Songs of the Arches (with Helicopters)

Utah’s famous bridges and spires hum with a deep, Earthly music, below the threshold of human perception. The wind that carved the sandstone of Arches National Park into spectacular arches and towers also plucks them, like giant guitar strings, making them ring at low frequencies. Geoscientist Riley Finnegan and her colleagues in the Geohazards research group at the University of Utah are recording these arch songs in the Park and around Utah with seismometers, the same basic technology geologists use to listen for earthquakes, to learn their characteristic vibration frequencies—and how human noise affects them. Passing helicopters can cause rock arches and spires to shake up to 100 times stronger than they do naturally. Why? Helicopters are loud. Below the distinctive chopper womp-womp, the blades produce sound waves at frequencies too low for humans to hear unaided. When these infrasound vibrations match pitches with the natural resonance of the rock feature, they reinforce the natural vibrations like a choir singing in unison. The extra push can amplify the hum to the level of a rock concert. In episode 37 of Third Pod from the Sun, Finnegan explains what she’s learned and helps us hear the songs of the arches. This episode was produced by Liza Lester and mixed by Kayla Surrey.