Indian Journal of Geo-Marine Sciences Vol. 41(6), December 2012, pp. 557-562 An investigation on the behaviour of PDMS as a membrane material for underwater acoustic sensing M. F. A. Rahman, M. R. Arshad, A.A. Manaf & M.I.H. Yaacob, Underwater Robotic Research Group(URRG), School of Electrical & Electronics Engineering, Universiti Sains Malaysia Engineering Campus, 14300 Nibong Tebal, Penang, MALAYSIA [E-mail:-mohdfaizal07@yahoo.com] Received 26 July 2012; revised 17 August 2012 Present study consists the behaviour of Polydimethylsiloxane (PDMS) as a vibrating membrane for acoustic signal detection application. Modelling and simulation work was performed using ANSYS 12.1. Theory of acoustic impedance showed that PDMS acting as an acoustic membrane is acoustically matched when operated in water rather than air with 96.7% energy transfer efficiency. Effect of hydrostatic pressure on the membrane deflection was studied for a very shallow application with depth level ranging from 0 to 1m. Effect of PDMS structural geometry on its deflection behaviour was also studied by varying the radius and thickness of the membrane structure. Finally, according to the deflection theory, the type of membrane deflection was classified based on the variation of radius to thickness ratio of the membrane as well as the applied pressure. [Keywords: Acoustic sensor, PDMS membrane, Hydrostatic effect, Deflection theory] Introduction In underwater or undersea technology, acoustic signal plays an important role in enabling many applications such as for the purpose of military, sea monitoring and exploration, imaging, control, underwater robotic and communication. Acoustic technology is a preferred method for underwater wireless application due to several advantages over its counterpart; radio and optical waves. Radio waves normally need large antenna and high power for its operation, while optical approach always suffers from scattering effect during underwater transmission 1 . The operation of acoustic system basically depends on how well the signal is to be transmitted and received between each subcomponent that forms the whole system. The performance criteria of underwater system depends on many parameters such as frequency, depth, range, power consumption, energy efficiency, transmit response and receive sensitivity 2,3 . Acoustic sensing is one of the main area that gains much attention in this underwater technology. It deals with the utilisation of a sensing device that is able to capture the transmitted signal at certain boundary condition and specification. Researchers have allocated much of their time and effort exploring in this area, owing to its prospective use in the future. Ability to successfully implement this technology will help to boost the performance of many underwater applications beyond the human capability. Apart from that, miniaturisation becomes another factor of consideration in acoustic sensor design due to its advantages in mass production, low power consumption and massive cost reduction. This miniaturization trend becomes viable due to the advancement in microfabrication technology and continues to be an active research in acoustic sensing field since a decade ago 4,5 . In microsensing technology, one of the main parameter that directly influences the performance of a sensor is the receive sensitivity. For a membrane- based acoustic sensor, receive sensitivity depends on the amount of membrane deflection in response to a signal. For such sensor, the more the membrane get deflected, the higher sensitivity it will achieve 6 . Such deflection not only depends solely on the signal pressure, but other parameters such as bias voltage, noise and hydrostatic pressure are among the main contributing factors that need to be addressed properly. Membrane material and structural geometry are two design variables that contribute to the receive sensitivity and thus affects the operating performance. Therefore, proper selection of material used as a membrane and its design geometry are vital in designing a device with remarkable performance.