The revolutionary Planet Waves S.O.S. Bass Tuner pulses 2 out-of-phase LED light beams directly onto the vibrating string. When the string is out of tune, the 2 lights will visually dance on the string being tuned. As you approach in-tune status, the movement slows down and eventually stops when you are perfectly in tune. This revolutionary visual tuning system enables precision tuning in silent or noisy environments, without the need for audible sound or sensing of any kind.