in Chapter 3on the ITD standard deviation across subjects (¯σ= 40.1 µs) it can be seen that the differences between subjects are within the dimension of the ITD JND estimated in experiment III. Hence, in reverberant conditions the need for individual ITD information in the direct sound is reduced.
Spatial elevation perception of a spectral source cue
Abstract
Spectral scrambling is applied to the spectrum of the stimulus in localization experiments to prevent the subject from using spectral timbre variations as a cue. It has not been in-vestigated yet, if the spectral scrambling introduces a localization cue that affects the ap-parent stimulus position. The spectral scrambling of the source spectrum could introduce a monaural cue that influences the elevation perception. Therefore, in the experiment presented here, the influence of a spectral cue in the source spectrum on the perceived elevation was studied for a noise stimulus (500 ms length) that is projected to the hor-izontal plane by using virtual acoustics. The spectrum of the source sound contains a monaural spectral cue that points to an elevation in the range of−40◦ to60◦. The task of the subject was to judge the perceived elevation in an absolute localization paradigm as a function of the spectral cue in the source spectrum. The results show that the spectral cue in the source spectrum significantly influences the perceived elevation with a maximum effect of 20◦. Hence, there is a need for developing scrambling methods that only change the perceived timbre but not the perceived localization of a given sound.
6.1 Introduction
The localization performance of the auditory system of human subjects is normally mea-sured by presenting a sound source at a certain stimulus position and asking the subject to report the perceived source location (see Chapter 2.3). To estimate the source po-sition of the stimulus the subjects can used binaural cues (interaural time differences, ITD and interaural level differences, ILD) as well as monaural spectral cues that are introduced by interference effects and pinna filtering. However, the monaural cues can
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serve as a spectral timbre cue that could be used by subjects to learn the timbre that corresponds to a certain stimulus position. To prevent the subjects from using timbre cues for the identification of the stimulus position, the spectrum of the sound source is often randomly scrambled in a certain level range before the stimulus is presented to the subjects.
Spectral scrambling has been used in a variety of localization studies. For instance, Wightman and Kistler varied the spectral amplitude of the source stimulus in critical bands by up to 20 dB in absolute localization experiments (Wightman and Kistler, 1989b;
Wightman and Kistler, 1992; Wightman and Kistler, 1997). The same manipulation of the source spectrum was used by Wenzel et al. (1993). Kulkarni et al. scrambled the source spectrum in 1/3 octave bands by up to ±5dB to prevent the subject from using non-spatial cues in a real/virtual source discrimination task. Langendijk and Bronkhorst varied the stimulus spectrum in 1/3 octave bands in order to investigate if subjects are able to virtual stimuli generated with interpolated HRTFs (Langendijk and Bronkhorst, 2000). In the experiments described in Sections 4 and 5 the stimulus spectrum was scrambled in 1/6 octave bands by up to ±5dB.
However, the spectral shape, that is introduced to the source spectrum by scrambling, could contain spatial information that could be processed by the auditory system. There-fore, the perceived stimulus position could change depending on the scrambled source spectrum. Hence, to quantify the bias that could be introduced by spectral scrambling, the affect of a monaural spectral cue in the spectrum of a broadband stimulus is inves-tigated in the current study by performing an absolute localization experiment.
It is well known that narrow band stimuli can cause confusions to the auditory system.
The apparent source position can be determined by the center frequency of the nar-row band sound independent of the actual source position. For instance, Blauert (1969) found that the perceived source position of 1/3-octave band noise signals, emanated from locations in the median plane, is only determined by the center frequency of the stimuli.
Butler and Helwig (1983) varied the center frequency of 1 kHz wide noises and showed that the perceived spatial position in the median plane goes from front to back, as the center frequency is increased from 4 kHz to 12 kHz. Musicant and Butler (1985) showed that the center frequencies of 1 kHz wide noise also determine the perceived localization in the horizontal plane, where binaural cues are usable for the subjects.
The physical properties that relate to the judged locations of the narrow band stimuli can be found in the head related transfer functions (HRTFs). They describe the direc-tional dependent transformation of a sound from its source location to a point in the ear canal. The HRTFs for the judged locations have peaks at frequencies that correspond to the center frequencies of the narrow band signals. Depending on studies these peaks are called ’boosted bands’ (Blauert, 1969), ’covert peaks’ (Flannery and Butler, 1981;
Musicant and Butler, 1984) or ’proximal stimulus spectra’ (Middlebrooks, 1992). Thus, salient peaks in the spectra of a stimulus bias the apparent stimulus location to the
po-sition for which the HRTF spectra have a peak in the corresponding frequency range. It is likely that the spectrum of scrambled broadband stimuli has prominent peaks in some frequency bands. Therefore, the auditory system could relate the peaks introduced in the source spectrum by spectral scrambling to spectral filtering by the HRTFs. As a result the apparent position of the stimuli would vary randomly for each scrambled stimulus spectrum. Although this explanation seems to be plausible, it has not been investigated in a systematic way if monaural source cues in a broadband stimulus spectrum affect the spatial perception.
Hence, in the study presented here it is investigated if the elevation perception of a broad band stimulus is affected by a monaural cue in the source spectrum. A noise stim-ulus is randomly presented from one of five different azimuth positions in the horizontal plane by using the methods of virtual acoustics (e.g. (Wightman and Kistler, 1989a;
Hammershoi, 1995)). Before the virtual stimulus is generated the spectrum of the noise is multiplied with the spectrum of a HRTF of one ear measured at the same azimuth position. However, the elevation of the HRTF spectrum was chosen from θ = −40◦,−20◦,0◦,20◦,40◦,60◦. Hence, a broad band stimulus is projected to the hor-izontal plane that contains a monaural source cue that points to a different elevation at the same azimuth. The task of the subjects was to judge the source location as a function of the monaural source cue in an absolute localization task. If the perception of the stimulus is independent from its spectrum then the subjects should localize each stimulus in the horizontal plane. If, however, the monaural source cue influences the perception of the stimulus, the judged elevation should increase as the elevation of the monaural source cue is increased. Two experiments were conducted. In the first experi-ment the monaural spectrum of the left ear HRTF was applied to the white noise source and in the second experiment the monaural spectrum of the right ear HRTF was applied to the stimulus spectrum.