ACOUSTIC GUITAR STRING SET String Sets Acoustic Guitar EJ Series Phosphos Bronze Medium (013 - 017 - 024 - 032 - 042 - 056). D'Addario Phosphor Bronze acoustic guitar strings are recognized the world over for their magnificent and durable tone. Phosphor Bronze was introduced for string making by D'Addario in 1974 and has become synonymous with warmth, brilliance, and well-balanced acoustic tone. They are D'Addario's most popular acoustic guitar strings.