The affix ap comes optionally after the verb kvan 'to jingle' when ni is in composition with it, as well as when it is without any upasarga or when 'flute' is meant.,
This debars the affix घञ्. Thus निक्वणः or निक्वाणः 'a musical tone'. क्वणः or क्वाणः 'sound'. When it refers to 'flute', it must have some upasarga not necessarily नि; as कल्याणप्रक्वणा वीणा.
But not so here, अतिक्वाणो वर्तते, where the upasarga being अति and the word not referring to 'a flute', no option is allowed. When flute is not meant the upasarga must be नि or no upasarga at all, for the optional application of the affix अप्. To sum up, क्वण् takes अप् optionally under three conditions:- Ist when preceded by नि, or 2ndly when having no upasarga, or 3rdly when it refers to 'flute', and then it must have some upasarga.,