All 5 Uses

acoustic
in
A Separate Peace
(Edited)

The Assembly Hall was used for large lectures, debates, plays, and concerts; it had the worst acoustics in the school.
p. 166.8 *

acoustics = sound quality
The acoustics were so bad and the light so dim that I could rarely tell who was speaking, except for Finny and Brinker who were isolated on the wide strip of marble floor between us in the seats and the others on the platform.
p. 169.1
The acoustics in the Assembly Room were so poor that silences there had a heavy hum of their own.
p. 169.8
But although the acoustics in the Assembly Hall were poor, those outside the room were admirable.
p. 173.5
The excellent exterior acoustics recorded his rushing steps and the quick rapping of his cane along the corridor and on the first steps of the marble stairway.
p. 177.7

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)