The active control of random sound inside cars

Mohammad, Janatul Islah (2006) The active control of random sound inside cars. University of Southampton, Doctoral Thesis.

Abstract

One approach that has shown the potential to achieve substantial reduction of low frequency road noise is active control, which can be integrated into a car audio system for commercial applications.

This research considers the possibility of introducing a multi-channel feedforward control system using internal microphones as reference sensors in order to produce an affordable active control system for commercial vehicle that has better performance over a wider bandwidth.

An idealised car model, consisting of a rectangular acoustic enclosure with a thin, small flexible panel placed at the bottom of the enclosure, has been used to examine the structural-acoustic interaction.  The attenuation due to active control, however, is over-predicted when a weakly-coupled analysis is used.  When fully-coupled analysis is used, the vibration of the flexible panel is increased over a certain frequency range when active control is present, since the active noise control reduces the pressure in the enclosure, that otherwise limits the vibration.

When random force excitation is used, it has been shown that for an unconstrained (frequency domain) feedforward controller, a significantly smaller number of microphone or accelerometer reference signals are required compared with using the uncorrelated random primary focuses as reference sensors.

Further support for these findings was obtained by analysing road noise data measured in collaboration with Lotus Engineering.

While this suggests that good performance can potentially be obtained from a low-cost system using microphones as reference signals, a number of robustness issues still remain to be explored.

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