intensity has 8 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the amount of energy transmitted (as by acoustic or electromagnetic radiation)
“he adjusted the intensity of the sound”
the magnitude of sound (usually in a specified direction)
“the kids played their music at full volume”
chromatic purity: freedom from dilution with white and hence vivid in hue
Synonyms, antonyms and the broader and narrower relations for intensity come from Princeton University’s WordNet. Full sources and licences.