New Martin Kovar Acoustic Guitar Strings. Crafted with the innovative Kovar alloy, known for its superior strength and high resonance, these strings deliver a new level of sonic performance and reliability. They feature Martin’s patented SP core wire, adding another layer of richness to the robust and bright tone that Martin strings are famous for. Designed with longevity in mind, these strings resist corrosion better than others on the market, providing consistent, premium sound for longer. These have slightly less tension that Martin's Retro strings.