Hi All,  I see that someone is asking how a frequency meter works.
Okay, here is the most simplest one.

The sound goes into a pickup (a microphone), through a amplifier into
a rectifier (diode), then into a capacitor-resistor-meter circuit.  The
rectifier takes the sine wave sound from the amplifier and converts it
into direct current pulses.  The pulses charge up the capacitor and the
meter starts to move since it is across the capacitor.  At the same
time the resistor is draining off the voltage on the capacitor.

The value of the resistor vs. the capacitor sets the frequency.  If
the frequency increases so does the voltage across the capacitor.  The
resistor is connected to the dial on the unit.

Newer units contain a phase lock loop (PLL) and/or digital circuits.
If you can not find a frequency meter circuit in the library, try
looking for an automobile tachometer circuit.  The two circuits are
very similar in nature.

John Conrad Kleinbauer