Seriously (but probably not usefully :-)) an instrument tuning aid
may help - sold for guitars etc with mic input and display. A
standard frequency meter with a mic and preamp is probably more
normal though.

    Russell.


From: Andy Kunz <mtdesign@FAST.NET>
>I'm looking for a frequency measuring device (which directly
correlates to
>RPM) which uses a microphone as the input.  No chain yanking <G>
>
>I need to get an idea of the RPM my boats motors are turning,
without
>putting something in the boat.