Well, yes, the "White Sound Device" (or "Heartbeat Reader" as FJ called it.) But, it seems to be lying around in all kinds of surgical places, so it must have some function we don't really understand. ("Quickly! We need to mask the sound of this patient's heart during the operation!").
The James Blish novelization of "Court Martial" called it a modified 'white noise generator.' Which makes perfect sense to have during a delicate operation in a hospital setting, filtering out various environmental sounds (read: onboard ship noises) to allow McCoy and Chapel to concentrate on what they're doing and (post-op) allowing their patients to rest without the constant thrum of the ship's systems in their ears.
Well, yes. I'm just not sure why you want to toggle that functionality on and off with a hand-held device. Why not just mount it into the ceiling or something to make the room quiet? Dr. McCoy tells Christine to "Initiate sterile field" with the flip of a switch in "Journey to Babel." You'd think he'd just say "Initiate quiet field" if he wanted it quiet. And how would it have to work? Do you have to take the doohickey around the room to each individual air conditioner vent and speaker, and computer and overbed monitor to mask the sound of each one? I mean, you have to mask each heartbeat
individually on the bridge and don't seem to be able to do it en masse
. So do you have to do it individually with every sound you want to mask? All in all, I'm not convinced that this thing is merely an interoperative or postoperative emergent white sound device. I think it does something else.
(Also, technically, I think a white sound device and/or generator doesn't filter
unwanted sounds; it merely masks them--although I'm not sure of what the practical difference would be.)