This invention relates to a system (300) for measuring acoustic properties of a vent (126) in a hearing aid. The system (300) comprises: a microphone (202) converting ambient sound pressure to an electric sound signal; a signal processing unit (302) connected to the microphone (202) and generating a processed electric sound signal; and a speaker (204) converting the processed electric sound signal to a processed sound pressure. In addition, the system comprises a determining means, which is adapted to determine the acoustic properties by measuring the acoustic feedback from the speaker 204 to the microphone (202).