Abstract: An automatic equalizer sequentially adds subdued narrow band audio reference signals of different frequencies to an audio program signal being broadcast, and measures the magnitude of each successive broadcast reference signal to determine a measured frequency response of an audio amplification system and its environment. The gains, or attenuations, of various frequency bands in the broadcast signal are adjusted as needed to produce a desired frequency response. In one embodiment the added reference signals are masked by adding the reference signals only when the test frequency is adjacent to a relatively large magnitude frequency component of the program signal. Additional masking is performed by the reference signal being relative quiet and of a short duration such as two to twenty cycles. A narrow band notch filter operates in synchronism with the reference signal adder to eliminate any program signal component at the frequency of each reference signal being added.