In this paper, a Mach–Zehnder interferometer-based membrane-free acoustic sensing method is developed. The sensing principle relies on direct detection of sound-pressure-induced changes of the refractive index in the open cavity. The detectable frequency range and sound pressure range of such a sensor have limitations because they are influenced by the membrane or a. Abstract: This paper investigates analytically the performance of MZI sensor for acoustic detection in terms of light power, fiber characteristic and detectable acoustically/induced phase in the terms of output current at the photomultiplier tube (PMT).