subaudio

Subaudio refers to sound energy with frequencies below the lower limit of human hearing, typically under 20 Hz. In some contexts it is used interchangeably with infrasonic, though subaudio can also refer to signals just below the audible range that are measurable with specialized equipment.

Low-frequency waves have long wavelengths and propagate differently from audible sounds. They attenuate slowly in air but require large sensors. Detection relies on calibrated instruments such as barographs, hydrophones, seismometers, or accelerometers, often arranged in arrays to locate sources.

Natural sources include weather systems, ocean activity, volcanoes, and earthquakes; human sources include engines, explosions, ships, and rocket launches. Subaudio energy may be felt as vibration or pressure changes even when it is not heard.

Applications include atmospheric and geophysical research, structural health monitoring, and treaty verification efforts that monitor for clandestine explosions. Subaudio data contribute to studies of long-range propagation, coupling between the atmosphere and oceans, and resonant effects in buildings.

Challenges include distinguishing subaudio signals from environmental noise, requiring long-term data collection and processing. Safety concerns are generally low for typical environments, but intense low-frequency energy can cause discomfort or audible flutter in some cases.