All 4 Uses

acoustic
in
If I Stay
(Edited)

It was at a hall in town, a place that usually showcased local bands, so the acoustics were terrible for unamplified classical.
Chpt 3 *

acoustics = sound quality (resulting from such things as the surfaces off of which sound bounces)
Just acoustic melodies.
Chpt 4

acoustic = not using electricity to amplify sound
I mean Dad might hammer on his drums for a few hours by himself or write songs alone at the kitchen table, plinking out the notes on his beat-up acoustic guitar, but he always said that songs really got written as you played them.
Chpt 11 *
He was riffing about on his acoustic guitar.
Chpt 15

Definitions:

(1) (acoustic as in: auditorium acoustics)
sound quality or its study, based on how sound bounces off surfaces

For example, acoustics affect whether a voice on stage can be heard clearly in the back of an auditorium, or how loud and echoey a busy restaurant feels.
(2) (acoustic as in: acoustic guitar)
of a guitar or other musical instrument: not using electricity to amplify sound
(3) (meaning too rare to warrant focus)