nonacoustical

The story

1. Proto-Indo-European*né
2. Proto-Germanic*ne
3. Proto-Indo-European*ís?
4. Proto-Indo-European*h₁óynos
5. Proto-Germanic*ainaz
6. Proto-Germanic*nainaz
7. Proto-West Germanic*nain
8. Old Englishnān
9. Middle Englishnon
10. Old Englishnān
11. Old Englishnān-
12. Middle Englishnon-
13. Englishnon-
14. Proto-Indo-European*h₂eḱ-
15. Proto-Indo-European*h₂ew-
16. Proto-Indo-European*h₂ṓws
17. Proto-Indo-European*h₂ḱh₂owsyéti
18. Proto-Hellenic*akóuhō
19. Ancient Greekἀκούω (akoúō)
20. Ancient Greekᾰ̓κουστῐκός (ăkoustĭkós)
21. Medieval Latinacousticus
22. Englishacoustic
23. Proto-Indo-European*h₂el-
24. Proto-Indo-European*h₂élisder.?
25. Englishacoustical
26. English todaynonacoustical

From non- + acoustical.