Infrasounds are low-frequency sound waves that are below the hearing threshold (20 Hz), little perturbations of atmospheric pressure which, due to the low atmospheric absorption of these low frequencies, can propagate for thousands of kilometers and be detected by pressure sensors.
HERMES is a differential infrasonic sensor based on differential pressure measurement and the use of an acoustic filter to eliminate the stationary component of atmospheric pressure. Hermes performs high precision / high reliability measurements over a wide frequency bandwidth and it’s capable of detecting low-frequency (non audible) sound waves.
In addition to strong volcanic eruptions, there are many sources of infrasonic waves, most of which are produced by natural phenomena such as adverse weather conditions, lightning, asteroids, wave motion, outgassing and minor volcanic explosions, debris-flow, rapid mass flows such as avalanches and landslides, earthquakes and related ground shaking, sea storms, mountain waves. Artificial sources of infrasonic waves that can be measured are quarry blasting, TNT/Gas blasting during avalanche control, nuclear explosions and shock waves.
A range of sensors with different sensitivity and dynamics as well as frequency ranges are able to cover the amplitude and frequency ranges of most of these infrasound sources, allowing effective monitoring.
The rugged casing together with the low power consumption guarantee the employment of HERMES sensors also in hostile environment.