sounding board

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Music A thin board forming the upper portion of the resonant chamber in an instrument, such as a violin or piano, and serving to increase resonance.
2. n. Music A structure placed behind or over a podium or platform to reflect music or a speaker's voice to an audience. Also called soundboard.
3. n. A person or group whose reactions to an idea, opinion, or point of view serve as a measure of its effectiveness or acceptability.
4. n. A device or means serving to spread or popularize an idea or a point of view.

Wiktionary

1. n. A thin board that forms part of the resonating chamber of a musical instrument and serves to reinforce its sound
2. n. A structure that reflects sound towards the intended listeners
3. n. by extension Any device or means used to spread an idea or point of view
4. n. A person, or group, whose reactions to a new idea or proposal serve to assess its acceptability