sounding board

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun A structure placed behind or over a podium or platform to reflect music or a speaker's voice to an audience.
• noun A person or group whose reactions to an idea, opinion, or point of view serve as a measure of its effectiveness or acceptability.
• noun A device or means serving to spread or popularize an idea or a point of view.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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