Sonic Pro the newest string innovation from HEAD is the ideal string for the new Extreme racquets. Made from an uniquely refined co-polymer polyester with a soft molecular construction the Sonic Pro string offers superior touch extra pop and spin without sacrificing control. This exceptionally durable string offers excellent response and playability that will make a real difference in today’s fast game. Head Sonic Pro 17g. Color(s): White Orange. Composition: Co-polymer polyester. Gauge: 17g. Length: 40′.