EJ17s offer heavier tension resulting in a bolder more resonant acoustic tone with higher resistance for heavy strumming and flat-picking. Buy 3 sets and save. Phosphor Bronze was introduced to string making by D Addario in 1974 and has become synonymous with warm bright and well balanced acoustic tone. D Addario Phosphor Bronze strings are precision wound with corrosion resistant phosphor bronze onto a carefully drawn hexagonally shaped high carbon steel core. The result is long lasting bright sounding tone with excellent intonation.