sound unit vs decibel

sound unit and decibel both are nouns.

Word	Noun	Adjective	Verb	Adverb
sound unit	Yes	No	No	No
decibel	Yes	No	No	No

As nouns, decibel is a hyponym of sound unit; that is, decibel is a word with a more specific, narrower meaning than sound unit:

sound unit: any acoustic unit of measurement
decibel: a logarithmic unit of sound intensity; 10 times the logarithm of the ratio of the sound intensity to some reference intensity

Other hyponyms of sound unit include B, Bel, dB, sone, phon.

sound unit (noun)	decibel (noun)
any acoustic unit of measurement	a logarithmic unit of sound intensity; 10 times the logarithm of the ratio of the sound intensity to some reference intensity