What's the "een" in Halloween?
c.1600, from French acoustique, from Greek akoustikos "pertaining to hearing," from akoustos "heard, audible," verbal adjective from akouein "to hear," probably from copulative prefix a- + koein "to mark, perceive, hear," from PIE *kous- "to hear," perhaps from root *(s)keu- "to notice, observe" (see caveat). Acoustic guitar (as opposed to electric) attested by 1958. Related: Acoustical; acoustically.
acoustic a·cous·tic (ə-kōō'stĭk) or a·cous·ti·cal (-stĭ-kəl)
Of or relating to sound, the sense of hearing, or the perception of sound.
acoustics a·cous·tics (ə-kōō'stĭks)
The scientific study of sound, especially of its generation, transmission, and reception.