sounding board

• sounding-board

sounding board (plural sounding boards)

1. A thin board that forms part of the resonating chamber of a musical instrument and serves to reinforce its sound.
2. A structure that reflects sound towards the intended listeners.
3. (by extension) Any device or means used to spread an idea or point of view.
   • 2019 May 5, Danette Chavez, “Campaigns are Waged On and Off the Game Of Thrones Battlefield (Newbies)”, in The A.V. Club‎, archived from the original on 28 January 2021:

     Missandei had little interiority beyond being Dany’s sounding board and Grey Worm’s lover, and she had to die to give them both a reason to keep killing, even though you’d think the looming Last War is reason enough.

4. A person, or group, whose reactions to a new idea or proposal serve to assess its acceptability.

• soundboard