MA540 Phosphor Bronze -light 12-54 Designed to cope with demanding practices and schedules. Martin Authentic Acoustic SP Phosphor Bronze strings feature their highest tensile-strength core wire, and are tin-plated for enhanced corrosion resistance. These Acoustic SP strings provide players with good tuning stability as well as a true and consistent tone, throughout gigs. Manufactured with copper, tin and phosphorous, Phosphor Bronze strings yield a dark and slightly warmer tone that 80/20 strings.