Polk Audio’s laser interferometer research has helped us develop and discover new materials and combinations of materials that strike the best balance between stiffness, damping and mass. The best material we have found to date is Aerated Polypropylene (APP). We use this material for the cones in our top of the line award-winning LSi in-home loudspeakers that have been widely praised for their exceptional accuracy. SR Series mobile speakers also get this special treatment as well as LCi and RTS in-ceiling/in-wall series.
So what exactly is Aerated Polypropylene (APP) and why is it so good? It is mineral filled Polypropylene that has been puffed up with injected air to form a honeycomb-like structure. Take a look at the scanning electron microscope photo of the cross section of an APP cone (below). You can see that near the surfaces the material is densely packed. In fact at that layer the material is hard and stiff. We call it the crunchy outside. The added thickness of the material contributes to stiffening the structure, because stiffness increases with the cube of thickness.