Earplugs may be handy for blocking out unwanted noises, but what happens when you want to hear some of what's going on, or even hear every little bit of it? Well, you pull the plugs out of your ears, of course. In the case of Knops, however, you can leave them in and just "turn up the volume."
Knops are a purely mechanical device, requiring no batteries.
You just stick 'em in your ears, then twist their adjustment dial to one of four "steps." Doing so turns their sound hole (or whatever you want to call it) farther and farther around, aligning it with an increasing number of acoustic filters with each step.
Step 1 has no sound-blocking effect, allowing you to hear everything. Step 2 is designed to reduce city noise, Step 3 is intended to reduce the volume of live music, and Step 4 is made to block all but the loudest of noises completely.
Knops is currently crowdfunding on Indiegogo, where a pledge of US$69 will get you a set. Shipping is estimated for November, if all goes according to plans.