Abstract: A microphone picks up a signal in response to both the acoustic output of the speaker and the ambient noise at the loudspeaker location, for example, in an automobile. The microphone output signal and the signal which is to be provided to the loudspeaker with variable amplification are each subjected to envelope detection. The two envelope curve signals are compared to produce a signal representative of their similarity, the similarity being high when ambient noise is low or absent. The similarity signal is then processed with reference to a reference voltage which varies with the setting of the manual volume control of the amplifier interposed ahead of the loudspeaker. That amplifier has an automatic volume control input to which a control signal generated in response to the envelope similarity signal is applied to raise the amplification and the loudspeaker output level when ambient noise, detected by the similarity of the envelope signals, is present or increases.