Effect of microphone position in hearing instruments on binaural masking level differences

Transcription

1 Effect of microphone position in hearing instruments on binaural masking level differences Fredrik Gran, Jesper Udesen and Andrew B. Dittberner GN ReSound A/S, Research R&D, Lautrupbjerg 7, 2750 Ballerup, Denmark, The aim of this paper is to investigate the effects of microphone position for digital hearing instruments on the binaural masking level differences (BMLD) compared to monaural listening. Bilateral impulse responses for 46 different microphone positions were measured in the concha and on and around the pinnae of a KEMAR manikin. It was assumed that the target signal was originating from zero degrees and that all other angles were considered potential masker directions. The BMLD was evaluated using the equalization and cancellation (EC) model described by Durlach (1963). The SNR, after the EC process, was computed for each microphone position and angle and consequently compared to the situation where the target and masker were incident from the same direction. The microphone positions were then grouped to represent typical hearing aid microphone positions and BMLD was compared across groups. The computations show large differences in BMLD. The difference can be as large as 2.8 db when comparing the positions with the best, to the positions with the worst BMLD. When comparing the best single position to the worst position, the difference was 5-6 db for certain frequencies and angles. In general, the positions in the concha and in front of the outer ear display better BMLD than the positions behind the ear. 1 Introduction This paper investigates the implications on Binaural Masking Level Differences (BMLD) of different microphone position candidates for hearing instruments. Traditional evaluation of microphone position in hearing aids relies on monaural performance metrics such as directivity index (DI) [1, 2, 3, 4] and does not take into account the binaural aspects of spatial listening. A key element in hearing and interpreting the acoustic wave field is binaural processing [5]. The two signals at the ears contain a multitude of information about the spatial nature of any of the sources in the acoustic wave field. The spatial information is encoded in Interaural Time Differences (ITD), Interaural Level Difference (ILD), spectral cues and reverberation cues. Binaural processing by the brain, when interpreting the spatially encoded information, results in several positive effects; better signal-to-noise ratio (SNR); direction of arrival (DOA) estimation; depth/distance perception and synergy between the visual and auditory systems [6, 7, 8]. Hearing aid solutions and in particular the microphone position affect the audio signal, constantly interfering with the integrity of the sound. The end users have been reported to have poorer ability to localize sounds and determine DOA in the aided situation compared to the unaided [9]. Theoretical models that predict human behaviour in different binaural listening tasks have been proposed in several publications, e.g. [10, 11, 12]. Of particular interest for this paper is the development of the Equalization and Cancelation (EC) model described in [13, 14] as this model will form the basis of the evaluation of binaural performance. The purpose of this paper is to incorporate a binaural listening model into the performance evaluation of a multitude of microphone positions. The experiments involved measuring the acoustic impulse response for 46 different microphone positions in and around the ear on a Knowles Electronic Manikin for Acoustic Research (KEMAR) in an anechoic chamber. The measurements were repeated for 180 different angles of incidence in the lateral plane. Data that would induce binaural processing was generated by combining a target signal from zero degrees with a masker from a different direction. The EC model was then applied to model the binaural signal-to-noise ratio (SNR) for each of the microphone positions and combinations of target and masker. The paper is

2 organized as follows; in section 2, the acoustic experiments are described, in section 0, the implementation and setup of the EC model is explained, in section 4, the results are presented and in section 5 the results are discussed. 2 Acoustic experiments The experiments involved measuring head related impulse responses (HRIR s) on KEMAR. In this paper we measure the HRIR s on a KEMAR manikin in the horizontal plane with an angular resolution of 2 degrees. We do this for 46 different microphone positions within and around the modelled human ear. The measurements are conducted in an anechoic chamber The measurement chamber An anechoic room was used for the HRIR measurements. The room is in accordance with ISO The distance from the speaker to the rotation axis of KEMAR was 1.5 m Microphones In this paper 8 QM Knowles microphones were used for the acquisition of acoustic signal. The microphones have approximately the same transfer functions and a bandwidth of nearly 20 khz. Furthermore, the physical dimensions of the microphones are only 4.3mmx3.0mmx1.47 mm which makes them suitable for measurement of different pinna positions. Each microphone is attached to a custom made (Larsen Electronics) preamplifier where the amplification is set to 58 db. The signals at KEMAR s eardrum were acquired with a 711 coupler Artificial ears and microphone positions An artificial ear was used for the measurements. The ear was reconstructed from 3D ear scans of a GN ReSound employee. The number of different microphone positions on each ear was 46. Each microphone was attached to either a jig or the pinna with a small needle mounted on the back of each QM microphone. The positions can be seen in Figure 1. Three jigs were constructed to ensure the right placement of the 24 emulated behind the ear (BTE) microphone positions. One of the jigs and how it is mounted on KEMAR is seen in Figure 2. Figure 1: Large male ear with 45 numbered microphone positions. The Blue positions are possible BTE positions. The yellow positions are in front of the ear. The white positions are inside pinna at locations easily recognized by features of pinna. The green positions are behind pinna, and considered to be lower BTE positions.

3 Figure 2: The largest jig with the up to 8 QM microphones attached and positioned on KEMAR The microphone positions can be divided into 4 categories. The positions marked in blue on Figure 1: These positions resemble possible locations on a fictive BTE device. The surface where the microphones are placed is on the smallest jig an offset by 2 mm of the base surface of the ear. The microphone surface of the medium jig is an intermediate line between the largest jig and the smallest. The offset is approximately 5 mm between each jig. The spacing between each microphone position on all the jigs is 6 mm. This group is further sub-divided into visible BTE positions (1-4, 9-12, 17-20) and invisible BTE positions (5-8, 13-16, 21-24) The positions marked in yellow in Figure 1: These 8 positions are placed in front of the ear in 2 vertical columns with 4 rows in each column. The spacing between each microphone position is 6 mm. The positions marked in white in Figure 1: The positions numbered and 45 are all placed inside the ear at various places defined by ear geometry/features. These positions are referred to as the ITE positions. The positions marked in green in Figure 1: These four positions are all placed behind pinna from the earlobe and up to a position where an imaginary horizontal line going from the opening of the ear canal intersects with the back of pinna. These positions are revered to as the lower BTE positions Speaker and power amplifier The speaker used in all experiments was a KEF Q85S (serial number: G). The phase is inverted by connecting (- ) on the speaker to (+) on the ROTEL RB-1050 power amplifier. The recoded microphone signals were convolved with the inverse of the speaker impulse response before further processing Sound hardware All measurements were performed at a sampling frequency of Hz using a Tucker Davis RX8 multiprocessor controlled by MATLAB, The MathWorks Inc., Natick, MA Excitation signal The signal presented through the speaker was a Maximum length sequence (MLS) signal [15]. In the anechoic room the code length was (2^11-1) = 2047 samples. This corresponds to an acoustic distance of 14.2 m. It was tested that the room reflections were below the noise floor at this distance. The corresponding intensity for the speaker signal at KEMAR s head position (when KEMAR was removed) was 74 db SPL.

4 3 Modelling binaural masking level differences Let the HRIRs for the target and masker in the right and left ear be and let the corresponding transfer functions be given by R L R L ht ( t), ht ( t), hm ( t), hm ( t) (1) R L R L H ( ω), H ( ω), H ( ω), H ( ω) (2) T T The target direction is for all experiments zero degrees but the masker direction can vary. We define the estimate of the residual target and masker components after the EC process (calculated according to equation 4 in [13]) as M M S ( ω) and N( ω) (3) where S (ω) is the target and N (ω) is the masker components respectively. As the estimates of the residual target and masker are stochastic variables, several independent simulations have to be carried out to provide a representative SNR after the EC process. We proceed and define the estimated SNR after the EC process as SNR EC Q 1 = q= 0 Q 1 q= 0 S( ω) 2 ( ω ) (4) 2 N( ω) where q is the experiment number and Q is the total number of experiments. The operator denotes average over the ensemble. The SNR for the two ears are similarly defined as SNR R,L R, L M 2 R, L H T ( ω) ( ω) = 2. (5) H ( ω) The BMLD was evaluated by comparing the SNR after the EC process for a target from zero degrees and a masker from an arbitrary angle with a target from zero degrees and the masker from the same direction. 4 Results The BMLD was evaluated for target from zero degrees and a masker with an arbitrary angle of incidence. The masker angle was varied from 0 to 360 degrees in 2 degree increments. The SNR after the EC process was compared to that of the situation where both the target and masker were incident from 0 degrees. The BMLD was evaluated for frequencies between 0 and 2 khz. The microphone positions were grouped in five sets; set number one represent the visible BTE positions on top of the ear, set number two represent the BTE positions covered by the pinna, set number three represent the positions in front of the ear, set number four represent the positions in the ear and set number five are the positions all the way down behind the ear. The BMLD was averaged over each set. In Figure 3, the BMLD is displayed for the frequency 300 Hz.It can be seen that BMLD is symmetric and the maximum value is achieved by the microphone set in front of the ear. The maximum BMLD is approximately 11 db for 300 Hz. The in the ear and visible BTE positions are better than the more hidden BTE positions. The BMLD as a function of angle for the frequency 600 Hz is displayed in Figure 4. The maximum BMLD is still attained by the microphones in front of the ear. The in the ear positions and the lower behind the ear positions display a slight degradation in BMLD whereas the visible and invisible BTE position give a degradation of up to 3 db. To get an impression of average performance across angle and as a function of frequency, the BMLD was averaged over angle. The resulting average BMLD can be seen in Figure 5. Again the trend is that the microphones in front of the ear generate the best BMLD closely followed by the in the ear positions. The worst average performance is given by the lower and invisible BTE positions.

5 If the positions generating the extreme BMLDs are chosen for a given frequency the difference will be larger than what is given when comparing across groups. This is illustrated in Figure 6 where the positions generating the best and worst BMLDs are shown for the frequency 600 Hz. It is shown that the difference can be as large as 5-6 db for certain angles. Figure 3: The BMLD as a function of angle for the frequency 300 Hz. Figure 4: The BMLD as a function of angle for the frequency 600 Hz.

6 Figure 5: The BMLD averaged over angle as a function of frequency. Figure 6: The BMLD for the positions generating the extreme BMLDs for the frequency 600 Hz. Position 44 is in front of the ear and position 24 is behind the ear. The difference between the two can be as large as 5 db.

7 5 Discussion In this paper the BMLD has been analysed for different microphone positions in and around the ear. The purpose was to use a binaural model to evaluate the different microphone positions rather than using monaural (single ear) metrics to assess listening performance. Instead of using real subjects, the EC model from [13] was used. This model is only considered to be valid up to 1200 Hz, wherefore the BMLD analysis was limited to this frequency range. To get a more precise understanding of how speech reception thresholds are affected, a wider frequency range would probably be needed. For this purpose, an extended EC model could be used [16] and this would be interesting to look into in the future. The SNR, after the EC process, was computed for each microphone position and angle and consequently compared to the situation where the target and masker were incident from the same direction. The microphone positions were then grouped to represent typical hearing aid microphone positions and BMLD was compared across groups. The computations show large differences in BMLD. The difference can be as large as 2.8 db when comparing the positions with the best to the positions with the worst BMLD. In general, the positions in the concha and in front of the outer ear display better BMLD than the positions behind the ear. If the extremes are analysed (i.e. the positions generating the best and worst BMLDs), the difference is even larger and it was shown that for the frequency 600 Hz and for certain angles the difference in BMLD can be around 5-6 db. References [1] Beranek, L.L. Acoustics. New York : McGraw-Hill Electrical and Electronic Engineering Series, [2] Real-ear polar patterns and aided directional sensitivity. Fortune, T.W. 8, s.l. : Journal of American Academy of Audiology, 1997, pp [3] Directivity quantification in hearing aids:fitting and measurement effects. Rickets, T.A. 21, s.l. : Ear Hear., 2000, pp [4] Measurement and intelligibility optimization of directional microphones for use in hearing aid devices. M., Roberts and R., Schulein : Audio Engineering Society, New York. AES 103rd Convention. [5] How we localize sound. Hartmann, W. M. 1999, Physics today, pp [6] The effect of head induced interaural time and level differences on speech intelligibility in noise. Bronkhorst, A. W. and Plomp, R. 1988, Journal of the Acoustical Society of America, Vol. 83(4), pp [7] Binaural speech intelligibility in noise for hearing impaired listeners. Bronkhorst, A. W. and Plomp, R. 1989, Journal of the Acoustical Society of America, Vol. 86(4), pp [8] The benefit of binaural hearing in a cocktail party: effect of location and type of interferer. Hawley, M. L., Litovsky, R. Y. and Culling, J. F. 2004, Journal of the Acoustical Society of America, Vol. 115(2), pp [9] Horizontal localization with bilateral hearing aids: without is better than with. Van Den Bogaert, T., et al., et al. 2006, Journal of the Acoustical Society of America, Vol. 119(1), pp [10] Colburn, H.S. and Durlach, N.I. Models of binaural interaction. [ed.] E. Carterette and M. Friedman. Handbook of Perception: Hearing. New York : Academic Press, 1978, Vol. 4, pp [11] Colburn, H.S. Computational models of binaural processing. [ed.] H. Hawkins and T. McMullin. Auditory Computation. New York : Springer Verlag, 1995, pp [12] Stern, R.M. and Trahiotis, C. Models of binaural perception. [ed.] R.H. Gilkey and T.R. Anderson. Binaural and Spatial Hearing in Real and Virtual Environments. New York : Lawrence Erlbaum Associates, 1996, pp [13] Equalization and cancellation theory of binaural masking level difference. Durlach, N. I. 1963, Journal of the Acoustical Society of America, Vol. 35, pp [14] Durlach, N.I. Binaural signal detection. [ed.] J.V. Tobias. Foundations of Modern Auditory Theory. New York : Academic Press, 1972, Vol. 2, 10, pp [15] Proakis, J.G. and Salehi, M. Communication Systems Engineering. s.l. : Prentice Hall, [16] Application of an extended equalization-cancellation model to speech intelligibility with spatially distributed maskers. Wan, R. and Durlach, N.I., Colburn, H.S. 6, s.l. : Journal of the Acoustical Society of America, 2010, Vol. 128, pp