A fiber-optic flexural disk microphone is developed to detect acoustic signals in the air. It consists of a Mach-Zehnder interferometer with an optimized sensing arm of 7.93 m. The disk’s resonance frequencies and their influence on the microphone’s sensitivity are investigated. The microphone’s frequency response is measured in the frequency range from 100 Hz to 5 kHz and the average phase sensitivity is about -120.7 dB re 1rad/μPa.