If you use someone as a sounding board, you discuss your ideas with them in order to get another opinion.
He needed a sounding board rather than thinking alone.
sounding board in British English
noun
1. Also called: soundboard
a thin wooden board in a piano or comprising the upper surface of a resonating chamber in a violin, cello, etc, serving to amplify the vibrations produced by the strings passing across it
See also belly (sense 6)
2. Also called: soundboard
a thin screen suspended over a pulpit, stage, etc, to reflect sound towards an audience
3.
a person, group, experiment, etc, used to test a new idea, policy, etc, for acceptance or applicability
sounding board in American English
1.
a.
soundboard (sense 1)
b.
a structure over or behind a rostrum, stage, etc. designed to reflect sound toward the audience
2.
a.
a person or thing used for spreading ideas around
b.
a person on whom one tests one's ideas, opinions, etc.
Examples of 'sounding board' in a sentence
sounding board
"He was staring down at the chart, using me as a sort of sounding board for his thoughts.
Innes, Hammond HIGH STAND (1990)
It was one of those occasions when he needed a sounding board rather than thinking or scribbling alone.
NULL DARE CALL IT TREASON (1990)
`Bunner had nothing to do with the case except as my sounding board.
Wood, Bari DOLL'S EYES (1990)
He missed the good times with Fancy, when she was a sounding board and friend as well as a lover.