Practical Implementation of Personal Audio in a Mobile Device

The widespread use of loudspeakers on mobile devices to reproduce audio in public spaces has led to issues of both user privacy and noise nuisance. Previous work has investigated the use of acoustic contrast control to optimize the performance of small arrays of loudspeakers to create a zone within which the audio program is audible, while minimizing the level reproduced elsewhere. These investigations have generally assumed that the dimensions of both the device within which the array is mounted and the loudspeaker drivers themselves are negligible, so that the array can be modeled as monopoles in the free field. Although this is reasonable at low frequencies the effect of both finite-sized baffle and sources on the optimized array performance is significant at higher frequencies. The effects of finite-sized baffle and sources are investigated for a two-source loudspeaker array mounted on a mobile phone-sized device through both finite element simulations and real-time implementation. The baffle is shown to reduce the performance of the array at frequencies greater than around 1 kHz for the geometry considered here, but the directivity of the individual drivers then enhances the performance at higher frequencies. The effects of implementing the optimal filters in the time-domain for a real-time system are also investigated.