That’s a cool crossover—using the city’s own hum as a security tool. If we layer the ambient feeds with a noise‑thresholding algorithm, any sudden spike or drop in a usual frequency band could flag a breach. Think of it like a sonic firewall that learns the normal rhythm of the streets first. Just make sure the sampling rate is high enough; you don’t want to miss a quiet click that’s actually a backdoor opening. Let's set up a test run in the subway tunnel tomorrow and see what anomalies pop up.