The affix ap comes optionally after the verbs 1. svan 2. has when no upasarga is in composition with them.,
Thus स्वनः or स्वानः 'sound'; हसः or हासः 'laughter'. But with upasargas, we have only one form; प्रस्वानः 'a loud noise'; प्रहासः 'a loud laughter'.,