In modern cars a lot of effort is put on reducing the background noise level. Despite these efforts it is often difficult for persons in the rear seat(s) to hear the persons in the front seat. This is partly due to the background noise, but also geometry and acoustics properties of the passenger compartment.
The aim of this project was to implement a speech enhancement system to increase the audibility between the driver and the rear passenger(s). The speech enhancement system should not affect the directivity of the speech or increase the background noise level.
A speech enhancement system has been implemented on a DSP in a test car. A microphone was placed in front of the driver to collect his/her speech. The microphone signal was bandpass filtered to remove the main part of the background noise and to avoid aliasing. The signal was delayed before it was sent out in the rear loudspeaker. The delay made the speech from the driver reaching the rear passenger before the sound the rear loudspeakers. This delay was enough to get the right directivity of the sound, i.e. making speech sounding as if it came from the driver instead of the rear loudspeakers.
In the project other methods to reduce background noise and get directivity of the sound were evaluated, but not implemented in the test car. The evaluations of the system showed that the audibility was increased. At the same time the background noise level was not noticeable increased.
Source: Linköping University
Author: Nygren, Mårten
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