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. Also called soundboard.