deriv SD cv (1338) ashtadhyayi.com hei.de L 1338 ETT STT a 5.4.57 ALPH OLDHOMEPAGE NEWHOMEPAGE

avyaktAnukaraNAd dvy-aj-avarArdhAd anitau DAc

अव्यक्तानुकरणाद्द्व्यजवरार्धादनितौ डाच् ONPANINI 54057

After an onomatopoeia consisting of a repeated sound whose half has more than one vowel, there is /DAc (before kR कृ bhU भू /as), unless before /iti.

So, paTat पटत् is an onomatopoeia and has more than one vowel. If we repeat it we get —

paTat पटत् + paTat पटत्
→ paTat पटत् + paTat पटत् + /DAc by this rule
→ paTapaTat पटपटत् + /DAc by nityamAmreD...
→ paTapaTA पटपटा by TeH

This only happens before kR कृ bhU भू /as —

!**paTapaTAkaroti पटपटाकरोति "he makes a sound like paTatpaTat पटत्पटत्"

And only if no /iti follows —

paTiti karoti पटिति करोति by avyaktAnuk... "he makes a sound like paTat पटत्"