Helmholtz resonance is the effect that makes air in a cavity and its neck oscillate strongly at a particular frequency. In simple terms, a hollow space with an opening can act like a tuned acoustic circuit: when sound matches its natural frequency, the response is amplified. The exact pitch depends on the cavity volume, neck shape, and opening size.
In cyber security, this matters because enclosures, vents, ducts, ports, and sensor housings can unintentionally amplify sound or vibration. Attackers may use resonant frequencies to inject noise into microphones, disturb MEMS sensors, or create side channels in devices that were not designed for acoustic coupling. Defenders look for the same effect when hardening hardware: they add damping, change geometry, test sensor limits, and check whether a device behaves differently under narrow-band acoustic or vibration exposure. A small resonance can become a real weakness if it is ignored.