Measurement of pinna flare angle and its effect on individualized head-related transfer functions

Transcription

1 PROCEEDINGS of the 22 nd International Congress on Acoustics Free-Field Virtual Psychoacoustics and Hearing Impairment: Paper ICA Measurement of pinna flare angle and its effect on individualized head-related transfer functions Guangzheng Yu (a), Yingyang He (b), Bosun Xie (c) (a) Acoustic Lab., School of Physics and Optoelectronics, South China University of Technology, China, (b) Acoustic Lab., School of Physics and Optoelectronics, South China University of Technology, China, (c) Acoustic Lab., School of Physics and Optoelectronics, South China University of Technology, China, Abstract Head-related transfer functions (HRTFs) are essential to the researches of binaural hearing and applications of virtual auditory display. Generally, HRTFs vary with frequency as well as source position relative to head centre. They also depend on anatomical structure and parameters of individual subject. The related anatomical parameters mainly include the dimensions of head and pinnae, and the position parameters between the pinna and head. In present work, the influence of pinna flare angle on HRTFs is investigated by using numerical calculation. Based on a combination model of ellipsoidal head and laser-scanned pinnae from KEMAR artificial head, the pinna flare angle in the model is changed from an original value with displacements of 6.7 degrees, and the corresponding horizontal HRTFs at various azimuths are calculated by using boundary element method. Results indicate that the changing pinna flare angle leads to a systemic variation on the frequency and azimuthal distribution of HRTF magnitude spectra. And approximately, the variation of azimuthal distribution of HRTF magnitude spectra is linearly related to the change of pinna flare angle. Therefore, individualized azimuthal HRTF magnitudes corresponding to various pinna flare angle can be predicted or customized by applying appropriate azimuthal rotation manipulation to a set of original HRTF data with certain pinna flare angle. The method in present work is applicable to customize a matched set of HRTFs for improving the perceived performance of virtual auditory display. Keywords: Head-related transfer function; anatomical parameters; customization.

2 Measurement of pinna flare angle and its effect on individualized head-related transfer functions 1 Introduction The acoustic transmission from a point source in free field to ears of subject can be described by head-related transfer functions (HRTFs). By definition, HRTF varies with the sound source position in the three dimensional space. The position of sound source can be specified by the azimuth, elevation and distance r. In addition, HRTFs depend on the individualized anatomical parameters of human subjects [1]. It is generally believed that different anatomical structures affect HRTFs at various frequency ranges. For example, the reflections from pinnae result in the spectral characteristics of HRTFs at high frequencies above about 5~6 khz [2], and the reflections of head and torso (except for the fine structures such as nose, eyes, pinnae, etc.) mainly impact on HRTFs at frequencies below about 3 khz [3]. In time domain, because the reflections from different anatomical structures occur in different time, the effects of pinnae, head and torso on HRTFs can be investigated separately. Based on the isolated anatomical structures, some structural models of individualized HRTFs were suggested [4]. In order to investigate the influence of pinnae on HRTFs, some parameters describing the fine structure of pinnae should be specified and measured. In an early study, 6 pinna-related parameters were specified [5]. Then, Algazi et al. used up to 12 parameters to describe pinnae [6]. In addition, Xie and Zhong et al. measured some head and pinna-related anatomical parameters for HRTF study [7]. More recently, Yu et al. further classed the pinna-related parameters into two groups. The first group with 8 parameters specifies the dimension of pinnae. And the second group with 4 parameters, including the pinna rotation angle, pinna flare angle, pinna frontward-offset and pinna downward-offset, specifies the position of pinna relative to head [8]. Previous research indicated that the pinna frontward- and downward-offsets are important to arrival of time (VOA) or inter-aural time difference (ITD) in HRTFs [9]. The pinna rotation angle was expected to influence the spectral features of HRTFs in different directions [10]. However, the effect of pinna flare angle on HRTFs is still unclear and lack of validation. Moreover, the result of pinna flare angle is sensitive to the measurement method. To address the aforementioned problem, the Frankfurt-plane is adopted in present work to improve the consistency in pinna flare angle measurement. Then pinna flare angles from 60 human subjects have been measured and statistically analysed. Then, the influence of pinna flare angle on HRTFs is investigated by numerical simulation and analysis on HRTFs of an ellipsoidal head model with various pinna flare angle. Finally, a coordinate transformation based method for customizing HRTFs with different pinna flare angle is suggested and evaluated. 2

3 2 Pinna flare angles 2.1 Definition and measurement The pinna flare angle is difficult to be measured directly, but it can be indirectly estimated by using triangle ABC in the horizontal plane [Fig.1(a)]. As shown in Fig.1(c), lines AB, AC and BC represent pinna flaring distance, tragus helix distance, and pinna posterior to tragus distance, respectively [1]. In practical measurement, however, it cannot be guaranteed that the points A, B, C actually locate in the horizontal plane because the operation of setting the horizontal plane is difficult for a human subject, and measured positions of points A, B and C could be unrepeatable because of unconscious head movements of subject during measurement. In current work, the head and pinnae of subject have been scanned by laser scanner in advance [4], so as to avoid the influence of head movement during measurement. In the scanned figure, the Frankfort plane is adopted to determine the horizontal plane. As shown in Fig. 1(a), the Frankfurt plane is determined by the left and right tragions as well as infraorbitales. The horizontal plane is parallel to the Frankfurt plane and goes through the centre of ear canal entrance. The section of Fig. 1(b) locates in the horizontal plane. Then, the pinna flare angle can be calculated by using the slopes of the auxiliary lines l 1 and l 2. The line l 1 goes through the tragion and is tangential to the front outline of the pinna; the auxiliary line l 2 is approximately tangential to the head surface back of pinna. All the measurement processes including setting the auxiliary lines are implemented in the software designed by ourselves. The aforementioned geometrical parameters are evaluated from the scanned image of human head and pinnae by Geomagic graphic software. The figure processing depends on a common skill, and thus it is omitted in the work. Figure 1: Method of pinna flare angle measurement: (a) the initial coordinate system and Frankfort plane; (b) the definition of flare angle on the section of the scanned head figure. 2.2 Statistical analysis The right-pinna flare angles of 60 human subjects have been measured using the method in section 2.1. The measured data is shown in Fig. 2. In total, all pinna flare angles are within the range of [29.1, 59.5 ], with the mean value of 39.0 and the standard deviation of about 6.7. The maximum individual difference of pinna flare angle reaches about 30. Generally, it is assumed that the pinna flare angel changes the pinna-related reflections in different azimuthal 3

4 directions. The azimuth resolution of sound source localization in horizontal plane varies from 1 to 5 [1], and thus is less than the standard deviation 6.7 of pinna flare angle. Therefore, the influence of pinna flare angle on HRTFs is worthy of more attention and investigation. Figure 2: The measured pinna flare angles for 60 subjects. For comparisons, three groups of pinna flare angles obtained from different measurements are listed in Tab. 1. They are from CIPIC database [6], SCUT1 database [7] and SCUT2 database measured in the current work, respectively. Generally, the standard deviations in three measurements are basically same (about 7 ). The mean values of SCUT1 and SCUT2 are about 40.0, which is 10 larger than that of CIPIC data. However, it cannot be concluded that there is statistically significant difference on pinna flare angle between the subjects in CIPIC and SCUT1/SCUT2, because the difference could also be induced by the different measurement methods and standards. Table 1: Comparisons between three groups of measured pinna flare angles. HRTF database Number of subjects Mean value ( ) Standard deviation ( ) CIPIC SCUT SCUT Effect of pinna flare angle on HRTFs In order to analyse the influence of pinna flare angle on HRTFs, HRTF calculation model with three different pinna angles is shown in Fig. 3(a), 3(b) and 3(c), respectively. The calculation model consists of an ellipsoid head and DB-060 pinna [11]. The dimensions of ellipsoid head were determined by the head depth m, width m and height m, which were approximately matched from the KEMAR artificial head. The geometric image of pinna was acquired by laser scanner and then fixed to the geometric model of ellipsoid head. The initial 4

5 pinna flare angle is 31.0, and the 6.7 offsets result in other two pinna flare angles of 37.7 and The 6.7 is just the statistical standard difference in SCUT2 database. Therefore, HRTFs for three pinna flare angles are analysed, and the only variable in the model is pinna flare angle. The HRTFs are calculated using boundary element method (BEM). Figure 3: The calculation models consist of the ellipsoid head and pinnae with flare angles: (a) 0 = 37.7 ; (b) 0 = 31 ; (c) 0 = The logarithmic magnitudes of right-ear HRTF in the horizontal plane, denoted by H 0, H 1 and H 2 for 0 = 37.7, 31 and 24.3, are plotted in Fig. 4(a), 4(b) and 4(c), respectively. In Fig. 4, horizontal azimuth of the sound source is specified by 0 < 360, with = 0 and 90 being the front and right directions, respectively. Figure 4: The calculated HRTF magnitudes (a) H 0 : for flare angle 0 = 37.7 ; (b) H 1 : for flare angle 0 = 31.0 ; (c) H 2 : for flare angle 0 =

6 The HRTF magnitudes for three pinna flare angles exhibit some common features. Due to the head shadow at directional range of 180 < 360, the contralateral HRTF magnitudes are attenuated at high frequencies and the influence of pinna is inconspicuous. However, in the ipsilateral directions, especially within 0 < 90, the difference in H 0, H 1 and H 2 are observed and thus the influence of pinna flare angle is obvious. Then, the differences of logarithmic HRTF magnitude for different pinna flare angles are calculated and denoted by SD 1 =H 1 H 0 and SD 2 = H 2 H 0. If SD 1 or SD 2 is close to 0 db, it means there is no difference. Fig. 5(a) and 5(b) shows the results. Because the influence of pinna on HRTF is small at contralateral direction, the figures only show the results for ipsilateral direction of [0, 180 ]. It is observed that, generally, the magnitude difference increase with frequency. The difference is within about ± 1.0 db and thus unobvious below the frequency of about 5 khz. Actually,the contribution of pinna to HRTF magnitude is small or even neglectable below 5 khz. Above about 5 khz, however, the magnitude difference becomes significant because of the enhancement of the pinna reflections. Especially, the difference is obvious within frontal and lateral direction 0º 120 º as well as at two narrow frequency bands varying between 7 khz and 14 khz. These two frequency band corresponds to the highfrequency notches of HRTF magnitudes and vital to elevation localization. Therefore, the change of pinna flare angle causes a variation on the frequency and azimuthal distribution of HRTF magnitude spectra. Figure 5: The differences of logarithmic HRTF magnitude for different pinna flare angles with (a) SD 1 = H 1 - H 0 ; (b) SD 2 = H 2 - H 0. Where, H 0, H 1 and H 2 correspond to pinna flare angle of 37.7, 31.0 and 24.3, respectively. 6

7 4 HRTF customization for different pinnae flare angles In order to analyze the detail effect of pinna flare angle on azimuthal variation of HRTF, the HRTF magnitudes for two pinna flare angles 0 = 37.7 and 24.3, two source azimuths = 46.9 and 60.3 are compared. The interval of two source azimuths, i.e., 13.4, is just equal to the interval of two pinna flare angles. The results are shown in Fig. 6 and denoted by H 0 (46.9 ), H 0 (60.3 ), H 2 (46.9 ), H 2 (60.3 ), respectively. The notch in magnitude spectra is called pinna notch of HRTF. It is observed that the HRTF magnitude spectra, especially the central frequencies of pinna notches varies with both source azimuth and pinna flare angle. However, magnitude spectra H 0 (46.9 ) and H 2 (60.3 ) are almost identical, with central frequencies of pinna notch being 9.15 khz and 9.20 khz, respectively. Figure 6: High-frequency HRTF magnitude spectra for two pinna flare angles 0 = 37.7 and 24.3, two source azimuths = 46.9 and The above result suggests that the variation of HRTF magnitudes caused by the change of pinna flare angle can be compensated or equalized by changing the azimuth of source. In other words, azimuthal HRTFs for various pinna flare can be estimated from those with known pinna flare by rotating the source azimuth. This may yield a method for customizing individualized HRTFs of different pinna flare angle. To further validate this conclusion, Fig.7 compares the differences of logarithmic HRTF magnitude for compensated and uncompensated cases, (a) SD 2 = H 2 (θ) H 0 (θ), and (b) SD 2 = H 2 (θ º) H 0 (θ). Overall, the magnitude difference SD 2 is significantly less than SD 2. Therefore, the method of customization is basically feasible. The azimuthal rotation-based method compensates for the change caused by the variation of pinna angle. At the same time, however, it changes source direction relative to head and inevitably changes inter-aural localization cues in HRTFs, such as inter-aural time difference (ITD) and inter-aural level difference (ILD). To alleviate this problem, the azimuthal rotation manipulation of sound source can be only applied to the HRTF magnitude spectra at high frequency above 5 khz, so as to reserve the ITD at low frequency, which is a dominant localization cue for azimuthal or lateral localization. 7

8 Figure 7: compares the differences of logarithmic HRTF magnitude for compensated and uncompensated cases, (a) SD 2 = H 2 (θ) H 0 (θ) (b) SD 2 = H 2 (θ) H 0 (θ + 6.7º). 5 Conclusions The pinna flare is one of anatomical parameters related to HRTFs. It varies with subject and can be reliably measured by using laser scanner and with the help of Frankfort plane. The mean value and standard deviation of pinna flare angles for 60 subjects are 39.0 and 6.7, respectively. The influence of pinna flare angle to HRTF is evaluated by using the BEMcalculation HRTFs from a combined ellipsoidal head and laser-scanned pinnae model with a various pinna flare angle. The variation range of pinna flare angle is within one standard difference around the mean value. The evaluated results indicate that the pinna flare angle mainly influence the frequency and azimuthal distribution of HRTF magnitude spectra above about 5 khz. In addition, the variation of HRTF magnitudes caused by the change of pinna flare angle can be compensated by changing the azimuth of source. Acknowledgments This work is supported by the National Natural Science Foundation of China ( ), the Guangdong Province Outstanding Young Teachers in Higher Education Institutions (Yq ), and Fundamental Research Funds for the Central Universities of South China University of Technology (2015ZZ135). References [1] Xie, B.; Head-related transfer function and virtual auditory display, J. Ross Publishing, New York (USA), The 2 nd edition, [2] Xie B., et al. A cluster and subjective selection-based HRTF customization scheme for improving binaural reproduction of 5.1 channel surround sound, the 134 th Convention of the Audio Engineering Society, Rome, Italy, May 4 7, [3] Zotkin, D. N., et al. HRTF personalization using anthropometric measurements, the proceedings of the 2003 IEEE workshop on the Applications of Signal Processing to Audio and Acoustics, New York, USA, October 19 22,

9 [4] Brown C. P., Duda R. O., A structural model for binaural sound synthesis, IEEE Trans. Speech and Audio Processing, Vol 6 (5),1998, pp [5] Middlebrooks, J. C. Individual differences in external-ear transfer functions reduced by scaling in frequency, the Journal of the Acoustical Society of America, Vol 106(3), 1999, pp [6] Algazi, V. R., et al. The CIPIC HRTF database, the proceedings of the2001 IEEE workshop on the Applications of Signal Processing to Audio and Acoustics, New York, USA, October 21 24, [7] Xie, B. S., et al., Head-related transfer function database and its analyses, the Sci. China, Ser. G: Physics, Mechanics & Astronomy, Vol 36(5), 2006, pp [8] Yu, G., et al. The initial head gesture and coordinate system in measuring anthropometric parameters and head-related transfer functions, the 170th Meeting of the Acoustical Society of America, Jacksonville, Florida, USA, November 2-6, [9] Yu, G.; Wu, Y. Variation of interaural time difference caused by head offset relative to coordinate origin, the 138th Convention of Audio Engineering Society, Warsaw, Poland, May 7 10, [10] Yu, G., et al. Effect of pinna rotation angle on the head-related transfer function, the Journal of Technical Acoustics (in Chinese), Vol 33(12), 2014, pp [11] Rui, Y., et al. Calculated individualized near-field head-related transfer function database using boundary element method, the 134th Convention of Audio Engineering Society, Rome, Italy, May 4 7,