The distance to which acoustic signals propagate is governed by how rapidly the energy is absorbed by the atmosphere. Absorption of acoustic energy varies roughly according to frequency. Lower frequency signals tend to travel further - one reason why low-frequency ship horns can be heard over several kilometres and high-frequency sirens become inaudible within a kilometre.
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| Figure 1. The 60-station IMS infrasound network, with the Australian-operated stations shown in red. |
Explosions provide a good broadband source of sound. High-frequency components in the hundreds to thousands of Hz range dissipate within a kilometre or so, but longer-lived infrasound components, if they are generated, can travel thousands of kilometres.
The long-lived infrasonic core generated by large atmospheric explosions is the focus of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in its efforts to monitor the environment for clandestine nuclear detonations. A global 60-station infrasound sensor network, a component of the International Monitoring System (IMS) of the CTBT, is being established to monitor compliance with the treaty when it enters into force (figure 1).
