Or search: unheard, breathed, supersonic, ultrasonic, infrasonic, audibility, unhearable, audibleness, imperceptible, unperceivable
3 Syllables: SUU
audible, doggerel, laudable, nautical, nominal, obstacle, plausible, possible, probable, topical, volatile
4 Syllables: USUU
abdominal, apocryphal, canonical, impossible, improbable, inaugural, methodical, phenomenal
5 Syllables: UUSUU
inexhaustible
6 Syllables: UUUSUU
anthropological, etymological

Found 22 words across 4 groupings. Skipped 1 excess word of a similar stem.