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Master-Vortrag: Cancelling of Non-Stationary Disturbances in Active Headphones
Freitag, 20. November 2020
The occlusion effect is a consequence of sealing the ear canals with an object such as a hearing aid or more generally a hearable. It results in an amplification of lower frequencies and attenuation of higher frequencies, which further leads to an unwanted distortion of the own-voice perception. To compensate this phenomenon, active and passive solutions have been presented in literature. Passive solutions include venting, where a ventilation hole is integrated into the hearing aid. However, this leads to unwanted feedback. An active approach has been presented by Liebich et al., in which a time-invariant robust feedback controller compensates the low frequency amplification. This solution is limited to the attenuation of the occlusion effect on the own-voice. Other body-conducted sounds, such as footsteps, chewing and swallowing are not explicitly considered by Liebich et al.
In this thesis, body-conducted sounds exceeding the own voice components are investigated and tackled. Measurements were conducted to identify the spectral distribution of different BC sounds due to the occlusion effect. Afterwards, a controller was designed to attenuate disturbances in the region of 40 Hz to 80 Hz. This is followed by the introduction of a stable controller interpolation scheme for switching between controllers. By use of this so called Youla-Kucera interpolation method, not only stability but also performance is guaranteed during switching for a nominal path. Additionally, robust stability for this interpolation scheme can be proven for discrete steps of delta and additional constraints on the choice of Q. This is supplemented by a simple and low complexity approach for detecting footsteps, based on a low-pass filter and recursive smoothing over time. Lastly, the controller was implemented in real-time on a dSPACE ulta low latency processing system to validate performance of the footsteps controller and the switching performance. While the controller worked in theory, real-time measurements revealed that additional tuning and possibly additional sensors are needed for a better footsteps controller. However, the switching scheme showed promising results, fading from one controller to the other in a stable and smooth fashion. It also revealed the possibility of real-time controller tuning with the help of the interpolation coefficient. As all combinations of the two controllers (with the interpolation coefficient 2) remain robust stable, switching to different configurations is made possible.
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