soundingboard

Noun

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. Abatsons.
3. (broadly) Any device or means used to spread an idea or point of view.
   • 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.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 Jan 2021
4. A person, or group, whose reactions to a new idea or proposal serve to assess its acceptability.