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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.