According to an embodiment, an apparatus for sensing vibrations produced by a bone conduction hearing aid is disclosed. The apparatus includes a proximal end, a distal end and a side surface. The proximal end comprising a proximal periphery comprising a material adapted to, during a measurement, contact a skin of a user of the bone conduction device and to enclose a skin area within the proximal periphery. The 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. The 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.