soundboard vs sounding board

soundboard and sounding board both are nouns.

Word	Noun	Adjective	Verb	Adverb
soundboard	Yes	No	No	No
sounding board	Yes	No	No	No

As nouns, soundboard and sounding board are synonyms defined as:

soundboard and sounding board: (music) resonator consisting of a thin board whose vibrations reinforce the sound of the instrument

soundboard (noun)	sounding board (noun)
(music) resonator consisting of a thin board whose vibrations reinforce the sound of the instrument	(music) resonator consisting of a thin board whose vibrations reinforce the sound of the instrument
	a person whose reactions to something serve as an indication of its acceptability