| 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. |
