An important way in which vehicles can be made more fuel-efficient is to make them lighter. Unfortunately, lighter vehicle structures inevitably transmit more low-frequency noise than heavier structures, and to make light-weight vehicles acceptable to passengers, this low-frequency noise has to be controlled. Conventional passive treatments for low-frequency noise tend to add weight to the structure, reducing the environmental benefits. Active noise control, in which the sound generated by an array of loudspeakers or shakers reduces vehicle noise by destructive interference, can, however, be achieved with much smaller increases in weight. There is thus a growing interest in reducing the low-frequency noise inside vehicles using active noise control.