The Precedence Effect for Noise Bursts of Different Bandwidths. I

Acoust. Sci. & Tech. 24, 5 (2003) INVITED PAPER

The precedence eﬀect for noise bursts of diﬀerent bandwidths. I. Psychoacoustical data

Jonas Braasch, Jens Blauert and Thomas Djelani Institut fu¨r Kommunikationsakustik, Ruhr-Universita¨t Bochum, Universita¨tsstr. 150, 44780 Bochum, Germany ( Received 27 January 2003, Accepted for publication 12 May 2003 )

Abstract: The human ability to localize a direct sound source in the presence of reflected sounds is well known as localization dominance due to the precedence effect, formerly also called ‘‘the law of the first wavefront.’’ The fact that the localization dominance partly fails for signals with a very narrow bandwidth raises the question of whether the localization dominance requires cross-frequency-band interaction. To investigate this, a psychoacoustic experiment was conducted in which the perceived lateralization of a noise burst (300-s ITD) in the presence of a reflection (300-s ITD) was measured. The parameters in this experiment were the inter-stimulus interval (ISI) and the bandwidth. The first was varied among the following values: 0.0 ms and then from 1.0 ms in steps of 0.5 ms to 4.0 ms. The bandwidth was adjusted to be either 100 Hz, 400 Hz, or 800 Hz. The data of the listeners clearly show that localization dominance becomes more stable, especially with regard to the dependence on the ISI, when the bandwidth is increased. At the smallest bandwidth under examination (100 Hz), localization dominance cannot be observed for some of the listeners anymore, while others perceive at least the sound coming from the lateral side where the direct sound source is positioned; nevertheless, their auditory event varies strongly with the ISI. A signal analysis reveals that in the first case, the precedence effect does not seem to have any influence. Here, the listeners rather base their judgements on the ongoing part of the sound. The signal analysis also indicates that it is not necessary to assume that the signals in the different frequency bands interact directly with each other while being processed (e.g., cross-frequency coincidence units). It rather seems to be sufficient to average across those bands.

Keywords: Binaural hearing, Precedence eﬀect, Localization dominance

PACS number: 43.66.Mk, 43.66.Pn, 43.55.Hy [DOI: 10.1250/ast.24.233]

more separate auditory events (echos). Extended reviews 1. INTRODUCTION on the precedence effect were written by Blauert [3], The precedence effect is thought to be essential for Hartmann [4], Litovsky et al. [5] and Zurek [6]. humans to localize a sound source in reverberant environ- In most investigations on the precedence effect, a single ments. Our auditory system achieves this by suppressing reflection and the direct sound source were used as stimuli. the directional information regarding the reflections of the Here, the direct sound is usually referred to as the lead, sound source (localization dominance). However, local- while the reflection is called the lag. In early investigations, ization dominance is only observed, when the reflections natural sounds were employed (e.g., Haas [7], Wallach et arrive within a certain time interval, the inter-stimulus al. [8]), while in most recent investigations click pairs or interval (ISI), after the primary source. If the time interval trains of clicks are utilized. The advantage of using clicks is shorter than 1 ms, the position of the auditory event is is the fact that lead and lag do not temporally overlap at the determined by both the primary source and the reflections. entrance of the ear canals, as was recently emphasized by This effect is referred to as summing localization. If the Litovsky et al. [5]. time interval is larger than the echo threshold, which is Another interesting fact about the precedence effect is typically between 4.5 ms and 80 ms, depending on the that the occurrence of localization dominance strongly featured signals (e.g., signal duration [1] and signal depends on the signal’s bandwidth and the gradient of its bandwidth [2]), the reflections are perceived as one or onset slope. When utilizing sinusoidal signals, the localization dominance even disappears when the onset slopes now with: Vodaphone D2 GmbH, Du¨sseldorf, Germany of the signals are shallow, as shown by Rakerd and

233 Acoust. Sci. & Tech. 24, 5 (2003)

Hartmann for a 500-Hz sinusoidal signal with an onset understanding of the bandwidth-dependent factors of the duration of 7 s [9]. Likewise, the so-called Franssen illusion precedence effect. For this purpose a listening experiment can only be observed when the bandwidth of the test signal similar to the one conducted by Blauert and Cobben [2] is very narrow. This illusion [10,11] can be demonstrated was carried out. Two important changes to the experiment using two loudspeakers that are placed a few meters apart of Blauert and Cobben were made; (i) the center frequency from each other in front of a listener in a reverberant was kept constant at 500 Hz and only the bandwidth of the environment. In the beginning, a sinusoidal tone is exposed signal was varied between 100 Hz and 800 Hz, and (ii) the to the listener through one of the loudspeakers. After a few duration of the signals was increased to 200 ms. The first seconds, the sound is cross-faded to the second loudspeaker modification is necessary to minimize frequency-dependent (fading time 20–40 ms) and subsequently faded back to the factors of the auditory system when processing the stimuli first one. During the whole demonstration procedure, the (e.g., the dominance region of the sensitivity to ITDs [16]). listener has the appearance that the sinusoidal tone is The second modification was implemented to rule out presented from the first loudspeaker only.1 The Franssen secondary effects that are likely to occur, as the duration of illusion fails, when the loudspeakers are set-up in an clicks rises considerably with decreasing bandwidth of the anechoic environment. It is, therefore, thought to be related bandpass filter (filter ringing). In addition, one has to to the precedence effect. As indicated previously, the consider that clicks are very unrealistic stimuli, and a illusion fails for ongoing signals2 of broader bandwidth. temporal overlap between lead and lag is observed in most Interestingly, only a few investigations were conducted typical listening situations. This is one of the reasons, why to determine the influence of the bandwidth on the ongoing signals were used in this investigation. precedence effect. Blauert and Col investigated the echo threshold for bandpass-filtered impulses with different 2. METHODS center frequencies and bandwidths [13,14]. These authors 2.1. Listeners were able to show that the echo threshold for the different Six listeners (1 female, 5 males) participated in the stimuli varied between 4.5 ms (20-s broadband impulses) experiment. Their ages ranged from 23 to 30 years. All and 25 ms (bandpass-filtered impulses with a frequency listeners reported normal hearing. Four of the six listeners range from 270 to 340 Hz). (1, 2, 5 and 6) had extensive experience with psycho- In the work of Blauert and Cobben, the effect of the acoustical experiments beforehand. signal’s bandwidth was investigated indirectly, because the resulting bandwidth, which was always chosen to be 1/3 2.2. Stimuli octave wide, changed with the center frequency of the A bandpass-filtered, frozen white-noise burst (200-ms signal [2]. The signals employed were bandpass-filtered duration, 20-ms cos2 ramps) was used for both lead and clicks of 25-s duration, with a center frequency of either lag. The bandpass-filtered noise was generated digitally in 500 Hz, 1 kHz, or 2 kHz. In their investigation, the authors the frequency domain and afterwards transferred into the observed that the auditory events of the listeners varied time domain (16-bit resolution, 48-kHz sampling rate). with the ISI periodically to be the inverse of the signal’s Next the signals were D/A converted and delivered to the center frequency. Similar results were observed by Linde- listener through headphones (STAX, SR-Lambda). The mann [15] for narrowband click pairs at 500-Hz and 2,000- center frequency of the bandpass filter was kept constant at Hz center frequency. For broadband signals, such a 500 Hz, while the following bandwidths were tested: variation could not be observed. 100 Hz, 400 Hz, and 800 Hz. The lead and the lag only The aim of the following investigation is a better differed by their ITDs and in most conditions by a time delay (ISI) regarding the reflection. The general time course of the stimuli is shown in Fig. 1. In half of the trials, 1It should be noted that, in general, the listeners do not localize the the ITD of the lead was adjusted to 300 s, while the ITD test sound during the whole demonstration at the position of the first loudspeaker, as was pointed out by Blauert years ago [12]. They only perceive the sinusoidal sound coming from the direction left of the first loudspeaker during its onset phase. Afterwards, the channel auditory event is spread throughout the whole room, and the sound right channel remains more or less unlocalizable. When the sound is turned off, ITD 1 ITD 2 time ISI the position of the auditory event corresponds to the position of the lead physical sound source, which makes it necessary to fade the sound lag back to the first loudspeaker to continue the illusion until the end of the stimulus presentation. Fig. 1 Time course of the lead and lag that was used 2In this investigation we use the term ‘‘ongoing signal’’ in the sense throughout this investigation (not drawn to scale). ITD of a ‘‘non-impulsive signal,’’ but do not necessarily refer to a 1 refers to the ITD of the lead and ITD 2 to the ITD of ‘‘never-ending signal.’’ the lag.

234 J. BRAASCH et al.: PRECEDENCE EFFECT. I. PSYCHOACOUSTICAL DATA of the lag was adjusted to 300 s. In the remaining trials, using an acoustic pointer as described in [17]. The acoustic the ITD of the lead was adjusted to 300 ms, and the ITD pointer consisted of a narrowband noise (500-Hz center of the lag was set at 300 s. This was done to unveil frequency, 200-Hz bandwidth, 200-ms duration, 20-ms eventual asymmetries in lateralization of those type of cos2 on- and offset ramps). The listeners could adjust the stimuli. The ISI was pseudo-randomly varied among the interaural level difference (ILD) of the acoustic pointer to following values: 0.0 ms and 1.0 ms to 4.0 ms in steps of any value between 30 dB and 30 dB using a slider. They 0.5 ms. In the reference condition, only the lead was were asked to adjust the ILD of the acoustic pointer until presented with an ITD of 300 s, 0 ms, or 300 s. the perceived lateralization of the pointer matched the perceived lateral position of the test stimulus. The listeners 2.3. Procedure could play the test stimulus or the pointer as often as they The experiment was divided into several sessions. wanted by pressing two assigned buttons. After adjusting During one session, the bandwidth of the signal was kept the perceived lateral position of the acoustic pointer, the constant, and only the ISI and the spatial positions of lead listeners were asked to press a third button. With pressing (300-sor300-s ITD) and lag (300-sor300-s this button, the actual position of the acoustic pointer was ITD) were pseudo-randomly varied. In each session, each recorded and the next test stimulus was played. During the stimulus was repeated twice. For each bandwidth, three data collection, the listeners were blind-folded to avoid any sessions were conducted, so altogether, each stimulus was influence from visual cues. In an informal pilot test, which repeated six times. One session lasted about 12 minutes. was carried out before the experiment, it was found that the The sessions for the different bandwidths (and the delayed offset of the lag can lead to a moving auditory repetitions) were conducted in random order. event at the end of the stimulus presentation. Therefore, the The listeners participated in additional experiments to listeners were instructed to base their judgements on the determine the perceived lateral position for the test signals beginning and ongoing part of the stimulus presentation in absence of a reflection (reference condition). Again, in and to ignore its ending. None of the listeners reported each session, the bandwidth was kept constant at 100 Hz, perceiving a moving auditory event during the onset and 400 Hz or 800 Hz. The ITD of the test stimulus was the ongoing part of the stimulus presentation. pseudo-randomly varied among the values 300 s, 0 s and 300 s. Each stimulus was repeated three times in each 3. RESULTS session. For each bandwidth, three sessions were conduct- The results for the stimuli with a bandwidth of 100 Hz ed. are shown in Fig. 2. In the left graph, the results for the The listeners were advised to report the lateral position individual listeners are shown for the conditions where the of their auditory events for each stimulus presentation ITD of the lead was set at 300 s, and in the right graph,

ITD: 300 µs (lead), –300 µs (lag) ITD: –300 µs (lead), 300 µs (lag)

10 L1 L2 10 L1 L2

0 0

–10 –10

10 L3 L4 10 L3 L4

0 0

–10 –10

perceived left/right [dB] perceived 10 L5 L6 left/right [dB] perceived 10 L5 L6

0 0

–10 –10

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 ISI [ms] ISI [ms]

Fig. 2 Lateralization performance of six listeners localizing a noise burst of 100-Hz bandwidth in the presence of a single reﬂection. The ‘’s show the median result for each listener, the error bars represent the upper and lower quartiles. The left graph shows the results for the conditions, in which the ITD of the lead was set at 300 s, the right graph those conditions, in which the ITD of the lead was set at 300 s.

235 Acoust. Sci. & Tech. 24, 5 (2003) the ITD of the lead was set at 300 s. The ISI is set along stimulus. the x-axis and the perceived lateralization is plotted along When the bandwidth is increased to 400 Hz (Fig. 3), the the y-axis in form of the adjusted ILDs of the acoustic variability of the responses in the localization dominance pointer. The results for the three reference conditions (lead region (ISI = 1–4 ms) decreases. All measured response only): 300-s, 0-s, and 300-s ITD are shown by the patterns now belong to Type I and only in a very few cases dotted lines in form of the median. The matching ILD of are median data points found to be close to the reference the acoustic pointer for the lateralized test signals varies stimulus at 0-s ITD. The variation of the responses considerably among the listeners between j4j dB (L5) and decreases furthermore, when the bandwidth is increased to j10j dB (L1), resulting in trading ratios from 30 to 75 ms/ 800 Hz (Fig. 4). For all three measured bandwidths, the dB. perceived lateral position at 0-ms ISI is very close to the The median data for the lead-lag pair are depicted by perceived lateral position of the 0-s reference stimulus the crosses. The error bars show the upper and lower (summing localization). quartiles. Unsurprisingly, the results for the ISI at 0 ms, Even though only Type-I response patterns were which represents the case of summing localization, are very measured for bandwidths of 400 Hz and 800 Hz, the similar to the result for the 0-s ITD reference stimulus. In variation of the responses at these conditions correlate to the region of localization dominance (ISI = 1–4 ms), the the Type-I or II response patterns in the 100-Hz conditions position of the auditory events varies periodically with the for most of the listeners. A good example is Listener 2. At a inverse of the signal’s center frequency, which is 2 ms. The bandwidth of 100 Hz, the response patterns belonged to responses of the listeners can be grouped into two types. In Type I when the lead was set at 300 s, and to Type II the first type, Type I, the range of all reported perceived when the lead was set to 300 s. Although, the response positions were at that lateral side where the lead was, with patterns are all of Type I for larger bandwidths, the the exception of the reference condition, of course. In the responses are considerably closer to the reference stimulus second type, Type II, median data points were also at 0-s ITD when the lead is set to 300 s than when it is measured to the side of the lag, and the responses cover set to 300 s. Hence, the asymmetric response is preserved a range from the lateral position of the lead to the lateral for Listener 2 at larger bandwidths. position of the lag. The responses of the listeners 1, 2, and In Fig. 5, the average values of all listeners are shown 6 belong to Type I, the responses of the remaining listeners for the three employed bandwidths in those conditions in to Type II. which the lead was set at 300 s. In order to be able to pool Asymmetrical effects are observed, when the position the data, the responses of the individual listeners were of lead and lag are interchanged (Fig. 2, right graph). For normalized. To this end, those data points i that were half of the listeners (2, 4, and 5), the responses now belong measured right of the median auditory event of the to the opposite type. Apart from this effect, the results are reference stimulus (0-s ITD) were transformed using very similar to the conditions shown in the left graph, when the following equation: mirrored with regard to the position of the 0-s reference

ITD: 300 µs (lead), –300 µs (lag) ITD: –300 µs (lead), 300 µs (lag)

10 L1 L2 10 L1 L2

0 0

–10 –10

10 L3 L4 10 L3 L4

0 0

–10 –10

perceived left/right [dB] perceived 10 L5 L6 left/right [dB] perceived 10 L5 L6

0 0

–10 –10

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 ISI [ms] ISI [ms]

Fig. 3 Same as Fig. 2, but for a bandwidth of 400 Hz.

236 J. BRAASCH et al.: PRECEDENCE EFFECT. I. PSYCHOACOUSTICAL DATA

ITD: 300 µs (lead), –300 µs (lag) ITD: –300 µs (lead), 300 µs (lag)

10 L1 L2 10 L1 L2

0 0

–10 –10

10 L3 L4 10 L3 L4

0 0

–10 –10

perceived left/right [dB] perceived 10 L5 L6 left/right [dB] perceived 10 L5 L6

0 0

–10 –10

0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 ISI [ms] ISI [ms]

Fig. 4 Same as Figs. 2 and 3, but for a bandwidth of 800 Hz.

The remaining data points i that were measured left of the median auditory event of the reference stimulus (0-s 1 ITD), were transformed using the equation: 0 Type I, Type II, y ¼ i 0 s ; ð2Þ –1 i f∆=100 Hz f∆=100 Hz 0 s 300 s

1 with 300 s, the measured ILD for the reference stimulus with an ITD of 300 s. 0 perceived left/right perceived Type I, Type I, In principle, the perceived lateral position of the 300- –1 f∆=400 Hz f∆=800 Hz s reference stimulus is scaled to 1, the perceived lateral 0 1 2 3 4 0 1 2 3 4 position for the 0-s reference stimulus is scaled to zero, ISI [ms] and the perceived lateral position for the 300-s reference stimulus is scaled to 1 for each individual listener after the Fig. 5 Lateralization performance of six listeners local- transformation. Figure 5 shows the median results over all izing a noise burst in the presence of a single reﬂection with the lead being set at 300 s. The panels depict the listeners. In the top panels, the data for the 100-Hz averages of the results obtained for six listeners as bandwidth condition are shown. The data belonging to shown in Figs. 2–4. The top-left panel displays the Type I were averaged separately from those that belong to Type-I results for the 100-Hz signal-bandwidth con- Type II and are depicted in the top left panel (Listeners 1, dition, the top-right panel those for the Type-II results. In the bottom-left panel, the results for the 400-Hz 2, and 6). The data for Type II are given in the top-right signal-bandwidth condition (all Type I) are given, and panel (Listeners 3, 4, and 5). The data for the 400-Hz in the bottom-right panel, the results for the 800-Hz bandwidth condition are shown in the bottom-left and signal-bandwidth condition (all Type I) are depicted. bottom-right panel. In these two conditions all measured The ‘ ’s show the median results over all listeners, the response patterns were of Type I. error bars represent the upper and lower quartiles. The positions for the three reference conditions: 300-s, The average data for the conditions, in which the the 0-s, and 300-s ITD are shown by the dotted lines. lead was set at 300-s ITD, are shown in Fig. 6. Here, the responses of Listeners 1, 4, 5, and 6 were of Type I, and responses of the remaining listeners of Type II. Again, the i 0 s yi ¼ ; ð1Þ responses for the 400-Hz and 800-Hz-signal-bandwidth 300 s 0 s conditions were all of Type I. The observations that were with yi, the transformed data point; i, the measured ILD of derived from the data of the individual listeners are also the adjusted acoustic pointer for the ith data point; 0 s, the clearly visible in the pooled data. Especially, the periodical measured ILD for the reference stimulus with an ITD of variation of the auditory event with the inverse of the 0 s, and 300 s, the measured ILD for the reference signal’s center frequency and the decrease of this variation stimulus with an ITD of 300 s. with increasing bandwidth are to be named.

237 Acoust. Sci. & Tech. 24, 5 (2003)

f∆=100 Hz, f∆=100 Hz, Analogously to the analysis of the ITDs, the ILDs were 1 Type I Type II determined after the test stimuli had been processed by the 0 gammatone ﬁlter bank. The common equation: –1 Pl ILD ¼ 10 log10 ð3Þ f∆=400 Hz, f∆=800 Hz, Pr 1 Type I Type I P P 0 was used, with l and r the average power in the left and perceived left/right perceived right channels of the analyzed frequency band. –1 The results for the ILD analysis are shown in Fig. 8 0 1 2 3 4 0 1 2 3 4 (top graph: 100-Hz bandwidth condition, center graph; ISI [ms]

Fig. 6 Same as Fig. 5, but for those conditions in which the lead was set at 300 s.

4. SIGNAL ANALYSIS In order to explain the outcome of the psychoacoustic experiment, the stimuli used were analyzed. Figure 7 shows the average ITD as a function of the center frequency for the three tested bandwidths: top panel, 100 Hz; center panel, 400 Hz; bottom panel, 800 Hz. In each panel, the solid line shows the median and the dotted lines depict the upper and lower range of all tested ISIs. To determine the ITDs for the different frequency bands, the same signals that were used in the psychoacoustic experiment were sent through a gammatone filter bank with 35 bands with center frequencies from 50 Hz to 1,500 Hz. Afterwards, the cross correlation was calculated in each frequency band over the whole stimulus duration and the average ITD of the signal was determined by the centroid of the cross-correlation function. Unsurprisingly, the average ITDs are approximately zero for all tested stimulus configurations with some exceptions during the lead onset and lag offset phases.

s] 25 µ 0

ITD [ –25 f∆=100 Hz –50 5 10 15 20 25 30 35 50

s] 25 µ 0

ITD [ –25 f∆=400 Hz –50 5 10 15 20 25 30 35 50

s] 25 µ 0

ITD [ –25 f∆=800 Hz –50 5 10 15 20 25 30 35 Fig. 8 In the right panels, the average ILDs in different Number of frequency band frequency bands, as given by the gray-scale maps, are shown for different ISIs (x-axis) and frequency bands Fig. 7 Average ITDs in different frequency bands (from (y-axis) from top to bottom: 100-Hz, 400-Hz and 800- top to bottom): 100-Hz, 400-Hz and 800-Hz signal- Hz signal-bandwidth conditions. The left panels show bandwidth conditions. The solid line shows the average the relative power-density level (PDL) in each fre- of all tested ISIs, the dotted line the maximum and quency band. In the panel below each gray-scale plot, minimum average ITDs that were measured for the the average ILDs over all frequency bands (weighted tested ISIs. with the signal amplitude in each band) are given.

238 J. BRAASCH et al.: PRECEDENCE EFFECT. I. PSYCHOACOUSTICAL DATA

400-Hz bandwidth condition, bottom graph; 800-Hz band- lead: 300 µs ITD lead: –300 µs ITD width condition). The gray-scale plots show the measured 20 ILDs for different ISIs and frequency bands. A light 10 shading signalizes a positive ILD, a dark shading a 0 negative one, according to the maps right of the figures. –10 For a better resolution, more ISIs were considered in the analysis than were tested in the psychoacoustic experiment. perceived left/right [dB] –20 0 1 2 3 4 0 1 2 3 4 The ILDs were calculated for all ISIs between 0.0 and ISI [ms] 4.0 ms in steps of 0.1 ms. The left panels show the average power density level (average over all calculated ISIs and Fig. 9 Measured ILDs for the critical-frequency band both channels) for a better orientation. around 500-Hz center frequency (dotted lines). The ‘’s show the perceived lateralization of the test In the following analysis, only the areas with a stimuli for different ISIs as the ILDs of the adjusted relatively high amount of signal energy are considered acoustic pointer. Each data point is the median of all (> 20 dB of the maximum value). In the 100-Hz Type-II responses collected for all listeners at 100-Hz bandwidth condition, it is clearly visible that large ILDs signal bandwidth. The error bars represent the upper and lower quartiles. The left panel shows the results for (up to 20 dB) occur. The ILD varies periodically to the the conditions in which the ITD of the lead was set at inverse of the signal’s center frequency. Large ILDs are 300 ms, the right panel the conditions in which the ITD also observed in the 400-Hz and 800-Hz bandwidth of the lead was set at 300 s. conditions, but in those conditions, a positive ILD in one frequency band is usually compensated by a negative ILD from the influence of the lag as the classical literature on in another frequency band, as can be seen in the plots the precedence effect and the outcome of the classical directly below the gray-scale plots that show the weighted experiment of Wallach et al. [8] using click pairs would averaged ILDs. predict. The movement of the auditory event varies with The analysis is in good agreement to the findings of increasing ISI periodically to the inverse of the signal’s Wendt [18], who estimated for a standard-stereo loud- center frequency. speaker set-up that, in the low-frequency approximation, an The reason for this becomes clearer when we analyze intensity difference in both loudspeaker signals results in the results for the response patterns for Type II. As shown an ITD at the receiver’s ears, while a delay of one of the in Fig. 9, in those cases, the ILDs of the acoustic pointer two loudspeaker signals results in an ILD at the receiver’s matches the physically measured ILDs of the stimuli, ears. Our test configuration is comparable to the latter case. which lets us conclude that the listeners base their To emphasize a possible impact of the ILDs on the judgements on the ILDs of the ongoing part of the sound. perceived lateral displacement of the stimuli, the ILDs The precedence effect does not seem to play a role under measured for the 100-Hz bandwidth condition in the this condition. frequency band at 500 Hz are shown again in Fig. 9 (left As previously mentioned, Rakerd and Hartmann [9] panel, lead at 300-s ITD; right panel, lead at 300-s have shown that the precedence effect is not observed for ITD). In addition, the average adjustment of the acoustic sinusoidal signals when the onset is as long as 7 s. pointer as the median of all listeners whose response However, in our data, the precedence effect even frequently patterns suits Type II is given by ‘’s. The errorbars show failed at a much shorter onset time of 20 ms. Years ago, the interquartile range. In those cases, the adjustment of the Kunov and Abel showed for 1-kHz sinusoidal sounds that acoustic pointer fits the physically measured ILDs of the onset times up to 200 ms influence the perceived lateral- stimuli very well. ization by trading the envelope of the signal against its carrier (no reflections present) [19]. Above this value, the 5. DISCUSSION AND CONCLUSIONS judgements of their listeners were based on the ITD of the As the results of the psychoacoustical experiments carrier signal only. The outcome of our investigation convincingly show, the localization dominance is not fully shows, however, that the precedence effect can even fail developed when stimuli with a very narrow bandwidth when it is expected that the onset of the sound will have (100 Hz) are used. Under this condition, the responses of influence on the perceived lateral position of the auditory the listeners can be grouped into two groups, Type I and event. Type II. In the response patterns that fall into Group I, Rakerd and Hartmann made another interesting finding localization dominance is observable as far as the side, left in their paper [9], which lead them to formulate their or right, of the auditory event is concerned; the auditory ‘‘plausibility hypothesis.’’ In this hypothesis they conclude event is always on the side of the lead (Figs. 2 and 5). On that the auditory system determines the sound-source the other hand, the auditory event is not as independent positions from plausible cues only. According to their

239 Acoust. Sci. & Tech. 24, 5 (2003) theory, localization cues that are implausible are subse- effect in some cases. The observation that the precedence quently ignored. Binaural cues can be accounted as being effect is not present in certain cases is in agreement with a implausible to the auditory system whenever their values series of papers by Braasch and Hartung [22,23], who were are unnatural or they do not match other cues (e.g., visual able to show that reflections in a small room can simply be cues). Rakerd and Hartmann assumed that the auditory treated as additive noise. In their investigation, the outcome system would weight the cues according to their plausi- of the localization experiment could be well simulated with bility. In their investigation, it turned out that the ITDs a cross-correlation model by averaging over the ongoing were frequently discounted when they had very large signal. values. So far, we cannot explain why two groups of response Preliminary analysis of the response patterns belonging patterns exist nor can we correlate the grouping to other to Type II seem to fit well into Rakerd and Hartmann’s features of the auditory system. Even though both types of hypothesis that the ITD cues are discounted in the presence response patterns occurred approximately equally often, six of reflections. However, we cannot conclude from our data listeners are not enough to provide a statistically reliable that the auditory system bases its analysis on the ILDs only. estimate of the percentages of the occurrence of Type-I and The data can be easily explained by assuming that the Type-II cases. There is evidence, however, that this effect precedence effect simply does not occur. The acoustic is constant for each hemisphere of the individual listener. pointer that was used in the experiments reported here had At least, we could not observe so far that the response a constant ITD of 0 s, while the ILD was variable. There patterns for a fixed condition changed during the experi- would be no reason to assume that the auditory system ment, even though there was usually at least one day would discount the ITD cues of the pointer when between two sessions for the same condition. determining its position. Based on a number of previous It should be noted that our results agree quite well with trading experiments [20,21], we would rather conclude that the results of Blauert and Cobben [2] and also with those of there is a trade-off between the ITD and the ILD cues. If Lindemann [15]. The explanation why Blauert and Cobben, the ILD is adjusted to a value other than zero, we would and Lindemann only observed Type-I response patterns expect the ILD to push the auditory event toward the side, with smaller deviation of the responses from the perceptual while the ITD cues would pull the auditory event toward position of the lead is easily found; Blauert and Cobben, the center of the head. Since the ILD of the adjusted and Lindemann used bandpass-filtered click trains that are acoustic pointer fits the ongoing ILDs of the lead-lag much shorter in duration than our 200-ms noise bursts. In stimuli almost perfectly well, there is no need to assume addition, the onset slopes of our stimuli were longer in that the ITDs are discounted. Furthermore, we would not duration. expect that the ILDs of the pointer match the ILDs of the With increasing bandwidth, the periodical variation of lead-lag stimulus when the ITDs of the latter are the auditory event with the ISI declines, and the local- discounted. In this case, trading could not occur and the ization dominance is observed in a form which is closer to ITD cues would not pull the auditory event toward the theory, namely that the perceived lateralization is inde- center of the head. pendent from the lag. Especially at a bandwidth of 800 Hz, It is obvious that the response patterns of Type I and the auditory events are close to the auditory event for the Type II correlate with each other (Fig. 5, top panels). In lead without lag. This phenomenon is easy to explain. Even both patterns, the listeners perceive the lead-lag stimulus though large ILDs occur also in those conditions (see closest to the position of the auditory event regarding the Fig. 8), the average ILD over the whole frequency range lead only at ISIs of 1.5 ms and 3.5 ms, while its deviation gets closer to zero the wider the frequency range is from the auditory events of the lead is greatest at an ISI of adjusted, because of the different periodicities of the center 2.5 ms. In the latter case, only the extent of the deviation is frequencies of the bands. In order to be able to create such different between both types. While in the response pattern high ILDs through interference, lead and lag have to of Type II, the auditory event at an ISI of 2.5 ms matches correlate greatly with each other. Since the reflection is the perceived position of the lag, the responses of Type I basically a time-delayed copy of the lead, the autocorre- are near the center of the head, but still on the side of the lation length has to be of considerable duration. For lead. One way to explain the occurrence of two types of Gaussian noise, the autocorrelation length increases with response patterns is to assume the precedence effect plays decreasing bandwidth. This explains why the resulting ILD only a role for the Type-I response patterns, while the was found to be much lower at larger bandwidths. The precedence effect is not needed to explain the data for the outcome of the ILD analysis lets us assume that interaction Type-II response patterns. Under this assumption, the between the frequency bands (e.g., second coincidence discounting of ITDs or ILDs for narrowband signals is not weighting) is not necessary, and it seems to be sufficient to observed, but rather the discounting of the precedence estimate the average over the involved frequency bands.

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ACKNOWLEDGEMENT [17] L. R. Bernstein and C. Trahiotis, ‘‘Lateralization of low- frequency, complex waveforms: The use of envelope-based We would like to thank Sebastian Schmidt for his temporal disparities,’’ J. Acoust. Soc. Am., 77, 1868–1880 assistance in programming the psychoacoustical experi- (1985). ¨ ments and Armin Kohlrausch for his helpful hints on the [18] K. Wendt, Das Richtungsho¨ren bei der Uberlagerung zweier Schallfelder bei Intensita¨ts- und Laufzeitstereophonie, Disser- physical appearance of ILDs for the type of stimuli used in tation, Technische Hochschule Aachen (1964). the experiment. We also wish to thank two anonymous [19] H. Kunov and S. M. Abel, ‘‘Effects of rise/decay on the reviewers for their helpful comments and suggestions lateralization of interaurally delayed 1-kHz tones,’’ J. Acoust. during the revision process and John Worley for proof- Soc. Am., 69, 769–773 (1981). [20] E. E. David, N. Guttman and W. A. van Bergeijk, ‘‘Binaural reading the manuscript. This work was financed by the interaction of high-frequency complex stimuli,’’ J. Acoust. Soc. Deutsche Forschungsgemeinschaft (Grant No. Bl 189/23- Am., 31, 774–782 (1959). 3). [21] G. G. Harris, ‘‘Binaural interaction of impulsive stimuli and pure tones,’’ J. Acoust. Soc. Am., 32, 685–692 (1960). REFERENCES [22] J. Braasch and K. Hartung, ‘‘Localization in the presence of a [1] M. Ebata, T. Sone and T. Nimura, ‘‘On the perception of distracter and reverberation in the frontal horizontal plane. I. direction of echo,’’ J. Acoust. Soc. Am., 44, 542–547 (1968). Psychoacoustical Data,’’ Acustica/Acta Acustica, 88, 942–955 [2] J. Blauert and W. Cobben, ‘‘Some consideration of binaural (2002). cross correlation analysis,’’ Acustica, 39, 96–104 (1978). [23] J. Braasch, ‘‘Localization in the presence of a distracter and [3] J. Blauert, Spatial Hearing: The Psychophysics of Human reverberation in the frontal horizontal plane. II. Model Sound Localization, 2nd Enlarged Edition (MIT Press, Cam- algorithms,’’ Acustica/Acta Acustica, 88, 956–969 (2002). bridge, Mass., 1996). [4] W. M. Hartmann, ‘‘Listening in a room and the precedence Jonas Braasch was born in 1971 in Wipper- effect,’’ in Binaural and Spatial Hearing in Real and Virtual fu¨rth, Germany. He received a diploma degree Environments, R. H. Gilkey and T. R. and Anderson, Eds. in physics from Dortmund University in 1998 (Lawrence Erlbaum Associates, Mahwah, 1997), pp. 191–210. and a Doctor-of-Engineering degree from Ruhr- [5] R. Y. Litovsky, H. S. Colburn, W. A. Yost and S. J. Guzman, University Bochum in 2001. Currently, he is ‘‘The precedence effect,’’ J. Acoust. Soc. Am., 106, 1633–1654 working at the Institute of Communication (1999). Acoustics at the Ruhr-University Bochum, [6] P. M. Zurek, ‘‘The precedence effect,’’ in Directional Hearing, managing the group ‘‘Auditory Signal Process- W. A. Yost and G. Gourevitch, Eds. (Springer-Verlag, New ing and Binaural Technology.’’ His research York, 1987), pp. 85–105. interests include binaural technology and perception and the [7] H. Haas, ‘‘U¨ ber den Einfluss eines Einfachechos auf die acoustics of organs. Ho¨rsamkeit von Sprache,’’ Acustica, 1, 49–58 (1951). [8] H. Wallach, E. B. Newman and M. R. Rosenzweig, ‘‘The Jens Blauert was born in 1938. He studied precedence effect in sound localization,’’ Am. J. Psychol., 62, communication engineering at Aachen, where 315–336 (1949). he received a Doctor-of-Engineering degree in [9] B. Rakerd and W. M. Hartmann, ‘‘Localization of sound in 1969. In 1973, he delivered an inaugural rooms. II. The effect of a single reflecting surface,’’ J. Acoust. dissertation to the Technical University of Soc. Am., 78, 524–533 (1985). Berlin (habilitation) and in 1994 he was award- [10] N. V. Franssen, Some Considerations on the Mechanism of ed an honorary Dr. technices degree by the Directional Hearing, Dissertation, Technische Hogeschool, University of Aalborg (DK). Since 1974 he Delft, The Netherlands (1960). holds a chair in electrical engineering and [11] N. V. Franssen, Stereophony (Philips Technical Library, acoustics at the Institute of Communication Acoustics of the Ruhr- Eindhoven, 1962), (English translation, 1964). University at Bochum. His major fields of current interest are [12] J. Blauert, Raümliches Ho¨ren (S. Hirzel Verlag, Stuttgart, binaural technology, models of binaural hearing, architectural 1974). acoustic, noise engineering, product-sound design, speech technol- [13] J. Blauert and J.-P. Col, ‘‘Etude de quelques aspects temporels ogy, virtual environments and telepresence. Currently, he is president de l’audition spatiale [A study of certain temporal effects of of the German Acoustical Society, DEGA. spatial hearing],’’ Note-laboratoire LMA, No. 118, CNRS, Marseille (1989). Thomas Djelani was born in 1970. He grad- [14] J. Blauert and J.-P. Col, ‘‘A study of certain temporal effects of uated from Dortmund University in physics in spatial hearing,’’ in Auditory Psychology, and Perception,Y. 1996. In 2001, he completed his doctoral Cazals, L. Demany and K. Horner, Eds. (Pergamon Press, dissertation in engineering at the Institute of Oxford, 1992), pp. 531–538. Communication Acoustics, Bochum, investigat- [15] W. Lindemann, ‘‘Extension of a binaural cross-correlation ing the precedence effect. Currently, he is model by contralateral inhibition. II. The law of the first wave employed at Vodafone D2 GmbH in Du¨sseldorf. front,’’ J. Acoust. Soc. Am., 80, 1623–1630 (1986). [16] F. A. Bilsen and J. Raatgever, ‘‘Spectral dominance in binaural lateralization,’’ Acustica, 28, 131–132 (1973).

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