We propose a fiber optics detector of new design able to perform human voice registration. The device is in principle a phase detector based on the well known effect of light diffraction. The light is highly diffracted at the end of the fiber. The diffraction pattern obtained depends strongly on the phase changes that the light can bear during its propagation throughout the fiber. An acoustic field impinging over the fiber can induce such a phase changes due to the photo-elasticity effect. The effect accumulates along the fiber during light propagation affecting the diffraction pattern when the light leaves the fiber at the exit end. If we located a small aperture near the exit end of the fiber and a small semiconductor detector behind this aperture we can register changes of the intensity proportional to these phase changes. The detected signal can be filtered from DC components and then amplified before sending it to a sound reproduction system. The sensitivity of the detector is high enough to register the weak acoustic field of a human voice. The system can also detect ultra and infrasound. We report detection of ultrasound of frequency up to several MHz. Besides acoustic field the detector can register any kind of perturbation able to affect the phase of the propagating light beam.