Practical audio system design for private speech reproduction

Abstract

Multi-zone sound field control allows individuals to listen to personalised audio content whilst sharing a physical space. Applications of this technology include home entertainment, audio reproduction in public spaces such as museums, shops or exhibitions, and providing areas where the privacy of sensitive communication can be safeguarded without the need for physical barriers. The problem of transmitting a speech signal to a single listener and reducing the intelligibility of that signal elsewhere is the focus of the present thesis. The motivation behind the presented experiments and simulations is to identify the practical trade-offs that must be considered in the design of these Speech Privacy Control systems. Conventional personal audio systems use loudspeaker array processing to produce a bright zone for the intended user of the system and a dark zone where silence is desired. However, established performance metrics and system optimisation techniques do not necessarily yield privacy for the target listener, as attenuated speech may remain intelligible within the dark zone. A system is proposed that focusses a synthetic masking signal into the dark zone to selectively reduce the intelligibility of the leaked speech. Privacy is ensured by adjusting the masker to meet predefined constraints on the speech intelligibility in each zone. This design methodology utilises information from speech intelligibility tests and subjective preference evaluations in order to improve the utility and acceptability of such systems for all nearby listeners. In addition to the design of the masking signal, the performance of a speech privacy control system is affected by the loudspeaker array design and the location of the listening zones. These effects are explored using experimental measurements of a loudspeaker array in a room, and the results are used to select two system configurations for additional evaluation using listening tests. The perceived performance of a system is also affected by the surrounding acoustic environment, notably due to reverberation and background noise, which may change over time. The effects of room reverberation are investigated using image source simulations and acoustical measurements within a room, and the performance is evaluated in terms of the achievable level of acoustic contrast, the difference in speech intelligibility between zones, and the masking signal levels that are required to achieve privacy. A proposal is made to further enhance privacy by combining the effects of background noise and artificial masking signals. This method reduces the level of acoustic contrast that is required to achieve a given level of privacy, compared to the case where the masking is provided by the background noise alone.

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Identifiers

ORCID for Daniel Wallace: orcid.org/0000-0003-0212-5395

ORCID for Jordan Cheer: orcid.org/0000-0002-0552-5506

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