An adapter configured to connect to a probe tube for calibrating a hearing aid device is disclosed. The adapter comprises a tubular pipe member and an opening through which the adapter is adapted to receive the probe tube. The adapter comprises an input member configured to connect to a connection member for providing fluid communication between the probe tube and the connection member through the adapter. An outer diameter of the probe tube is 5-20%, preferably 8-15% smaller than an inner diameter of the cylindrical space of the pipe member and that length of the cylindrical space of the pipe member is at least five times, preferably at least ten times larger than the outer diameter of the probe tube. Hereby it is achieved that a long narrow gab can be provided between probe tube and the pipe member of the adapter so that a sufficient acoustic seal can be accomplished.