The CM-300 is a contact microphone that can be attached to the bell of a brass instrument, the bridge of a violin, or even to a guitar or ukulele. The microphone uses a piezo element to directly capture the vibrations and send them to your tuner. The CM-200 is an indispensable item in situations where the mic built into your tuner might have difficulty detecting the sound of your instrument, such as when you're practicing with an entire brass band.