From Longman Dictionary of Contemporary Englishsounding boardˈsounding board noun [countable usually singular]TRY something TO SEE IF IT IS GOOD someone you discuss your ideas with in order to see if they think your ideas are goodsounding board for John always used her as a sounding board for new ideas.
Examples from the Corpus
sounding board• Now he was happy enough to let Rickards impose on his solitude to use him as a sounding board.• She had been a check and balance for her husband, a sounding board and humanizing influence.• The role which the tutor should adopt is that of a sounding board for students' ideas and suggestions.• They had a special role in relation to stock-rearing and stock health and as the confidante and sounding board for the farmers' problems.• Elfed, a sober and upright man who was to become unfairly the sounding board for Richard's first rebellions. 