Polypropylene, next to paper, is probably one of the most popular materials used for speaker driver construction. It offers many benefits over paper however, including a being extremely light weight while being resistant to moisture and mildue. On the sonic front, it is a softer material and although not quite as precise as more rigid materials like kevlar is less prone to sonic breakup distortion at the crossover points. Because of it's natural tendancy to dampen breakup at the frequency extremes you get a smoother transition at the crossover. This results in a more mellow and less tinny sound than you would normally get from other types of speakers.