double bass vs string bass

double bass and string bass both are nouns.

Word	Noun	Adjective	Verb	Adverb
double bass	Yes	No	No	No
string bass	Yes	No	No	No

As nouns, double bass and string bass are synonyms defined as:

double bass and string bass: largest and lowest member of the violin family

Other synonyms of double bass include bass fiddle, bass viol, bull fiddle, contrabass.

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