Spectral directional cues captured by hearing device microphones in individual human ears

Abstract

Spatial hearing abilities with hearing devices ultimately depend on how well acoustic directional cues are captured by the microphone(s) of the device. A comprehensive objective evaluation of monaural spectral directional cues captured at 9 microphone locations integrated in 5 hearing device styles is presented, utilizing a recent database of head-related transfer functions (HRTFs) that includes data from 16 human and 3 artificial ear pairs. Differences between HRTFs to the eardrum and hearing device microphones were assessed by descriptive analyses and quantitative metrics, and compared to differences between individual ears. Directional information exploited for vertical sound localization was evaluated by means of computational models. Directional information at microphone locations inside the pinna is significantly biased and qualitatively poorer compared to locations in the ear canal; behind-the-ear microphones capture almost no directional cues. These errors are expected to impair vertical sound localization, even if the new cues would be optimally mapped to locations. Differences between HRTFs to the eardrum and hearing device microphones are qualitatively different from between-subject differences and can be described as a partial destruction rather than an alteration of relevant cues, although spectral difference metrics produce similar results. Dummy heads do not fully reflect the results with individual subjects.

The Hearpiece database of individual transfer functions of an in-the-ear earpiece for hearing device research

“…The between-participant variation of HRTFs measured in the ear canal has been studied extensively and is usually larger than in the HRTFs of the present work [42]. However, it has been shown before that the between-participant differences decrease at hearing device microphones, due to partial destruction of pinna cues that are very individual [6]. Variations both between participants and insertions are also well-known for headphone transfer functions, and the variations of the driver responses seen here are consistent with previous literature on the issue [43].…”

“…The most prominent difference is an amplification around 2-10 kHz originating from ear cavity resonances, which is seen at the eardrum but not at the Concha microphone [3,39]. While the differences between left and right hemispheres are evident at both microphone locations, the eardrum HRTFs also contains more spatial dependences than the Concha microphone that originate from directional pinna filtering effects, which are largely destroyed by inserting the device [6,7]. Figure 5 shows the diffuse-field insertion loss, the attenuation of external sounds reaching the eardrum by inserting the passive device.…”

“…In conclusion, KEMAR with anthropometric pinnae seems like a suitable tool to assess the acoustic properties of in-ear hearing devices in a median adult human earno less, but no more. For the development and robustness analysis of algorithms, we suggest measurements on multiple human participants (as provided in this database) rather than several repeated measurements in KEMAR [6,9].…”

“…To simulate input signals of hearing devices, signals can be convolved with appropriate Headrelated Transfer Functions (HRTF) that describe the acoustic free-field transmission to the hearing device microphone from a certain incidence direction. Several researchers have presented measurements of hearing device HRTFs [2][3][4][5], and it has been shown that there are significant differences in the HRTFs between hearing device styles and microphone positions, as well as differences and perceptual consequences with respect to HRTF measured in the unobstructed ear [6][7][8]. Also, the differences between individuals and the implications for designing signal processing algorithms has been recognized [3,9].…”

Hybrid multi-harmonic model for the prediction of interaural time differences in individual behind-the-ear hearing-aid-related transfer functions

“…When wearing HAs, the sound field is captured by the microphones of an HA, with the corresponding spatial filters referred to as HARTFs or by similar terms [3][4][5][6]. It is well understood from systematic investigations that the HA-device style and the microphone positions have considerable effects on HARTFs and directional hearing [7][8][9]. In part, a reduced localisation performance in the horizontal plane when using behind-the-ear receiver-in-ear HAs can be attributed to modified binaural cues, and limited access to ITDs owing to the reduced bandwidth of the HA receivers [10].…”