contrabass vs double-bass

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

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

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

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

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

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