Spratt504, via Seattle Weekly’s Flickr PoolFor all our festival coverage, visit Bumbershoot.SeattleWeekly.com.Who: Cave SingersWhere: Starbucks StageWhen: 5 p.m. It’s hard to believe that the sound The Cave Singers create comes from just three musicians. The heavy guitar opening the song “At the Cut” sounds like it’s made by two guitar players and a bassist, not just Derek Fudesco’s strumming. Even in an outdoor venue like the Starbucks Stage, and even when playing live, the Cave Singers’ music fills the air. Every beat drummer Marty Lund doesn’t hit is filled with Fudesco’s guitar or lead singer Peter Quirk’s husky warble. Maybe it’s musicianship–or maybe it’s just turning the volume to 11–but once the Cave Singers take the stage, it’s hard to pay attention to anything else.