Apparatus, method and computer program for controlling an acoustic signal

An apparatus comprising: at least one filter configured to filter an electrical input signal and provide a filtered electrical input signal to at least one speaker module configured to convert the filtered electrical input signal to an acoustic signal; and a detector configured to receive the filtered electrical input signal as a first input and an electrical output signal provided by at least one microphone as a second input; wherein the detector is configured to determine at least one difference between the electrical output signal provided by the at least one microphone and the filtered electrical input signal provided to said speaker module and, in response to the at least one difference provide a control signal to the filter to control the filter.