Several types of ultrasound sensors have been developed and are used in the field of medical imaging. Conventional transducers are made of piezo-electric material and show good practical performance. However, when the piezo-electric elements need to be small (below 100 μm × 100 μm), these transducers face challenges in fabrication as well as the electrical impedance matching of the elements. As an alternative, we fabricated an optical micro-machined ultrasound transducer (OMUT). This sensor contains an optical micro-ring resonator, which is coupled to a photonic waveguide, and integrated onto an acoustical membrane. The OMUT is build with standard silicon-on-insulator (SOI) technology, allowing for easy fabrication. In this paper, we present the first measurement results of the sensor. Our prototype has a -6 dB bandwidth of 19% and a noise equivalent pressure (NEP) of 0.5 Pa. These first acoustical measurements show that this prototype may form the basis of future ultrasound transducers.