sounding-board Definition

1. A thin board or piece of wood in a musical instrument such as a piano or violin that strengthens or amplifies the sound of the strings which pass over it.

Form: soundboard (also)
2. A board over a stage or pulpit that is designed to direct the speaker's voice towards the audience.
3a. A means of testing the acceptability or popularity of ideas or opinions;
3b. Someone or a group that is used for this purpose.