HEAD Sonic Pro 17 660' Reel The HEAD Sonic Pro 17 660' Reel is a high performance tennis string that supplies players with more control and touch on the courts. It is a uniquely refined co-polymer polyester with a soft molecular construction for extra pop and spin. For club players who need a durable string for their weekly training sessions, choose the HEAD Sonic Pro 17 660' Reel! Provides exceptional durability string. Offers excellent response and playability to make a difference in today's fast game.