An apparatus comprising: a filter configured to filter an electrical input signal (x) and provide a filtered electrical input signal (y); an audio output device configured to receive the filtered electrical input signal (y) and convert the filtered electrical input signal (y) to an acoustic output signal; a microphone configured to detect an acoustic signal and provide an electrical output signal (r) corresponding to the detected acoustic signal; and a detector configured to receive the filtered electrical input signal (y) provided to the audio output device as a first input and the electrical output signal (r) provided by the microphone as a second input; wherein the detector is configured to detect a change in the signal power across multiple frequency bands of the electrical output signal (r) provided by the microphone relative to a distribution across multiple frequency bands of the filtered electrical input signal (y) provided to the audio output device and, in response to the detection of the change in the signal power, provide a control signal (k) to the filter to control the filter to compensate for the detected change in signal power; wherein the apparatus is configured to be positioned adjacent to a user's ear and the control signal (k) provided by the detector controls the filter to filter the electrical input signal (x) to compensate for distortion of the acoustic output signal that is caused based on a change in position of the apparatus relative to the user's ear; and wherein the change in signal power of the electrical output signal provided by the microphone relative to the filtered electrical input signal that is provided to the audio output device and detected by the detector provides a measure of the frequency response of the system comprising the user's ear and the apparatus. a filter configured to filter an electrical input signal (x) and provide a filtered electrical input signal (y); an audio output device configured to receive the filtered electrical input signal (y) and convert the filtered electrical input signal (y) to an acoustic output signal; a microphone configured to detect an acoustic signal and provide an electrical output signal (r) corresponding to the detected acoustic signal; and a detector configured to receive the filtered electrical input signal (y) provided to the audio output device as a first input and the electrical output signal (r) provided by the microphone as a second input; wherein the detector is configured to detect a change in the signal power across multiple frequency bands of the electrical output signal (r) provided by the microphone relative to a distribution across multiple frequency bands of the filtered electrical input signal (y) provided to the audio output device and, in response to the detection of the change in the signal power, provide a control signal (k) to the filter to control the filter to compensate for the detected change in signal power; wherein the apparatus is configured to be positioned adjacent to a user's ear and the control signal (k) provided by the detector controls the filter to filter the electrical input signal (x) to compensate for distortion of the acoustic output signal that is caused based on a change in position of the apparatus relative to the user's ear; and wherein the change in signal power of the electrical output signal provided by the microphone relative to the filtered electrical input signal that is provided to the audio output device and detected by the detector provides a measure of the frequency response of the system comprising the user's ear and the apparatus.