sounding board

コロナ感染による緊急事態状況は長引くような気配がします。
さて、宮部みゆきの小説 "The Sleeping Dragon" を読んでいます。
Listen to what he says and be patient with him. It's different from when I was young; it's even harder now. You'll never understand what he has to go through. Watch out for him, and listen to him whenever he needs a sounding board. That's all you can do.
"sounding board" この表現は7/26/2017に取り上げていました。
今回はその語源についてMerriam-Webster's Online Dictionaryが触れているので、それを紹介します。
The Literal Origins of Sounding Board
Beginning during the Renaissance, a flat wooden canopy called a sounding board was placed over church pulpits, amplifying the sound of the preacher's voice, carrying it to the farthest reaches of the church. Similarly, ideas can be spread by a figurative sounding board. A publication, for example, can be a sounding board for propaganda. That's one modern sense of the word. But when sound bounces off a literal sounding board, not only does it reach more people, it also comes across more clearly. Likewise, bouncing ideas off another person can lend clarity to one's thought processes. If someone comes to you and says "How does this sound?" and leaves with his or her mind made up (whether or not you've ventured a word), you have served as a very effective sounding board.