open mike, this might help:

ounding board
n.
1. Music
a. A thin board forming the upper portion of the resonant chamber in an instrument, such as a violin or piano, and serving to increase resonance.
b. A structure placed behind or over a podium or platform to reflect music or a speaker's voice to an audience. Also called soundboard.
2. A person or group whose reactions to an idea, opinion, or point of view serve as a measure of its effectiveness or acceptability.
3. A device or means serving to spread or popularize an idea or a point of view.

I remember hearing about the sounding board of our piano whilst growing up; it was very important, esp. when moving.:-)