I don't know but something involving light (perhaps the police color of blue) would help. Are there laws about sticking something under a door? There are hock

I don't know but something involving light (perhaps the police color of blue) would help. Are there laws about sticking something under a door? There are hockey-puck sized devices you can attach to surfaces, which can turn them into speakers. Perhaps you can make clearer, calmer announcements compared to yelling, especially if the device can take a second to calibrate itself against the material. Once it's attached, your hand is free to do something other than knocking. And the device can play some standard announcement, so your voice is freed up as well to communicate with other officers or whatever. Yes such a device wouldn't be cheap, but if I were an officer and there's a risk of being misunderstood, I'd be interested.