Sounding-board

noun
1.
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.
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.
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 belly (sense 6)
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