Sounding board

I was emailing my big Yankee friend today, and among other things informed him that he was my 'sounding board'.

Now we all know what that means but how did it originate? Why would a board sound? Am I taking this too literally? Does it mean a board of the wooden variety or the corporate variety or something completely different?

Merriam-Webster online:

Main Entry: sounding board
Function: noun
1 a : a structure behind or over a pulpit, rostrum, or platform to give distinctness and sonority to sound b : a device or agency that helps propagate opinions or utterances c : a person or group on whom one tries out an idea or opinion as a means of evaluating it.