sounding board

US: /ˈsaʊndɪŋ ˌbɔrd/

WordReference Random House Learner's Dictionary of American English © 2025

sound•ing board /ˈsaʊndɪŋ ˌbɔrd/  n. [countable]

1. Music and Dance Also called ˈsoundˌboard. a thin plate of wood forming part of a musical instrument, used to improve the power and quality of the tone.
2. one whose reactions serve as a measure of whether or not an idea will be acceptable to others:
   Her husband was her sounding board for the plan.

WordReference Random House Unabridged Dictionary of American English © 2025

sound′ing board′, 

1. Music and Dance a thin, resonant plate of wood forming part of a musical instrument, and so placed as to enhance the power and quality of the tone.
2. a structure over or behind and above a speaker, orchestra, etc., to reflect the sound toward the audience.
3. Building a board used in floors, partitions, etc., for deadening sound.
4. a person or persons whose reactions serve as a measure of the effectiveness of the methods, ideas, etc., put forth.
5. a person or group that propagates ideas, opinions, etc.:
   He was more of a sounding board than a novelist.
   Also called soundboard. 
• 1760–70

Collins Concise English Dictionary © HarperCollins Publishers::

sounding board noun

1. Also called: soundboard a thin wooden board in a piano or comprising the upper surface of a resonating chamber in a violin, cello, etc, serving to amplify the vibrations produced by the strings passing across it
   See also belly6
2. Also called: soundboard a thin screen suspended over a pulpit, stage, etc, to reflect sound towards an audience
3. a person, group, experiment, etc, used to test a new idea, policy, etc, for acceptance or applicability