subacoustic

The story

1. Proto-Indo-European*upó
2. Proto-Italic*supo
3. Latinsub
4. Latinsub-
5. Englishsub-
6. Proto-Indo-European*h₂eḱ-
7. Proto-Indo-European*h₂ew-
8. Proto-Indo-European*h₂ṓws
9. Proto-Indo-European*h₂ḱh₂owsyéti
10. Proto-Hellenic*akóuhō
11. Ancient Greekἀκούω (akoúō)
12. Ancient Greekᾰ̓κουστῐκός (ăkoustĭkós)
13. Medieval Latinacousticus
14. Englishacoustic
15. English todaysubacoustic

From sub- + acoustic.