sounding board 
noun
1A board or screen placed over or behind a pulpit or stage to reflect a speaker’s voice forward.The original garland was hung on the sounding board of the pulpit, but the present garland now hangs on the church wall in the north-west corner of the Nave....
- It represents the most lavish use of rosewood veneer known on any Charleston surface with the exception of the sounding board of Saint Michael's pulpit, which dates from around 1760.
- A sounding board on the ceiling at Notre Dame Basilica is at the root of a major local mystery.
1.1 another term for soundboard.
2A person or group whose reactions to suggested ideas are used as a test of their validity or likely success before they are made public: friends are sounding boards who let you know how far you can go...
- The mentorship matches a budding entrepreneur with a successful businesswoman who shares her experience of entrepreneurship, acts as a sounding board for ideas, suggests ways to solve problems, encourages and advises.
- Bench coaches exist primarily to help managers, reducing their workloads, acting as their sounding boards, suggesting in-game strategies.
- We are looking for some volunteers to act as a sounding board when we want to test new ideas, products, or services, and to collect more feedback on existing products and services.
2.1A channel through which ideas are disseminated: neighbourhood councils provide a sounding board for local opinion...
- It has served for more than thirty years as a unifying news portal, providing a communication medium and a sounding board for Canadian geoscientists.
- We also hope to connect local communities by providing a space and sounding board for existing social relationships.
- The chance to publish freely to the online community provides another sounding board for disgruntled, negligent or even malicious members of staff.