sounding board vs soundboard

sounding board and soundboard both are nouns.

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

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

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

sounding board (noun)	soundboard (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