Three filters
Three 100 nF capacitors decide what this block hears. Taken one at a time they are filters with corners near 190 Hz, 140 Hz and 1.6 kHz; worked out together they pass a band from about 75 Hz to 970 Hz from 5V, strongest near 280 Hz. That is the thump of a clap, a knock or a voice, not the hiss above it.
A resistor and a capacitor make a filter
A capacitor from a signal to ground shorts away fast changes and leaves slow ones alone: a low-pass filter. A capacitor in series with a signal blocks slow changes and passes fast ones: a high-pass. Either way the corner, where it lets through about 71 %, is one divided by 2π times the resistance around it times the capacitance.
This board has three capacitors, all 100 nF, and each makes a filter with the resistance around it.
Pick C2 alone. It sits across the capsule's output, where the 10 kΩ and the 51 kΩ in parallel make about 8.4 kΩ. That is a low-pass with its corner near 190 Hz: higher sounds are turned down.
Pick C6 alone. It is the coupling capacitor, and it sees the 8.4 kΩ on one side and the transistor's base on the other. The base behaves like about 3 kΩ from 5V, 5 kΩ from 3V3, so the corner is near 140 Hz from 5V and 120 Hz from 3V3, and it is a high-pass: lower sounds are turned down.
Pick C7 alone. It sits across the output with the 1 kΩ above it: a low-pass near 1.6 kHz. The schematic carries a note beside it, that every analog output gets a 100 nF near the connector. It keeps noise off the wire to your board.
Together they are not three corners
Pick All three. The curve is not simply the three drawn on top of each other, because they share parts: once C6 passes a sound, the base's few kΩ load the capsule's node and move C2's corner up. So the figure works the whole chain out at once, from the capsule's current to SIGNAL.
From 5V it passes a band from about 75 Hz to about 970 Hz and is strongest near 280 Hz. From 3V3 the band is about 67 Hz to 750 Hz, strongest near 230 Hz. The shape does not depend on which transistor your board has; only the height does. All of it is worked out from the part values, not measured.
What that means for a sketch
This block is built to react to loud, low sounds: a clap, a knock on the desk, a raised voice near it. They all have plenty of energy in the low hundreds of hertz. It is not a recording microphone: the band ends near 1 kHz, and ordinary speech makes a swing of a few millivolts. For recording, the ESP32's I2S audio with a digital microphone is the tool.
When it does not work
Most of a whistle is above 1 kHz, where this block passes a fraction of what it passes at 280 Hz, and falls off further with every step up. Claps, knocks and voices have plenty of energy in the low hundreds of hertz, which is why they read so much larger.
Not usefully. The band stops near 1 kHz, the swing for ordinary speech is a few millivolts, and a board would have to sample thousands of times a second with steady timing. For recording, use a digital I2S microphone and a board with I2S.
In principle a smaller C2 and C7 would move both low-pass corners up. They are surface-mount parts and this book assumes the board as it ships. The numbers here are worked out from the part values with a simple model, not measured, so a change would need measuring too.
Three wires, a sketch, and the Serial Plotter showing a clap.
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