Abstract. An extrinsic Fabry–Perot interferometer (EFPI) acoustic sensor based on a gold diaphragm with an about 100 nm thickness has been proposed and demonstrated. The Fabry–Perot (F-P) cavity consists of a gold diaphragm and a fiber endface. Experimental results illustrate a static pressure sensitivity of about 19.5 nm / kPa in the range from 0 to 100 kPa with linearity of about 0.99. Meanwhile, the flat acoustic frequency response of about 35.0 mV / Pa between 0.2 and 2.0 kHz is achieved, and the largest acoustic pressure sensitivity is about 80.6 mV / Pa at 2.6 kHz. The noise-limited minimum detectable pressure is 1.3 mPa / Hz1/2 @ 2.6 kHz. The proposed EFPI acoustic sensor may have potential application in security and surveillance systems.