sounding board
noun phrase

- a person or group whose reactions to an idea, opinion, or point of view serve as a measure of its effectiveness or acceptability

- a device or means serving to spread or popularize an idea or a point of view

- 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

- a structure placed behind or over a podium or platform to reflect music or a speaker's voice to an audience

(American Heritage Dictionary)

---
WORD ORIGIN

A sounding board is the part of a string instrument that transmits the vibrations of the strings to the air, greatly increasing the loudness of sound over that of the string alone. It operates on the principle of forced vibration; the board is gently vibrated by the string, and despite their differences in size and composition, the board is forced to vibrate at the exact same frequency, producing the same sound as the string alone, differing only in the quality of the tone

Figuratively speaking, a sounding board is a person who can provide an opinion or feedback regarding an idea of concept. Like an instrument sounding board, the human sounding board reflects or "bounces" the idea or concept back so that the originator can hear how it sounds. People often use the expression "Can I bounce something off you?" to ask for someone's opinion.

Sounding also describes a measurement of atmospheric conditions at various heights as well as a measurement of depth, especially in the ocean.

British physicist and engineer Lord Kelvin once worked out a sounding method for sea depths using piano wire and a narrow glass tube plugged at the upper end.

One day, in the course of experimenting with his invention, he was interrupted by a colleague who - astonished by the unusual object - asked Kelvin what on earth he was doing.

"Sounding," said Kelvin.
"What note?" Joule asked.
Kelvin's reply? "The deep C!"

---
Practice OWAD in a conversation:

"Bret is happy to have his wife as a SOUNDING BOARD before making important decisions."

---
Thanks to Thomas for suggesting today's word!