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This paper presents a micromachined vibration sensor working in the audible frequency range [1-5] kHz that is intended to be used during an ORL (Oto-Rhino-Laryngology) surgery. The sensor must give the surgeon feed back on his intervention related with the middle ear acoustic transmission properties by measuring the ossicles vibration amplitude. Since vibration amplitudes of the lightweight ossicles are very low (from 10 nm to 100 nm), high sensitivity and low mechanical impedance are required. The hand-held device will be easy to position on small middle-ear ossicles (millimeter scale). The use of such a device could be extended to other types of hand-held vibration measuring tools where a low mechanical impedance and high sensitivity are required. An electromechanical equivalent model is used to predict the influence of the sensor on the ossicular chain behaviour. A strain gauge that is suitable for this type of sensor is discussed. Finally, sensor dimensions and mechanical properties are optimised, taking into account specific constraints imposed by the human middle ear on the sensor dynamic behaviour.