The signal from the sensor is filtered to remove interference and amplified, then frequency divided using a counter. A Schmitt trigger circuit prevents false counts. An audio amplifier takes the filtered output from the counter, and feeds headphones. Loudspeakers aren't used with frequency division detectors because the output signal contains an amplified copy of the input signal, and this produces oscillation if picked up by the sensor.
The pages that follow provide additional detail on each section of the block diagram