Hi! I found a nice thing via google for a measuring forces by using a light pipe. The light gets coupled into a light pipe and the amount of light is measured by a diode (http://www.dr-iguana.com/prj_LightPipe/). I think you can also use such a device for measuring the deflection of a membrane, made out of polycarbonate or silicone rubber or something which acts like a light pipe. This would be a nice thing to measure the defection of a audio membranes or of all kinds of other membranes which get deformed or stretched by a external force.