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An active sound control system has been developed for automobiles which uses interior loudspeakers to counteract the low-frequency rumble of road noise when driving on typical road surfaces. Reductions of around 7 dB of A-weighted sound pressure level in the range 100-200 Hz were measured using the system. Six accelerometers were attached to the vehicle structure to detect road-induced vibration and to provide reference signals for a feedforward control strategy. In this paper the design basis of the control system is set out and the theoretical and practical limitations inherent in this approach are examined. It is seen that key issues are "1… the placement of reference accelerometers for maximum multiple coherence with respect to the interior noise to be attenuated, and "2… adequate time advance of the reference signals to allow the controller to act in real time.

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The paper considers the active control of harmonic sound transmitted through a double-leaf partition by cancelling the volume velocity of the radiating panel. The double-leaf partition consists of a pair of small plates, 300ϫ380 mm, separated by a 100-mm air-gap. The panel volume velocity can be sensed by a single shaped film of piezoelectric PVDF material attached to the plate. Cancellation of volume velocity using a single point force is compared with the result using a matched, distributed actuator which applies a uniform force to the plate and does not give rise to control spillover. Comparison with earlier work in which the volume velocity of a single plate was cancelled ͓Johnson and Elliott, J. Acoust. Soc. Am. 98, 2174-2186 ͑1995͔͒ shows that substantial reductions in the transmitted sound power are only possible up to around 350 Hz, as opposed to 600 Hz in the single panel case. A radiation mode analysis of the panels shows that the double-leaf construction provides good passive attenuation of the first radiation mode at high frequencies, so that inefficiently radiating even modes of the radiating panel make a dominant contribution to the radiated sound power. Thus there is no advantage in controlling volume velocity in this frequency range.

The article describes a nonlinear adaptive controller for the attenuation of harmonic disturbances in nonlinear systems. The steepest descent algorithm is used to adapt the controller coefficients and synthesize the optimum periodic waveform at the input of the nonlinear system, giving the best attenuation of the disturbance. A simple frequencydomain model of the system is required to implement the steepest descent approach. The scheme is particularly applicable where the secondary actuator used for active vibration control is a nonlinear device such as a magnetostrictive actuator. Simulation studies show that an adaptive scheme of this type is robust to the choice of frequency-domain model when the nonlinearity is hysteretic, but much more sensitive when the nonlinearity is a saturation function. A real-time adaptive harmonic controller has been built and used to control the motion of a magnetostrictive actuator. With dc and 7 harmonics under control the system was able to overcome the inherent nonlinearity of the actuator at fundamental frequencies in excess of 3 kHz.