The aim of this paper is to determine the acceleration due to gravity "g", using a simple and low-cost experimental device. The time taken for a metallic ball to travel a predetermined distance is measured and recorded by a series of optical sensors. Four pairs of sensors are placed along the external surface of a vertical methacrylate tube at regular intervals. The metallic ball is fired by a simple spring device placed at the bottom of the tube. As the metallic ball passes through the tube it interrupts the path of infrared light radiating from each sensor and its corresponding photo-diode. As a result, an electric signal is received by an input microphone, digitized and subsequently processed by computer software (Audacity). The program allows us to view a frequency graph and therefore allows us to identify the exact time between two consecutive events, which is necessary to determine the value of "g". In this paper, the Audacity measurements were exported as a .wav file. (Contains 1 table, 4 figures and 3 footnotes.)