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Master-Vortrag: Characterization of Vibration Transfer Paths for Earphones
José Luis Conesa Pérez
Freitag, 23. August 2024
10:00 Uhr
IKS 4G | zoom
Devices that seal the ear canal from the outside, such as insert earphones, create what is usually referred to as the Occlusion Effect. This effect generates a coloration of the signals, other than the ones produced by the device, that reach the eardrum. The consequences of this coloration are a reduction of the higher components of the spectrum of air-borne sounds, and an amplification of low-frequency sounds that reach the ear canal through bone vibration. In the efforts to cancel the Occlusion Effect, there is new research into adding vibration sensors to the earphones. These sensors can record the body-conducted sounds to counter the coloration introduced on the voice by the Occlusion Effect and enhance speech.
Due to the closeness of the vibration sensor to the earphone, the Feedback Path has to be characterized in order to ensure stability of the system and avoid possible echoes. Therefore, this project’s objective is to study the feedback of the system. To this aim, a series of experiments have been conducted to test how the feedback from the loudspeaker is affected by different variables such as the tightness of the fit (how well the earphone seals the ear canal), the silicone tips used or changing the mass of the earphone. With these tests some insights into how the earphones work was gained, such as how the feedback can be reduced due to the ear canal not being perfectly sealed or by making the earphone heavier. The reduction caused by both previous examples is due to the loudspeaker having increased difficulties in moving the earphone due to energy escaping from the ear canal or a higher inertia of the earphone. In addition to these experiments, a measurement of a group of participants was conducted to inspect how the Feedback Path varies between individuals. To finalize the project, the basic mechanical behavior of the earphone when it reproduces inside of the ear canal was analyzed with the aim of creating a basic lumped element model that roughly describes its movements. This model proved to be only good as a first approximation, as it could not represent all the complexities of the vibration paths due to its simplicity. A system identification was also performed, with N4SID, on the lower parts of the spectrum of the Feedback Path to obtain the various poles and zeroes that represent the filter and observe their placement in the complex plane.
