Proceedings of Meetings on Acoustics

Transcription

1 Proceedings of Meetings on Acoustics Volume 19, 13 ICA 13 Montreal Montreal, Canada - 7 June 13 Engineering Acoustics Session 4pEAa: Sound Field Control in the Ear Canal 4pEAa13. Secondary path variation in human listeners and its effect on an active noise cancellation system Jinjun Xiao*, Buye Xu and Tao Zhang *Corresponding author's address: Starkey Hearing Technologies Inc., 66 Washington Ave. S., Eden Prairie, MN 55344, jinjun_xiao@starkey.com Active noise cancellation (ANC) has been applied to cancel the penetrated ambient noise in the ear canal for hearing impaired listeners (Zhang et al, 1). The performance of the proposed ANC system depends on the characteristics of the secondary path (SP). In this study, we developed an in-ear ANC system where the error microphone and the miniature loudspeaker were both placed in the ear canal. In such a case, the SP response depended on the error microphone response, the loudspeaker response and how the microphone, the loudspeaker and the ear canal are acoustically coupled. A robust method was proposed to measure the SP response of the proposed ANC system under various coupling conditions using x human listeners. Variations of the measured SP responses were analyzed both within each individual and across different individuals. The effect of the SP variations on the performance of the proposed ANC system was evaluated. The implications for improving the proposed ANC system is discussed. Published by the Acoustical Society of America through the American Institute of Physics 13 Acoustical Society of America [DOI: 1.111/ ] Received Jan 13; published Jun 13 Proceedings of Meetings on Acoustics, Vol. 19, 313 (13) Page 1

2 INTRODUCTION Ambient noise can penetrate through the shell of a hearing aid into even an occluded ear canal especially at low frequencies. As a result, hearing aid wearers can still be exposed to significant amount of ambient noise in a very noisy environment. Typical noise reduction algorithms for hearing aids are ineffective for the penetrated ambient noise. Active noise cancellation (ANC) has been applied to cancel the penetrated low-frequency ambient noise in the ear canal for hearing impaired listeners [1]. It is well-known that the performance of the ANC system depends on the characteristics of the secondary path (SP) []. In this study, we measure the SP of an in-ear ANC system where the error microphone and the miniature loudspeaker (aka receiver) are both placed in the ear canal. We analyze the variations of the measured SP responses within each individual and across different individuals. The implication of the SP variation on the performance of a proposed ANC system is also discussed. SECONDARY PATH MEASUREMENT In an in-ear ANC system for the hearing aids application, the error microphone and cancelation receiver are both placed in the ear canal and connected to a digital signal processor (DSP) [Fig. 1]. In such a case, the acoustic part of the SP response depends on the transducer responses, the ear canal response, and how the microphone, the receiver and the ear canal are acoustically coupled. On the electrical side, the SP includes the responses from both the front-end and the back-end of the DSP (Fig. 1). Digital Processing DSP ADC DAC Microphone Preamp Receiver AMP Error Microphone Cancellation speaker Acoustic Propagation FIGURE 1. The secondary path components in an active noise cancellation system for a digital hearing aids application; In-ear microphone/receiver system. In the proposed ANC system, both the microphone and receiver are placed in a standard ear mold. Once inserted into an ear canal, the coupling between the ear mold and the ear canal can vary depending on how well the shapes match each other. To understand how such coupling variation impacts the SP, we propose a method to measure the complex response of the SP and then analyze its variation within or across individuals. The proposed approach can be described as follows: a broadband digital stimulus is sent from the DSP memory to the receiver. After it goes through the rest of the SP, the response is stored by the DSP on its memory. Due to the memory size constraints on the DSP, the stimulus is preferred to be chosen as a periodic signal. Specifically we choose a tone complex that consists of tones at frequencies of interest. Fig. gives an example of the stimulus with a sampling frequency of 3kHz consisting of tones of multiples of 1Hz. The magnitudes of these tones have more emphasis on low frequencies to improve the poor signal-to-noise ratio (SNR) at low frequencies. The complex response of the SP is obtained taking the transfer function between the stimulus and the response stored on the DSP memory. Averaging in the time domain was utilized to further increase the SNR. Proceedings of Meetings on Acoustics, Vol. 19, 313 (13) Page

3 The SP responses were measured on 1 subjects. They were all employees of Starkey Hearing Technologies. The measurement was done during two sessions. In each session, the ear mold was inserted into a subject s ear and the stimulus was played for 1s to obtain the averaged SP response..4. Time-domain Signal Taps FFT Magnitude FIGURE. Stimulus used in the SP measurement. Fig. 3 and 4 show the gains and phases of the SP of the system measured during the first session. It can be seen from Fig. 3 that the SP has different gains at 1Hz, ranging from -5 to 5 across the subjects. In our proposed application, the noise cancellation is mainly aimed at the low frequency range from 1Hz to 1kHz, it will be demonstrate later in this paper that the gain near 1Hz relative to the overall gain of the SP plays a critical role in the performance of the noise cancellation. To quantify this, we define the leakage of the SP as follows: SP leakage (at 1Hz) = SP gain (at 1Hz) max SP gain between 1Hz and 1kHz Following such definition, we plot the histograms of the SP leakage of the left and right ears of the 1 subjects in Fig. 4. The leakage can be generally divided into three groups: Small leakage: within -1; Medium leakage: between -1 than -; and Large leakage: over -. Figure 5 shows the leakage difference between left and right ear of the 1 subjects. Although most of the subjects have small leakage difference between the two ears, 6 of the 1 subjects have difference larger than 1. 5 Left ear responses of 1 subjects 5 Right ear responses of 1 subjects FIGURE 3. SP gains of the ANC system in the left and right ears of 1 subjects: (i) small leakage (green); (ii) medium leakage (blue); and (iii) large leakage (red). Proceedings of Meetings on Acoustics, Vol. 19, 313 (13) Page 3

4 Left ear responses of 1 subjects Right ear responses of 1 subjects FIGURE 4. SP phases of the ANC system in the left and right ears of 1 subjects: (i) small leakage (green); (ii) medium leakage (blue); and (iii) large leakage (red). 1 Leakage of left ears 1 Leakage of right ears Leakage () Leakage () FIGURE 5. Histograms of SP leakage: left ears; right ears. 1 Leakage difference per subject Leakage () FIGURE 6. Histogram of the SP leakage difference between the left and right ears. Proceedings of Meetings on Acoustics, Vol. 19, 313 (13) Page 4

5 Even for the same user and on the same ear, the SP may vary each time when the user removes and inserts the inear module. Therefore, the second session evaluation was conducted and the measured SP responses were compared with those obtained from the first session. To quantify the SP variation between sessions, we define the SP mismatch (between the first and second session) as follows: SP mismatch = max ( SP gain from 1 st session SP gain from nd session ) where the maximization is over the frequency range from 1Hz to 1kHz only. Fig. 7 shows the distribution of the SP mismatch for both left and right ears of the 1 subjects. It can be seen that while the majority of SP mismatch falls within a couple of, on some ears, the mismatch can be as large as or more. Fig. 8 shows three examples of the SPs measured from the two sessions. The differences are mostly notable in the low frequency range and/or at the resonance peaks. 15 Left 15 Right Difference () Difference () FIGURE 7. Histograms of SP mismatch: left ears; right ears. Subject 4: Left Subject 7: Left Subject 7: Right (c) FIGURE 8. Examples of SP measured during two sessions with mismatch: 1.3; 5.1, and (c) 11.. Proceedings of Meetings on Acoustics, Vol. 19, 313 (13) Page 5

6 IMPACT ON ACTIVE NOISE CANCELLATION In this section, we perform MATLAB simulation to illustrate the impact of SP variation on the performance of an adaptive feedback ANC system [3]. The ANC algorithm diagram is shown in Fig. 6, where W is the adaptive filter on the cancellation path, SP represents the real-time secondary path, and SPm is the filter representation of the measured secondary path on the feedback path and the adaptation side branch. The system in Fig. 6 is simulated in MATLAB to cancel three noise recordings (Airplane, Motorcycle engine, and Bathroom fan). FIGURE 9. Diagram of the proposed adaptive feedback ANC system. The first simulation example illustrates the impact of the SP leakage on the noise cancellation performance. The attenuation results in terms of cancellation in over the frequency range from 1Hz to 8kHz are presented in Table 1. It is clear that the cancellation performance degrades as leakage grows. TABLE 1. Noise cancellation of the system vs. SP leakage. Noise Type Small leakage (-5) Medium leakage (-13) Large leakage (-) Airplane Motorcycle engine Bathroom fan In the second example, we studied the ANC performance variation due to the mismatch between the SP and its filter representation SPm. The is defined to be the maximum gain difference between the SP and the SPm in the frequency range between 1 Hz and 1kHz. In a system without online SP identification, the SPm on the feedback path is initialized at beginning but not updated afterwards. However, as shown in the SP mismatch study, the actual SP on the cancellation path could change due to the reinsertion of the device and thus differ from the initialization. We chose 4 mismatch values and the attenuation results are shown in Table. The results indicate that when the was within 1, the system was stable but the cancellation degraded as mismatch increased. However, when the mismatch was beyond 1, the system became unstable. These results indicate that at least for some users, the SPm initialization should be taken each time when the in-ear module is inserted to the ear canal. In fact, the change of the SP may also be caused by a loose fit of the in-ear module so that the module would move in the ear canal while wearing. In that case the SPm should be updated more frequently. TABLE. Noise cancellation of the system vs. SP mismatch. Noise Type () (5) (1) (15) Airplane Unstable Motorcycle engine Unstable Bathroom fan Unstable Proceedings of Meetings on Acoustics, Vol. 19, 313 (13) Page 6

7 SUMMARY In this study, we proposed an approach to measure the SP of an ear mold ANC system to understand how the coupling between the ear mold and ear canals impacts the ANC system. We measured the SP response on the ears of 1 subjects. Variations of the measured SP response were analyzed both within each individual and across different individuals. For the majority of the subjects we studied, such variations are small (within 5), while for a small number of subjects, the SP on their ears either differs greatly from the rest, or changes significantly between sessions. MATLAB simulation implies that for the majority of the subjects, effective noise cancellation can be achieved even when the SP variation is present. ACKNOWLEDGMENTS The authors would like to thank Karrie Recker, Au.D. for help in collecting the experiment data and subjects who participated in this study. REFERENCES [1]. T. Zhang, B. Xu, S. Vishnubhotla, J. Xiao, and M. McKinney, "Applications of an annoyance perception model to noise reduction for hearing aids," International Hearing Aid Research Conference (IHCON), Lake Tahoe, California (1). []. S. J. Elliott. Signal Processing for Active Control, Academic Press, London, UK (1). [3]. S. M. Kuo and D. R. Morgan, Active noise control: a tutorial review, Proceedings of the IEEE, 87, (1999). Proceedings of Meetings on Acoustics, Vol. 19, 313 (13) Page 7