Fibre optic sensors have demonstrated a broad range of commercial potential due to their intrinsic characteristics such as low loss, very small size, light weight and immunity to electromagnetic interference. Representing one type of optical fibre sensors, long period gratings (LPGs) have shown a high sensitivity to a number of parameters, including temperature, strain, refractive index and bending, therefore they have been explored widely for a range of potential sensing applications. This thesis is focused on the design, implementation and evaluation of LPGs for acoustic wave detection. In doing so, the LPG-based sensor has been evaluated and optimized under low frequency conditions (up to 3 kHz) both in air and underwater, with varying acoustic pressure values. This complements the research widely reported for the detection of ultrasounds. The LPG-based sensor, fixed between two pillars with one pillar being movable, is found to be sensitive over a specific frequency range with a minimum detectable sound pressure to be 63dB (ref 1μPa) in water and 66.8dB (ref 20μPa) in air. The sensor has demonstrated a linear response to the variation of the amplitude of the acoustic pressure applied. The sensor performance, by varying the acoustic frequencies, acoustic pressure amplitude and the bending curvature, fits well with the theoretical model derived from the bending effect of the LPG. Both the in-air and underwater tests of the LPG-based sensor have confirmed the potential of using optical fibre sensors for acoustic signal detection and for working in harsh working conditions, where conventional acoustic sensors have shown some limitations.