Abstract: An audio system for a vehicle comprises an audio amplifier 14 and a manually adjustable volume control 18 for setting the output level of the sound generated by a loudspeaker 20 connected to the output of the amplifier 14. The amplifier 14 includes distortion sensing means for detecting when the level of distortion of the output signal of the amplifier 14 exceeds a reference threshold and a control circuit 16 reduces the sound output level of the amplifier in response to a signal from the distortion sensing means. The reference threshold of the distortion sensing means 14 is varied in dependence upon the output level setting of the volume control 18 so that the allowable output distortion can be varied based upon user input.