In Michigan, the DNR uses a sound meter and tachometer to check sound levels. Whats unique (to me) is that the tach is a resonance tach. They dial in a RPM by extending or retracting a spring to a calibrated length as I remember it. They set the unit against your transmission or engine a have you gas it til the spring tip vibrates wildly, thats the rpm level they dialed in, then they do the sound check. Does anyone know where to get one, I have searched the net with out any luck. The thing thats nice is you don't have to hunt for the plug wire to get readings for an electronic tach. Help!