Custom molded ear plugs. I can play for hours and still hear the full spectrum of frequencies and no ringing.
I'm also interested. My work provides moulded ear plugs, but they definitely don't let the same range of frequencies through.
I have molded plugs, the filters are etymotics. Cost was about 200$ at an audiologist that did the molding. You can get filters with varying levels of attenuation, I think mine are 20db and they recommend 25 for drummers.
I had etymotic musicians earplugs before, but they never worked that well. I couldn't understand lyrics with them in, for instance. But I can with these.