Listening tests were conducted in order to analyze the localization of band-limited stimuli in vertical stereophony. The test stimuli were seven octave bands of pink noise, with center frequencies ranging from 125-8000 Hz, as well as broadband pink noise. Stimuli were presented from vertically arranged loudspeakers either monophonically or as vertical phantom images, created with the upper loudspeaker delayed with respect to the lower by 0, 0.5, 1, 5, and 10 ms (i.e., interchannel time difference). The experimental data obtained showed that localization under the aforementioned conditions is generally governed by the so-called "pitch-height" effect, with the high frequency stimuli generally being localized significantly higher than the low frequency stimuli for all conditions. The effect of interchannel time difference was found to be significant on localization judgments for both the 1000-4000 Hz octave bands and the broadband pink noise; it is suggested that this was related to the effects of comb filtering. Additionally, no evidence could be found to support the existence of the precedence effect in vertical stereophony. INTRODUCTIONThe mechanisms used to localize sound sources incident from the median plane are fundamentally different from those used in horizontal plane localization. In the horizontal plane, localization is reliant on a combination of the time and level differences between a given source arriving at each ear (binaural cues) as well as on the directional filtering of the sound source by the pinnae (spectral cues) [1]. However, in the median plane binaural cues are absent as sound sources arrive at each ear simultaneously. As a result, median plane localization relies solely on spectral cues [2].Median plane localization is a topic that has received much attention in the literature, with numerous studies being particularly concerned with the localization of tonal and band limited stimuli. In early experiments using tonal stimuli presented from vertically arranged loudspeakers, Pratt [3] concluded that localization is governed solely by frequency, with high tones being localized physically higher in space than low tones. A similar observation was made by Trimble [4], who presented tonal stimuli both singularly and in succession to listeners via receiving phones positioned 15 cm from each ear. A more expansive study by Roffler and Butler [5], also using tonal stimuli presented from vertically arranged loudspeakers, affirmed the results presented in [3] and [4], with the authors noting that the effect was maintained irrelevant of listener orientation, visual bias, and whether or not subjects had prior knowledge of the terms "high" and "low" in describing pitch. Subsequent experiments by Roffler and Butler [6] and Cabrera and Tiley [7] demonstrated that the relationship between pitch and height is maintained for the localization of bandpassed noise signals and moreover that the perceptual range of pitch-height depends on the physical height of the loudspeaker that presents the signal. In [7] the ...
Abstract:In subjective listening tests, natural sound sources were presented to subjects as vertically-oriented phantom images from two layers of loudspeakers, 'height' and 'main'. Subjects were required to reduce the amplitude of the height layer until the position of the resultant sound source matched that of the same source presented from the main layer only (the localisation threshold). Delays of 0, 1 and 10 ms were applied to the height layer with respect to the main, with vertical stereophonic and quadraphonic conditions being tested. The results of the study showed that the localisation thresholds obtained were not significantly affected by sound source or presentation method. Instead, the only variable whose effect was significant was interchannel time difference (ICTD). For ICTD of 0 ms, the median threshold was −9.5 dB, which was significantly lower than the −7 dB found for both 1 and 10 ms. The results of the study have implications both for the recording of sound sources for three-dimensional (3D) audio reproduction formats and also for the rendering of 3D images.
Listening tests were conducted in order to investigate the frequency dependency of localization thresholds in relation to vertical interchannel crosstalk. Octave band and broadband pink noise stimuli were presented to subjects as phantom images from vertically arranged stereophonic loudspeakers located directly in front of the listening position. With respect to the listening position the lower loudspeaker was not elevated; the upper loudspeaker was elevated by 30• . Subjects completed a method of adjustment task in which they were required to reduce the amplitude of the upper loudspeaker until the resultant phantom image matched the position of the same stimulus presented from the lower loudspeaker alone. The upper loudspeaker was delayed with respect to the lower by 0, 0.5, 1, 5, and 10 ms. The experimental data demonstrated that the main effect of frequency on the localization threshold was significant, with the low frequency stimuli (125 and 250 Hz) requiring significantly less level reduction (less than 6 dB) than the mid-high (1, 2, and 8 kHz) frequency stimuli (9-10.5 dB reduction). The main effect of interchannel time difference (ICTD) on the localization thresholds for each octave band was found to be non-significant. For all stimuli interchannel level difference (ICLD) was always necessary, indicating that the precedence effect is not a feature of median plane localization.
Direct sound that is captured by the upper layer of a three-dimensional (3D) microphone array is typically regarded as vertical interchannel crosstalk (VIC), since it tends to produce an undesired effect of the sound source image being elevated from the ear-level loudspeaker layer position (0°) in reproduction. The present study examined the effectiveness of band-limited VIC attenuation methods on preventing the vertical image shift problem. In a subjective experiment, five natural sound sources were presented as vertically-oriented phantom images while using two stereophonic loudspeaker pairs elevated at 0° and 30° in front of the listener. The upper layer signal (i.e., VIC) was attenuated in various octave-band-dependent conditions that were based on vertical localisation thresholds obtained from previous studies. The results showed that it was possible to achieve the goal of panning the phantom image at the same height as the image produced by the main loudspeaker layer by attenuating only a single octave band with the centre frequency of 4 kHz or 8 kHz or multiple bands at 1 kHz and above. This has a useful practical implication in 3D sound recording and mixing where a vertically oriented phantom image is rendered.
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