An apparatus, method and computer program, the apparatus including includes a housing configured to be positioned in a user's external ear, a loudspeaker located at a first position within the housing and configured to provide an acoustic signal, a microphone configured to detect an acoustic signal located at a second position within the housing, a filter configured to filter an input signal provided to the loudspeaker; and a controller configured to enable the acoustic signal detected by the microphone to be used to provide a control signal to the filter.