Ernie Ball Earthwood Medium Phosphor Bronze Acoustic Guitar Strings - 13-56 Gauge. Ernie Ball Earthwood Phosphor Bronze Acoustics are made with phosphor bronze wire wrapped around brass plated high carbon steel hex cores. Engineered to meet the demands of acoustic musicians worldwide, these concert quality strings provide deep, rich bass notes with clear bright trebles. Made with the finest and freshest raw materials, all Ernie Ball strings are hermetically sealed to ensure your strings stay as fresh as the day they were made.