CONTROL OF SPATIAL IMPRESSION BY MEANS OF … of Spatial Impression... · binaural cues, IACC, ... CONTROL OF SPATIAL IMPRESSION BY . ... values between 0.5 and 0.88 at the listening

The apparent source width (ASW) describes how focused a sound source is heard by a listener and therefore resembles a kind of acoustical lens of the human auditory system. This attribute is assessed by investigating the perceived spatial extent of a sound source on an individual listener basis which gained insights into the listener’s spatial perception. ASW measurements have been performed by making use of a loudspeaker line array and a source widening algorithm. This has been done for two groups of listeners, normal-hearing (NH) and hearing-impaired (HI) listeners. It was found that HI listeners have, compared to normal-hearing listeners, a reduced sensitivity towards this percept, i.e. a compressed sensing of their acoustical environment. Further, it was tested, how much a hearing aid using wide dynamic-range compression, distorts the perception of ASW. While results for NH listeners indicated a slight compression in ASW perception due to the applied signal processing, the HI listeners’ perception was almost unaltered. The used signals were analysed regarding statistics of the binaural cues, IACC, ITDs and ILDs. IACC and ITDs could account for most of the changes in ASW, whereas changes in perception due to hearing aid processing could be accounted for by the ILD fluctuations.

I N N O V A T I O N P R O J E C T

CONTROL OF SPATIAL IMPRESSION BY MEANS OF APPARENT SOURCE WIDTH Technical University of Denmark (DTU), Johannes Käsbach, Alan Wiinberg, Tobias May and Torsten Dau

Widex A/S, Morten Løve Jepsen

Bang&Olufsen (B&O), Søren Bech and Neo Kaplanis

COLOPHON

Publisher

Danish Sound Innovation Network Technical University of Denmark Matematiktorvet, Building 303B, 2800 Kongens Lyngby, Denmark +45 45253411 www.danishsound.org

August 2013

About the publication

This publication and possible comments and discussions can be downloaded from www.danishsound.org.

The content of this publication reflects the authors’ point of view and not necessarily the view of the Danish Sound Innovation Network as such.

Copyright of the publication belongs to the authors. Possible agreements between the authors might regulate the copyright in detail.

About the network Danish Sound Innovation Network is an innovation network funded by the Danish Agency for Science, Technology and Innovation. The Network is hosted by the Technical University of Denmark and is headed by Director, Associate Professor, PhD Jan Larsen. Danish Sound is the facilitator of the national ecosystem for SOUND, creates value for all parts in the value chain and contributes to growth and wealth in Denmark.

Network membership is free of charge and open for all. Registration at www.danishsound.org

This publication is the result of an innovation project, an instrument to strengthen the cooperation between knowledge institutions and private companies. The primary goal is to promote innovation by combining accessible/existing research and technologies with creative uses in order to facilitate the creation of new products, services or experiences. Innovation projects are mainly short term feasibility studies conducted on a pre-competitive level.

2 Dansk Sound Innovation Network

http://www.danishsound.org/



PREFACE

This publication is the result of an innovation project entitled Control of spatial impression by means of apparent source width. The project is financed by the Danish Sound Technology Network through a grant from the Danish Agency for Science, Technology and Innovation. The project is completed in the period from September 01 until December 19, 2014 and managed by the Centre for Applied Hearing Research (CAHR) at the Technical University of Denmark (DTU) with project manager Johannes Käsbach. Additional project participants are: Alan Wiinberg, Tobias May and Torsten Dau (CAHR, DTU), Morten Løve Jepsen (Widex A/S), Søren Bech and Neo Kaplanis (Bang&Olufsen).

SPECIAL CIRCUMSTANCES

There were no special circumstances.

INTRODUCTION

BACKGROUND AND OBJECTIVES

The present study was divided into 4 milestones that are outlined together with background information in the following.

The apparent source width (ASW) describes the perceptual spatial extent of sound sources in the horizontal dimension. It is mainly affected by the degree of decorrelation between the ear signals, namely the interaural cross-correlation (IACC). Compared to anechoic conditions, reflections in a room cause the binaural cues, namely interaural time differences (ITD) and interaural level differences (ILD), to fluctuate over time. This leads to the decorrelation of the ear signals (ICears). In Käsbach et al. (2014a), ASW was measured for bandlimited noise stimuli as a function of ICears and as a function of center frequency for normal hearing (NH) listeners using a stereo loudspeaker setup. It was shown that ICears and ASW are in inverse proportion and that low frequency stimuli produce a wider ASW than high frequency stimuli for a fixed ICears value which is consistent with other studies (Blauert et al., 1986a) using headphone-based experiments. However, it remains unclear how the perception of ASW is affected for more natural stimuli, such as music and speech, in such a setup. Zotter et al. (2011) used a phantom source widening algorithm which allowed the manipulation of ICears for any kind of source signal, including speech and music. They tested their algorithm for noise and speech, but not music.

The initial aim of this study was to

(1) quantify ASW for natural stimuli (such as music and speech).

Whitmer et al. (2012 and 2014) showed that hearing-impaired (HI) listeners have a reduced dynamic range of ASW perception in comparison to normal-hearing (NH) listeners, for headphone-based and loudspeaker-based presentations, respectively. There, the spatial impression appears to be distorted what might be one reason for the difficulties in separating sound sources in multi-talker environments. Likewise, an altered spatial perception is expected for these listeners, e.g. when using home-entertainment systems. To obtain


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a better understanding of individual ASW perception for different groups of listeners, the present study investigated

(2) the individual differences in ASW perception both in NH and HI listeners.

A hearing device, such as a hearing aid, might alter the spatial impression of the surrounding due to the applied signal processing such as the use of wide dynamic range compression. Wiggins and Seeber (2011) studied the influence of dynamic range compression on the lateralization of a sound source and on ASW perception. They found a diminished lateralization, but no influence on the ASW percept. However, their tests used non-individualised HRTFs for binaural simulations, so that the influence of a room was neglected. Further, they used a fast-acting dynamic-range compression applied only to high frequencies above 2kHz. In the study of Whitmer and Akeroyd (2013), a virtual sound environment was used to test ASW perception and they found that the use of hearing aids did not have an effect on the perception of ASW. Their participants did, however, wear their own hearing aids, such that the control over the presented sound was weak. Therefore, this study further investigated

(3) the influence of wide dynamic range compression in hearing-aids on ASW.

It would be desirable to develop a signal processing algorithm that allows for targeting a specific spatial impression for an individual listener. For instance, it would be beneficial to modify the binaural signal in a way that the spatial impression of a sound source as perceived by a healthy auditory system is restored for a HI listener. This could potentially aid the HI listener in complex listening situations.

In Käsbach et al. (2014b), instantaneous ITD fluctuations of bandlimited noise signals were compressed in different frequency bands to study their influence on ASW. It was found for NH listeners that the frequency band centered at 1 kHz was mostly contributing to the perception of ASW, whereas above 2 kHz ITDs did not have an influence on ASW. A similar study, which investigated the influence of instantaneous ILD fluctuations on the perception of ASW is not known by the knowledge of the author.

A processing scheme that modifies ITD and ILD fluctuations at the ears of a listener, could be used in order to target or to compensate for a specific perception of ASW on an individual basis. The final goal of this study was therefore to suggest

(4) a signal processing algorithm that is based on binaural cue statistics, i.e. fluctuations in ITDs and ILDs, to manipulate ASW perception.

The suggested algorithm could then be applied to either

a) a loudspeaker setup (for instance in home-entertainment systems) or b) a hearing device such as a hearing aid.

The results of milestones (1)-(3) were important in order to suggest a compensation algorithm as specified under milestone (4). This was done in the present study in a perceptual evaluation of ASW for music, speech and noise signals presented over a loudspeaker setup.


IMPACT/EFFECT

The outcome of this project is a loudspeaker-based setup that can create a given spatial auditory experience by controlling the perception of ASW in natural acoustic environments. Any loudspeaker based setup (stereo or multi-channel) is, in principle, suitable to provide the binaural cues required to create the desired ASW percept. This will be valuable for sound field control strategies in home entertainment systems creating a specific spatial impression in a defined sound zone around the listeners. Moreover, the results can be transferred to signal processing algorithms in hearing instruments for restoring natural spatial impression for hearing impaired listeners. A variety of other products focusing on applications in the field of spatial hearing - such as teleconference and navigation systems, headphone and mobile phone applications - could profit from the findings by directly addressing listeners’ auditory spatial perception.

METHODS AND RESULTS

THEORY

In this study, loudspeaker pairs arranged in a line array were used to simulate differences in apparent source width (ASW) perception, i.e. from narrow to wide sources. The setup produced a phantom source for the listener in the center of the loudspeaker arrangement. The ASW of this phantom source was dependent on the opening angle of the loudspeaker pair (Frank, 2013, p.50). Its ASW was varied by changing the correlation between the loudspeaker signals (ICLS) which effectively altered the interaural cross-correlation at the ears of a listener (ICears). A large dynamic range of ASW perception could be achieved by using three loudspeaker pairs with differing opening angles in combination with appropriately adjusting the ICears.

To generate differing ICears values for mono-signals, the use of a source widening algorithm was required. This procedure is also known as pseudo stereo. For this purpose, the “Efficient phantom source widening” algorithm described in Zotter et al. (2013) was used. It is based on either a modulated phase or a modulated magnitude spectrum and the advantage of this decorrelation algorithm compared to others is that it is easy to implement, cost efficient and optimised in terms of audible artifacts such as coloration. In their study, it was shown that in a typical stereo setup (i.e. with an opening angle of 30°) this algorithm can generate IACCE3

1 values between 0.5 and 0.88 at the listening position, depending on the specified modulation depth ɸ. . In their study, it was shown that IACCE3 correlates well with psychoacoustic data. The algorithm outputs two FIR filters, one for the left and one for the right loudspeaker channel, which are convolved with the mono signal. Each filter consists of 5 filter tabs separated with a gap of N = Tfs samples, with T being the periodic modulation interval and fs being the sampling frequency. Therefore, the filtering process can be interpreted as adding early reflections to the signal that differ for the two loudspeaker channels. This causes fluctuations in the binaural cues over time.

1 IACCE3 is a frequency-averaged interaural cross-correlation (IACC) over the octave bands 500 Hz, 1 kHz and 2 kHz as defined in Okano et al. (1995). By definition, only the early part (first 80ms) of an impulse response is considered in the calculation. However, in this report the IACCE3 is calculated over the entire signal duration. 5 Dansk Sound Innovation Network

Compared to noise, natural signals, such as music and speech contain inherent fluctuations such as envelope modulation. Therefore, the perception of ASW was expected to be different for these signals. To test this, all three kinds of signals were used in this study.

METHOD

LISTENING ROOM AND LOUDSPEAKER SETUP

The listening experiment was performed in the standardised IEC listening room (according to the ISO 268-13 standard) at the acoustical laboratories in building 354 at DTU. This room measured (LxWxH) 7.52m x 4.74m x 2.76m, resulting in a volume of 98.3m3, and resembled the acoustics of a living room with a reverberation time of T30 ≈ 0.4 s. The reverberation distance is about 0.9m.

Three loudspeaker pairs with differing opening angles were installed in a line array in the room. Their opening angles and distances to the listening position are specified in Table 1. In addition a center loudspeaker was installed which served as well as a reference condition. The entire loudspeaker setup was covered with an acoustical transparent curtain to reduce any visual influence of the loudspeakers on the listener (see Figure 3).

Loudspeaker pair Opening angle [°] Distance to listening position [m]

LS pair 1 (outer pair) ±42.5° 2,29m

LS pair 2 (middle pair) ±30° (standard for stereo) 1,95m

LS pair 3 (inner pair) ±16° 1.74m

Reference LS (center LS) 0° 1.69m

Table 1 Opening angles and distances of the listening position to the loudspeaker setup.

STIMULI

In the present study, an anechoic speech (male talker) and an anechoic guitar excerpt were used as input signals to the source widening algorithm. In addition, pink noise (non-frozen) was used for comparison reasons. The signals were passed through the source widening algorithm which was operated in the modulated phase spectrum mode. In total, 5 distinct source widenings were set. The variable parameters of the algorithm were set to N = 132 samples at the sample frequency fs and parameter ɸ was set individually for each loudspeaker pair. The signal flow for sounds #1 to sound #5, parameter ɸ and the achieved ICears, measured by the IACCE3 via a head and torso simulator (HATS) at the listening position, are listed for each stimulus individually in Table 2. The chosen values were found by pre-listening of 4 expert listeners and pilot-testing with 3 normal-hearing listeners. The processed signals were presented to the listeners at 70 dB SPL (with a maximal deviation of +0.3/-1 dB SPL) based on the long-term rms-value of the individual signals. Only the reference loudspeaker (sound #5) was set to a presentation level of 69 dB SPL. The duration of each stimulus was 6s. Each loudspeaker was calibrated individually to a calibrated microphone positioned at the listening position. This was done separately for each of the three source signals, such that a gain-


factor was obtained per loudspeaker and source signal. It was not compensated for the magnitude and phase differences of the individual loudspeakers.

Sound Played back on... Parameter ɸ

[x pi/180°] IACCE3 pink noise IACCE3 male speech IACCE3 guitar

#1 LS pair 1 (outer pair) 30° 0.49* 0.43 0.61*

#2 LS pair 2 (middle pair) 40° 0.47 0.53 0.54

#3 LS pair 2 (middle pair) 20° 0.62 0.67 0.63

#4 LS pair 3 (inner pair) 20° 0.71 0.73 0.80

#5 Reference LS (center LS) - 0.82 0.85 0.80

Table 2 Signal flow for sounds #1 to #5, settings for parameter phi and the achieved IACCE3 values for each signal. *Note that the marked IACCE3 value was higher for sounds #1 compared to #2. However, they produced a wider ASW perception on average.

HEARING AID SETUP

In the listening experiment a hearing aid (HA in the following) of type Widex Dream Fusion (see Figure 1, left) and the master hearing aid MHA30ch (MHA in the following) were used (see Figure 1, right). The HA was a standard consumer hearing aid with two microphones to capture the sound in an omnidirectional mode. The MHA was a power PC (speedgoat) designed for fast real-time processing and was controlled via Matlab Simulink (for more details see Buchholz, 2010). The satellites were based on a modified Phonak Exelia hearing aid using a single microphone and were connected to the power PC via cables. Specifications for both hearing aids are given in Table 3. Hearing aid type Bandwidth [Hz] Lower cut-off [Hz] Upper cut-off [Hz] Latency

time [ms]

Transduction to ear-plug

MHA 5000Hz 6.3ms Acoustical

HA 8000Hz Electrical

Table 3 Technical specifications of the hearing aids HA and MHA

The following HA/MHA conditions were tested:

(1) without HA/MHA [w/o HA]

The test was performed without the use of the HA/MHA which dealt as a reference test.

(2) with HA/MHA using a filterbank + gain [HA/MHA lin] The listener was provided with the hearing aid with a typical filterbank implementation and a linear gain dependent on the audiogram.

(3) with HA/MHA using compression [HA/MHA comp] In addition to the frequency bank, a wide dynamic-range compression algorithm was tested.


Both devices were fitted to the individual group of listeners, i.e. normal hearing (NH) and hearing impaired (HI) listeners. For the NH listener group, no amplification was applied and for the HI group, about half the average across all listeners' audiograms was applied as insertion gain following the NAL-R(P) rationale (see Figure A.1). This could be done due to the homogenous audiograms of the HI group. The wide dynamic-range compression was set for both devices (HA and MHA) to a compressor ratio of 2:1 and an attack and release time of 11.5ms and 100ms, respectively, which is a rather fast-acting compressor setting. The threshold was defined as 35 dB SPL to ensure an activated dynamic range compression throughout the entire stimulus presentation. Both hearing aids used an omnidirectional microphone for the sound capturing and no further signal processing, such as e.g. beam-forming, noise-reduction or feedback control were applied. The ear-plug was inserted in a listener’s ear with a closed fitting.

Figure 1 Photos of the hearing aids. Left: Hearing aid HA on the HATS. Right: Hearing aid MHA on a listener (note that the satellites are connected to the power PC via cables).

EXPERIMENTS

EXPERIMENTAL PROCEDURE

The experimental setup and procedure were adapted from Käsbach et al. (2014a). In order to measure ASW, listeners were asked to indicate the apparent opening angle (or boundary) of the perceived sound in degrees, separately for the left and right side. A scale with a 5° resolution was installed in front of the loudspeaker setup and the response was given via a touchscreen as indicated in Fig. 1. The stimuli were presented in random order and were each repeated three times. A reference sound was available to the listener during the entire experiment which was the pink noise signal presented over the center loudspeaker (equal to sound #5 for pink noise). This produced a narrow sound source. Both, stimulus and reference, could be played back as often as desired by the listener. A photo of the setup is shown in Figure 3.

The experiment was subdivided in a demonstration, training and experimental phase. The demonstration phase was done once for each new listener. Here, the listener was familiarized with the concept of ASW perception and the use of the interface by evaluating only sound #1 and #5 for the three source signals (pink noise, music and speech) without repetitions. The training phase was performed prior to each hearing aid condition where an evaluation only for the pink noise signal for all 5 sounds was chosen, resulting in 15 8 Dansk Sound Innovation Network

evaluations. In the experimental phase, all stimuli had to be evaluated, resulting in a total of 45 stimuli per hearing aid condition. The hearing aid conditions were arranged by a latin-rectangular design across listeners to avoid order effects.

Figure 2 Sketch of the experimental set-up and procedure. The loudspeaker pairs - positioned as specified in Table 1 - generate a phantom source at 0°. Subjects are asked to indicate ASW in degree on the given scale via a touchpad, separately for the left(-) and right(+) boundary of the source image. The loudspeaker in the center of the setup is used to play back the reference condition representing a narrow source.

Figure 3 Photo of the listening setup in the IEC listening room. Left: HATS in front of the loudspeaker line array covered with an acoustically transparent curtain. Right: Listener in front of the setup holding the touchpad for the evaluation procedure.

LISTENERS

In the listening experiment, 6 NH (5 male and 1 female) and 6 HI (male) listeners participated. The NH listeners were 27 to 32 years old and had a hearing threshold below +20 dB HL. Four of the participants were familiar with psychoacoustic experiments and two were unexperienced listeners. The HI listeners ranged from 59 to 75 in age and were hearing aid users for at least 4 month. All HI listeners were all diagnosed with a cochlear hearing loss at the audiology department at Bispebjerg hospital. All listeners participate regularly in listening tests. Their audiograms were homogeneous and are shown in the Appendix in Figure A.1. Both groups of listeners performed the experiment for all hearing aid conditions. Listeners were allowed to rotate their head during an evaluation of a stimulus since this can facilitate the evaluation of ASW (Kim et al., 2013).


SUMMARY OF EXPERIMENTAL CONDITIONS

• 3 anechoic stimuli: 1x noise, 1x speech, 1x music • 5 predefined IACCE3 values per signal resulting in 5 distinct ASW perceptions (for NH

listeners) • 3 repetitions per stimulus • Hearing aid (HA) and master hearing aid (MHA) conditions: (1) without HA/MHA [w/o HA],

(2) with HA/MHA using a filterbank + gain [HA/MHA lin], (3) with HA/MHA using compression [HA/MHA comp] -> 5 conditions in total per listener

• In total: 45 Stimuli to evaluate per hearing aid condition (av. duration 13min.) -> 225 stimuli per listener

• Listeners: 6 NH and 6 HI • Level at 70 dB SPL (Reference at 69 dB SPL) • Duration of 6s per stimulus • Total experimental duration: 2.5 hours per listener (including breaks)

RESULTS

LISTENER RESULTS

In the following the experimental results are presented for both groups of listeners. They are shown as averaged data across repeated measures which were obtained by first calculating the median across stimuli repetitions per listener (repeated measures) and then calculating the mean value across the individual repeated measures. The error bars are represented by 25th and 75th percentiles which were obtained in the same manner.

Figure 2 shows, the differences in ASW perception for the three source signals (represented by the different colors) in the condition without hearing aids [w/o HA] for NH (left panel) and HI (right panel) listeners. On the y-axis, the predefined source wideners, sounds #1 (wide) to #5 (narrow), are shown. The averages of the measured psychoacoustic responses for ASW in degrees are shown on the x-axis, indicating the responses for the left and right boundary, separately. Considering first the results for the pink noise signal (red curve), for NH listeners (left panel), ASW decreases symmetrical for the left and right boundary with increasing the sound number resulting in a V-shaped pattern. The dynamic range, i.e. the difference between wide and narrow sound sources, ranges from ±40 (wide) to ±10 degrees (narrow). An overall reduction in dynamic range of ASW perception (5 to 10 degrees on both sides) is present for the two more natural signals, speech and guitar (blue and red curve, respectively).

For HI listeners (right panel), the slope of the two boundaries and the dynamic range is significantly reduced for the pink noise signal, here from -20 degrees left and + 15 degrees right (for wide sources) to ±10 degrees (for narrow sources). This indicates clearly a reduced sensitivity of the HI listeners towards ASW perception compared to NH listeners. Their data results in an asymmetric pattern between both boundaries which is due to larger individual differences in ASW perception (see Figure 8). For the other two source signals, a minimal reduction in ASW for broader sources only (sounds #1 and #2) is observable,


though less pronounced as for the NH listeners. This results into a complete insensitivity towards ASW perception for these two source signals.

Besides a change in ASW, both groups of listeners mentioned an increase in the perception of distance/depth and a more reverberant character of the sound for wide (sound #1) compared to narrow (sound #5) stimuli.

In the following for a better overview, only results for the pink noise signal are considered, since here the largest dynamic range in ASW perception was observed. However, the remaining data is shown in the Appendix (Figure A.2).

Figure 4 Averaged ASW results for the condition without hearing aid for all source signals (represented by the different colors). ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median and respective 25th and 75th percentiles. Left panel: NH listeners. Right panel: HI listeners.

To provide a better overview on the effect of hearing aid processing on ASW perception, the data without hearing aid [w/o HA] is first compared to the condition with hearing aid HA with dynamic range compression [HAcomp] in Figure 3. It shows the averaged data for these two hearing aid conditions ([w/o HA] and [HAcomp], represented by the different colors), for the pink noise signal for NH (left panel) and HI (right panel) listeners.

Considering the results for the HA condition with wide dynamic-range compression ([HAcomp], green curve), different impacts for both listener groups are noticeable: For NH listeners, the compression causes an overall reduction of ASW, i.e. a narrower sound source, especially for sound #1 and #3. For HI, the wide dynamic range compression rather causes a further asymmetric pattern due to individual perceptual differences: While for the left boundary the compression either reduces or maintains ASW, the compression has the effect of expanding ASW for the right boundary. The individual results in Figure 8 provide more details for the obtained data.

Similar observations can be made for the other hearing aid conditions (represented by the different colors) shown in Figure 6 for the pink noise signal. For NH listeners, all hearing aid conditions cause rather a reduction in ASW perception for wider sources. For HI listeners, the differences between the individual hearing aid conditions are larger, especially for the left boundary. For the HA, similar results are obtained without compression [HAlin] as with compression [HAcomp]. For the MHA, ASW is enlarged to both sides in both cases with [MHAcomp] and without [MHAlin] compression. Note that for the other two source signals, speech [BOspeech] and guitar [guitar2], the perception of ASW with hearing aids was identical to the data


shown here (a change in ASW perception was only given in the condition without hearing aids). More results comparing the different hearing aid conditions are shown in the Appendix (Figure A.3 to Figure A.6).

Figure 5 Averaged ASW results for the selected hearing aid conditions [w/o HA] and [HAcomp] (represented by the red and green curve, respectively) for the pink noise signal. ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median and respective 25th and 75th percentiles. Left panel: NH listeners. Right panel: HI listeners.

Figure 6 Averaged ASW results for all hearing aid conditions (represented by the different colors) for the pink noise signal. ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median and respective 25th and 75th percentiles. Left panel: NH listeners. Right panel: HI listeners.

In the following individual results are shown for both listener groups in Figure 7 (NH listeners) and Figure 8 (HI listeners).To highlight the influence of wide dynamic-range compression in hearing aid processing, condition [HAcomp] has been selected and is compared to the condition without hearing aid [w/o HA].

In Figure 7, a large dynamic range for ASW perception was measured for all NH listeners without hearing aids [w/o HA]. For wide sources (sound #1) ASW varies from ±35 to ±55 and for narrow sources (sound #5) from ±5 to ±15 between individual listeners. Concerning the impact of the wide dynamic-range compression, two groups of listeners can be identified among the results: For listeners KSt and AP (first row in Figure 7), the use of compression reduced the dynamic range of ASW perception significantly for all sounds, whereas for the remaining listeners either no reduction or small reductions for individual sounds were present. In individual cases, the compression caused also an enlargement of the ASW perception, as e.g. for listener PC. Note that for the other two source signals, speech and guitar, the differences between


the two hearing aid conditions were smaller, besides a few individual listeners that showed an enlargement of the sound sources due to the hearing aid processing (see Appendix).

Figure 7 Individual ASW results for NH listeners for the selected hearing aid conditions [w/o HA] and [HAcomp] (represented by the red and green curve, respectively) for the pink noise signal. ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median (repeated measures) and respective 25th and 75th percentiles across the repeated stimuli presentations per listener. Individual runs are indicated by dashed lines.


Figure 8 Individual ASW results for HI listeners for the selected hearing aid conditions [w/o HA] and [HAcomp] (represented by the red and green curve, respectively) for the pink noise signal. ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median (repeated measures) and respective 25th and 75th percentiles across the repeated stimuli presentations per listener. Individual runs are indicated by dashed lines. Note that for listener JG measurements for hearing aid HA could not be obtained.


The individual results for the HI listeners (Figure 9) show that two listeners, listener JG and KM (first row), had an increased dynamic range in ASW perception compared to the other listeners and hence were more sensitive towards ASW perception. The remaining listeners show a rather constant ASW perception where listeners KS and ET (last row) indicated an asymmetric ASW perception. For this group of listeners, dynamic-range compression was affectless on ASW or showed only small enlargements of the sound source. Exceptions are listeners KS and ET (last row) where a large asymmetric perception was caused by the hearing aid processing. Note that no differences in ASW perception were found for the other two sources. signals, speech and music, with exception of listener JG who showed a reduced dynamic range in case of speech and an inverse proportional ASW perception for sounds #1 to #3 (see Appendix).


ANALYSIS OF BINAURAL CUE STATISTICS

In this section an analysis of the binaural cues, namely the interaural cross-correlation (IACC), interaural time and level differences (ITDs and ILDs, respectively), will be presented.

In Figure 9, the interaural cross-correlation is presented as 1-IACCE3 (left) and 1-IACC (right). The illustration of the data follows the one of the psychoacoustic data. The 1-IACCE3 values (exact values for IACCE3 can be found in Table 2) decrease with increasing the sound number in case of [w/o HA] (red color), representing a V-shaped curve, with exception of sound #1 where a smaller value than for sound #2 is calculated. This represents well (with exception of sound #1) the psychoacoustic data for NH listeners. In case of condition [HAcomp] for NH and HI users (blue and green color, respectively), the graph reveals that the wide dynamic range compression does not alter the IACCE3. However, individual results showed a perceptual effect on ASW for this condition, especially for NH listeners. This fact and the divergence of IACCE3 predictor from perceptual data at #sound1 for condition [w/o HA] indicate that IACCE3 does not capture all properties of the sound that elicit the measured ASW perception.

Considering the IACC measurements, alternating values for the different sounds are observed. A proper representation of the psychoacoustic data is no longer given (for a NH listeners) with this predictor. A reason for this is the missing frequency selection as in the IACCE3 calculation. In that sense, the IACC represents more the psychoacoustic data of HI listeners. This highlights the fact that for a loss of frequency selectivity, the interaural cross-correlation is less accurately estimated for the given stimuli, both for a prediction measure and probably also in case of an impaired system which might explain the loss of ASW sensitivity.

Figure 9 The interaural cross-correlation presented as 1-IACCE3 (left) and 1-IACC (right) measured with a head and torso simulator (HATS) at the listening position. Besides condition without hearing aids [w/o HA], condition HA with the dynamic range compression [HAcomp] for NH and HI listeners (represented by the different colors) is shown. The measures are displayed as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). In consistence with the psychoacoustic data, the measurements are presented here symmetrical, i.e. representing a left (-)

and right (+) boundary. Note that for the calculations of IACCE3 and IACC, no actual negative values were obtained since the measures were

calculated as the absolute value of the maximum in the cross-correlation function.

In the following a detailed analysis of ITDs and ILDs is shown in form of histograms, i.e. measuring the relative occurrence of the two binaural cues after performing a time-frequency analysis. This analysis was performed on the binaural HATS recordings which were processed by a fourth order gammatone filterbank with 1-ERB wide filters for center frequencies ranging from 131 to 13563 Hz. Subsequently, ITDs and ILDs for each frequency channel were computed for frames of 20ms with 50% overlap.


The ITD and ILD histograms are shown in Figure 10 and Figure 11, respectively. Columns display (from left to right) the histograms for a wide sound (sound #1, left column) and a narrow sound (sound #5, right column) and rows represent (from top to bottom) the different hearing aid conditions, [w/o HA] and [HAcomp] for NH and HI listeners, respectively. Results for the hearing aid condition [MHAcomp] are shown in the Appendix (Figure A.7 and Figure A.8)

Figure 10 Relative occurrences of ITDs per frequency channel shown in form of histograms. These results for the oink noise source signal were obtained from a time-frequency analysis of ITDs for the widest (sound #1, left panels) and the narrowest (sound #5, right panels) source width. A dark color represents a high relative occurrence of ITDs. Comparing signals from the left (wide) and right panels (narrow), it is apparent that the spread in ITDs is reduced especially at low frequencies. To the right of each panel the accumulated distribution across all frequency channels (center of gravity) is shown. From top to bottom panel: conditions [w/o HA], [HAcomp] for NH listeners, [HAcomp] for HI listeners.


Figure 11 Relative occurrences of ILDs per frequency channel shown in form of histograms. These results for the oink noise source signal were obtained from a time-frequency analysis of ITDs for the widest (sound #1, left panels) and the narrowest (sound #5, right panels) source width. A dark color represents a high relative occurrence of ILDs. To the right of each panel the accumulated distribution across all frequency channels (center of gravity) is shown. From top to bottom panel: conditions [w/o HA], [HAcomp] for NH listeners, [HAcomp] for HI listeners.


The relative occurrence of large ITDs (Figure 11) decreases with reducing the width of the sound source (from left to right), especially at low frequencies. This is expected since the source widening algorithm was chosen to operate on the phase spectrum which introduced the ITD fluctuations. Of course, due to room reflections and diffraction at head and torso, further ITD fluctuations are introduced. The three rows, representing the different hearing aid conditions, reveal similar patterns, so that it can be concluded that the ITD statistics were not changed by the hearing aid processing.

Considering the ILD histograms, the differences between a wide (left) and narrow (right) sound source becomes less prominent than in the ITD histograms. However, a reduction in the spread of ILDs is visible. The only difference between the hearing aid conditions (rows) is an introduction of large ILDs at high frequencies that might have caused a lateralization shift for individual listeners and a slight distortion (modulation) in the ILD distribution pattern which might explain the enlargement of ASW for individual listeners.

The analysis of ITDs is strongly linked to the analysis of the IACC, since the values for the ITDs are determined by identifying the lag of the absolute maximum in the cross-correlation function. Therefore, their results correlate well with each other. The differences obtained in the ILD analysis, reveal further information on the used stimuli and might account for psychoacoustic data that could not be accounted for by the other two binaural cues.

DISCUSSION

In the previous section it was shown that HI listeners have a reduced sensitivity for the perception of ASW compared to NH listeners. These results are in agreement with Whitmer et al. (2012 and 2014). They could show that the reduced sensitivity for individual listeners correlates with a lower threshold in the binaural temporal fine structure (TFS) of these listeners and an insensitivity to changes in the IACC. Listeners with these disabilities were found to be older, such that the explanation lies in aging of the auditory system. In an impaired auditory system the frequency selectivity is reduced due to broadened auditory filters, hence causing a less accurate use of the temporal binaural cues, IACC and ITDs.

The here chosen signal processing to generate different ASW perceptions was based on generating fluctuations of ITDs over time. Therefore, it is not surprising that HI listeners could not resolve the cues as well as NH listeners did. The wide dynamic-range compression elicited, though minimal, a change in ASW perception. A change in ILD fluctuations was identified to be the prominent cue for the divergence in perception as also elaborately discussed in Wiggins and Seeber (2011). It remains however unclear which resolution for the resolve of ILD fluctuations is remaining in the impaired auditory system. A trade-off can be discussed here. On the one hand dynamic range compression is applied to fit the large dynamic-range of the source signal to the impaired system that shows level recruitment. On the other hand, the dynamic range compression decreases - in theory and based on Wiggins et al. (2011) – the dynamic range in ILD fluctuations which would cause a narrower ASW. Considering this, dynamic range compression would even further compress the dynamic range in ASW perception. However, this was not the case in the current study. Here the analysis of ILDs showed slightly larger ILD fluctuation statistics due to distorted ILD patterns. This nevertheless showed a compressed ASW perception for NH listeners, but rather an enlarged ASW for the HI listener group probably due to the given ILD patterns. In principle, it could be possible to partially restore ASW perception for HI listeners, i.e. enhancing their dynamic range of ASW perception, by making use of ILD cues (instead of ITD cues) given a sufficient sensitivity to ILD cues for the individual listener.


http://dict.leo.org/ende/index_de.html%23/search=elaborately&searchLoc=0&resultOrder=basic&multiwordShowSingle=on

This study highlights the fact that both cues, ITDs and ILDs, are contributing to the perception of ASW. The use of the provided cues by a listener depends on the frequency content of the signal as was pointed out in Wiggins and Seeber (2011). They claim that especially envelope ITDs are often used as beneficial cues and that a listener relies on ILDs when low frequency content is missing. Therefore, the question regarding the dominant cue is always context specific.

Overall, it remains further unclear, what a reduced sensitivity of ASW perception means for a listener in a multi-talker environment. One could argue that - even though a reduced dynamic range in ASW perception is given – a focused source image is present for such listener. On the other hand, introducing more than one source to this listener, limits the resolved space of all sound sources. Therefore, it might be more difficult for this listener to distinguish between the sources in the limited perceived space compared to a NH listener that has access to a larger resolved space and thus a better spatial resolution. Hence a reduced dynamic range in ASW perception could mean a reduced resolution in spatial perception. With other words, HI listeners show a compressed perception of ASW which might correspond to a compressed spatial perception.

It is important to consider the aspect of spatial perception on an individual basis to fit a certain strategy to the individual listener. In terms of fitting, the here applied method of measuring ASW perception could be a helpful clinical investigation tool.

CONCLUSION

In the following, conclusions are drawn along the predefined milestones of this project.

(1) Quantification of ASW for natural stimuli (such as music and speech). It was shown that the pink noise signal produced the largest dynamic range in ASW perception, i.e. the difference between wide and narrow source perception. The two natural stimuli in use showed a slightly reduced dynamic range for this percept.

(2) Individual differences in ASW perception both in NH and HI listeners. The obtained results illustrated that HI listeners have a strongly reduced sensitivity towards ASW perception compared to normal hearing listeners. This is in agreement with literature.

(3) The influence of wide dynamic-range compression in hearing-aids on ASW.

The impact of wide dynamic range compression on the perception of ASW was identified to be rather moderate. While for NH listeners, a reduction in the dynamic range of ASW was produced, for HI listeners no influence or a slight enlargement of ASW perception was caused. However, the signal processing, neither improved nor worsened the sensitivity of ASW perception for the impaired auditory system. Note, that the wide dynamic range compression under use was rather moderate, so that stronger impacts are possible for more fast-acting compressors.


(4) A signal processing algorithm that is based on binaural cue statistics, i.e. fluctuations in ITDs

and ILDs, to manipulate ASW perception. In terms of this project, it was not possible to develop a new algorithm with the specified properties. However, new insights were gained that will stimulate the development process for such algorithm. The chosen source widening algorithm is either based on a modulation of the magnitude or phase spectrum. This pronounces either ILD or ITD fluctuations over time. A hybrid solution with a better control of the individual binaural cue statistics could potentially lead to a “smart” and innovative control strategy. Considering individual listeners, the algorithm could then be fit to the sensitivities towards these two cues for the individual listener. This might benefit especially hearing-impaired listeners which suffer from a distorted spatial perception.

The impact of the current study for the two industrial partners are summarized in the following.

OUTCOME FOR WIDEX

Dynamic range compression does slightly compress the spatial impression, i.e. a narrower ASW is perceived for NH listeners. However, HI listeners are less sensitive to changes in ASW perception, i.e. they have a reduced dynamic range compared to NH listeners. Therefore, the application of wide dynamic-range compression does not worsen the limited spatial impression of the HI listener. An open question is though the impact of the reduced dynamic range in ASW perception for the HI listener for instance in a cocktail party scenario.

OUTCOME FOR B&O

Concerning objectively measuring ASW, IACC is almost insensitive towards the predefined source wideners (sound #1 to #5), whereas IACCE3 is correlated with the results of the NH listeners. However, IACCE3 cannot predict the reduced ASW caused by the wide dynamic-range compression. A more detailed analysis of the binaural cues, ITDs and ILDs, showed that mostly the variation of ITDs is reduced from wide to a narrow sound, i.e. that ITDs are the dominant cue here. However, fluctuations in ILDs do also change with changing width. The compression does mostly affect the ILD cues (especially at high frequencies), such that they contribute to the ASW perception by producing a more narrow ASW for NH listeners. Concerning an algorithm that controls ASW, the source widening algorithm was operating on the phase spectrum providing a corresponding ITD distribution. However, it needs to be clarified in which conditions ITDs or ILDs are dominating the perception of ASW. This knowledge will lead to the possibility of designing a “binaural cue inspired” algorithm and a robust analysis tool.

Regarding new products, services or processes, a solid evaluation method of ASW perception was developed in this project. The method is intuitive for a listener and provides a relatively fast measurement with reliable results. The developed procedure might be an interesting tool for clinical evaluations of spatial perception.


REFERENCES

1. Blauert, J. and Lindemann, W. (1986): Spatial Mapping of intracranical auditory events for various

degrees of interaural coherence. J. Acoust. Soc. Am. 79 (3).

2. Buchholz, J. M. (2010): Master hearing aid MHA30ch – User Manual. CAHR, DTU.

3. Frank, M. (2013): Phantom Sources using Multiple Loudspeakers in the Horizontal Plane. PhD thesis, Institute of Electronic Music and Acoustics and University of Music and Performing Arts Graz, Austria.

4. Käsbach, J., May, T., Le Goff, N. and Dau, T. (2014a): The importance of binaural cues for the perception of apparent source width at different sound pressure levels. Proc. of DAGA, Oldenburg, Germany.

5. Käsbach, J., May, T., Oskarsdottir, G., Jeong, C., Chang, H. (2014b): The effect of interaural-time-difference fluctuations on apparent source width. Proc. of Forum Acusticum, Krakow, Poland.

6. Kim, C., Mason, R. and Brooks, T. (2013): Head movements made by listeners in experimental and real-life listening activities. J. Audio Eng. Soc., Vol. 61, No. 6.

7. Okano, T., Beranek, L. L., Hidaka, T. (1995): Interaural cross-correlation, lateral fraction, and low- and high- frequency sound levels as measures of acoustical quality in concert halls, J. Acoust. Soc. Am. 98 (2), 988-1007.

8. Whitmer, M. W., Seeber, B. U., Akeroyd, M. A. (2012): Apparent auditory source width insensitivity in older hearing-impaired individuals. J. Acoust. Soc. Am. 132 (1).

9. Whitmer, M. W. and Akeroyd, M. A. (2014): The perception of apparent auditory source width in hearing-impaired adults. J. Acoust. Soc. Am. 135 (6).

10. Whitmer, M. W. and Akeroyd, M. A. (2013): The effects of simulated room acoustics on the perception of auditory source width by hearing impaired adults with and without amplification. Poster presentation at Proc. of BSA.

11. Wiggins, I. M. and Seebert, B. U. (2011): Dynamic-range compression affects the lateral position of sounds. J. Acoust. Soc. Am. 130 (6).

12. Zotter, F., Frank, M. (2013): Efficient phantom source widening. Archives of Acoustics, Vol. 38, No.1, pp. 27-37.


APPENDIX

Figure A. 1 Individual (dashed lines) and averaged (solid lines) audiograms of HI listeners for the left and right ear

(blue and red curve, respectively). The insertion gain used for the hearing aids is shown with the green dotted-dashed line.


Figure A. 2 Averaged ASW results for all hearing aid conditions (represented by the different colors) for the three source signals pink noise, speech and guitar (from top to down). ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median and respective 25th and 75th percentiles. Left panel: NH listeners. Right panel: HI listeners.


Figure A. 3 Individual ASW results for NH listeners for the hearing aid conditions [w/o HA], [HAlin] and [HAcomp] (represented by the red, blue and green curve, respectively) for the pink noise signal. ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median (repeated measures) and respective 25th and 75th percentiles across the repeated stimuli presentations per listener. Individual runs are indicated by dashed lines.


Figure A. 4 Individual ASW results for HI listeners for the hearing aid conditions [w/o HA], [HAlin] and [HAcomp] (represented by the red, blue and green curve, respectively) for the pink noise signal. ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median (repeated measures) and respective 25th and 75th percentiles across the repeated stimuli presentations per listener. Individual runs are indicated by dashed lines. Note that for listener JG measurements for hearing aid HA could not be obtained.


Figure A. 5 Individual ASW results for NH listeners for the hearing aid conditions [w/o HA], [MHAlin] and [MHAcomp] (represented by the red, purple and orange curve, respectively) for the pink noise signal. ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median (repeated measures) and respective 25th and 75th percentiles across the repeated stimuli presentations per listener. Individual runs are indicated by dashed lines. For some listeners, e.g. listener PC and MM, lateralization changes occur in the hearing aid condition MHA.


Figure A. 6 Individual ASW results for HI listeners for the hearing aid conditions [w/o HA], [MHAlin] and [MHAcomp] (represented by the red, purple and orange curve, respectively) for the pink noise signal. ASW is shown in degrees on the x-axis, as left and right expansion of the sound source, as a function of the predefined source widening, sound #1 (wide) to #5 (narrow). Plotted are the median (repeated measures) and respective 25th and 75th percentiles across the repeated stimuli presentations per listener. Individual runs are indicated by dashed lines.


Figure A. 7 Relative occurrences of ITDs per frequency channel shown in form of histograms. These results for the oink noise source signal were obtained from a time-frequency analysis of ITDs for the widest (sound #1, left panels) and the narrowest (sound #5, right panels) source width. A dark color represents a high relative occurrence of ITDs. Comparing signals from the left (wide) and right panels (narrow), it is apparent that the spread in ITDs is reduced especially at low frequencies. To the right of each panel the accumulated distribution across all frequency channels (center of gravity) is shown. From top to bottom panel: conditions [w/o HA], [MHAcomp] for NH listeners, [MHAcomp] for HI listeners. Note the asymmetry of ITD occurrences at low frequencies for the [MHAcomp] conditions which might have caused lateralization changes for the listener.


Figure A. 8 Relative occurrences of ILDs per frequency channel shown in form of histograms. These results for the oink noise source signal were obtained from a time-frequency analysis of ITDs for the widest (sound #1, left panels) and the narrowest (sound #5, right panels) source width. A dark color represents a high relative occurrence of ILDs. To the right of each panel the accumulated distribution across all frequency channels (center of gravity) is shown. From top to bottom panel: conditions [w/o HA], [MHAcomp] for NH listeners, [MHAcomp] for HI listeners.


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