Waterfall infrasound

Waterfalls are long-lasting sources of continuous infrasound with relevance to multiple acoustic fields. This paper presents infrasonic observations of 11 waterfalls spanning wide ranges of hydraulic power, discharge, and height. The waterfall recordings had similar spectral shapes, including an abrupt rise from low-frequency background noise, a mid-frequency plateau with small broad peaks, and high-frequency power-law decay. Parameters of waterfall infrasound signals depend on waterfall characteristics; in particular, infrasound power is proportional to waterfall hydraulic power, with an acoustic efficiency of approximately 10-6, and infrasound median frequency approximately relates to height and discharge via negative-slope power laws. Using Niagara Falls as an example, this paper evaluates a simple spherical spreading model for predicting infrasound power over a waterfall's surrounding region and finds that infrasound remains strong out to at least 10 km. Finally, the paper models waterfall audibility to pigeons as an example of an animal whose infrasound perception is good and well-studied and predicts that pigeons may hear waterfalls hundreds to thousands of meters away, depending on hydraulic power via another power law. The simple relationships between these infrasound characteristics and commonly available height and discharge measurements enables remote discharge monitoring by hydrologists and predictions of infrasound spatial extent for studying its ecological effects.