double-bass vs contrabass

double-bass is not a noun while contrabass is a noun.

Word	Noun	Adjective	Verb	Adverb
double-bass	No	Yes	No	No
contrabass	Yes	Yes	No	No

As adjectives, double-bass and contrabass are synonyms defined as:

double-bass and contrabass: pitched an octave below normal bass instrumental or vocal range

double-bass (noun)	contrabass (noun)
	largest and lowest member of the violin family

double-bass (adjective)	contrabass (adjective)
pitched an octave below normal bass instrumental or vocal range	pitched an octave below normal bass instrumental or vocal range