An apparatus for sensing vibrations produced by a bone conduction hearing aid, the apparatus comprising a proximal end comprising a proximal periphery comprising a vibration damping material adapted to, during a measurement, contact a skin of a user of the bone conduction hearing aid, and to enclose a skin area within the proximal periphery; a distal end comprising a measurement microphone adapted to, during the measurement, receive an acoustic signal in dependence of vibrations produced at the skin area, the vibrations being representative of skull vibrations produced within the user by the bone conduction hearing aid in response to a sound signal, and where the vibrations include a wide range of frequencies; and a side surface, in combination with the proximal periphery and the distal end, adapted to define an acoustic signal transmission cavity that allows transmission of the acoustic signal from the skin area to the measurement microphone during the measurement, and wherein the measurement microphone is configured to receive a leakage acoustic signal in response to a sound from a sound source and convert the leakage acoustic signal into a leakage electrical signal when the bone conduction hearing aid is switched off;the measurement microphone is configured to receive a mixed acoustic signal in response to the sound from the sound source and convert the mixed acoustic signal into a mixed electrical signal when the bone conduction hearing aid is in operation; a determination unit configured to receive the leakage electrical signal and determine characteristics of the leakage electrical signal; receive the mixed electrical signal and to determine a mixed characteristics of the mixed acoustic signal; access the determined characteristics of the leakage electrical signal; and apply a correction based on the determined characteristics of the leakage electrical signal to the mixed characteristics in order to cancel effect of the leakage acoustic signal in the mixed characteristics for obtaining characteristics of the acoustic signal that depends on the vibrations produced at the skin area enclosed within proximal periphery. a proximal end comprising a proximal periphery comprising a vibration damping material adapted to, during a measurement, contact a skin of a user of the bone conduction hearing aid, and to enclose a skin area within the proximal periphery; a distal end comprising a measurement microphone adapted to, during the measurement, receive an acoustic signal in dependence of vibrations produced at the skin area, the vibrations being representative of skull vibrations produced within the user by the bone conduction hearing aid in response to a sound signal, and where the vibrations include a wide range of frequencies; and a side surface, in combination with the proximal periphery and the distal end, adapted to define an acoustic signal transmission cavity that allows transmission of the acoustic signal from the skin area to the measurement microphone during the measurement, and wherein the measurement microphone is configured to receive a leakage acoustic signal in response to a sound from a sound source and convert the leakage acoustic signal into a leakage electrical signal when the bone conduction hearing aid is switched off;the measurement microphone is configured to receive a mixed acoustic signal in response to the sound from the sound source and convert the mixed acoustic signal into a mixed electrical signal when the bone conduction hearing aid is in operation; a determination unit configured to receive the leakage electrical signal and determine characteristics of the leakage electrical signal; receive the mixed electrical signal and to determine a mixed characteristics of the mixed acoustic signal; access the determined characteristics of the leakage electrical signal; and apply a correction based on the determined characteristics of the leakage electrical signal to the mixed characteristics in order to cancel effect of the leakage acoustic signal in the mixed characteristics for obtaining characteristics of the acoustic signal that depends on the vibrations produced at the skin area enclosed within proximal periphery.