Ernie Ball Earthwood Medium 12-String 80/20 Bronze Acoustic Guitar Strings - 11-28 Gauge. Ernie Ball 80/20 Bronze Acoustic strings are made from 80% copper, 20% zinc wire wrapped around brass plated high carbon steel hex cores. One of the most popular acoustic guitar strings as used by The Eagles, Elvis Costello, and Joe Bonamassa. Ernie Ball 80/20 bronze acoustic strings provide a crisp, bright sound with pleasing overtones.