We report a new fiber sensor utilizing a spring-loaded circular diaphragm to detect extremely weak acoustic waves. When exposed to an acoustic wave, the diaphragm vibrates parallel to itself relative to a stationary substrate. This vibration is detected interferometrically with a laser beam provided by a single-mode fiber placed ∼30 μm from the diaphragm. The beam straddles the reflective diaphragm and the substrate, is reflected, and recoupled into the fiber. When the diaphragm vibrates, it modulates the phase and amplitude profiles of the reflected beam, and therefore, the optical power recoupled into the fiber. A measurement of this power modulation provides the amplitude and frequency of the acoustic wave. This design presents several advantages over the previous generation. It does not require a lens, its sensitivity is much less dependent on angular misalignment between the chip and the fiber, assembly is faster and simpler, and the pressure resolution is improved by a factor of ∼2. A sensor prototype is described with a measured average pressure resolution of 7.6 μPa/√Hz between 300 Hz and 20 kHz and a resolution of 1.8 μPa/√Hz at 7.5 kHz. The measured acoustic sensitivity is independent of the polarization of the interrogating light. The measured wavelength dependence is larger than expected, as a result of a spurious Fabry–Perot interferometer caused by the weak Fresnel reflection on the fiber end. A second sensor with an anti-reflection-coated fiber end is reported with significantly reduced wavelength dependence, of great interest in multiplexed sensor arrays.