The circuit shows an inverting amplifier, which uses the source impedance of electret microfone (which is the output impedance of internal FET-Amp) as the one gain determining resistor. The other is the 47k pot.

This is a bit dangerous, because the circuit might not work with all available electret microfones then.

R2 and R3 provide a vitual ground, so that the inverting amplifier can handle AC-signals.

Behind the amplifier comes a comparator with two storage caps. C11 is charged and discharged slowlier than C10. By this the threshold of comparator "follows" the signal level and makes the circuit work for varying signal levels.

When a beat comes, C10 is charged faster than C11 and the output of comparator goes low. After the beat, C10 discharges faster than C11 and the output of comparator goes high again.

Take care, the two diodes shoudn't be too different. So, a matching could be helpful.

Two invert the logic of this beat detector just swap pin5 and pin6, as Oli already recommended.

Kai Klaas