sounding board

Short definition from Wiktionary

Noun

1. 1
   A thin board that forms part of the resonating chamber of a musical instrument and serves to reinforce its sound.
2. 2
   A structure that reflects sound towards the intended listeners. Abatsons.
3. 3
   Any device or means used to spread an idea or point of view.
4. 4
   A person, or group, whose reactions to a new idea or proposal serve to assess its acceptability.
5. 5
   Alternative spelling of sounding board.