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Master-Vortrag: Optimized Design of an Acoustic Front-End for Active Noise Cancelling Headphones
Montag, 21. Juni 2021
Headphones with active noise cancelling (ANC) capability become more and more common in everyday life. The acoustic front-end, consisting of the headphone enclosure, a speaker and at least two microphones, is a crucial part of the ANC system. It significantly influences the achievable degree of noise cancelling. The optimization of the acoustic front-end is therefore a key factor for a high ANC performance. Being able to completely predict the influence of every element of the acoustic front-end is a desirable goal.
This work comprises an investigation of the different components of the acoustic front-end. It includes the characterization of a set of microphones to evaluate the influence on the ANC performance. A development of a model of the electro-acoustic system of a headphone enclosure with an integrated dynamic speaker allows the simulation of relevant transfer paths. Every individual element of the enclosure is modeled, so that the effect of every parameter on the transfer paths can be investigated. This provides a great advantage over investigations on the basis of measurements. A comparison of the boundary element method and lumped element method shows their suitability for acoustic modeling.
The inability of the used implementation of the boundary element method to directly model viscothermal losses exhibits limitations, as these losses play an important role in the acoustic front-end. A 3D print of the prototype allowed the verification of the acoustic model with measurements. The different enclosure design parameters with regard to ANC performance were studied on the bases of both, simulations and measurement. An improvement of the prototype could be achieved by damping resonances. This reduces group delay in the path between the speaker and the inner microphone, which is an important limiting factor for ANC. The group delay of the inner microphone was found to be significant for low frequencies. The investigations deliver valuable knowledge about the influence of the different parameters of the acoustic front-end on the important factors for ANC. An acoustic model was obtained that allows the prediction of various transfer paths and an easy fine tuning of the elements of the acoustic front-end.
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