Helicopter generated infrasound

Did you know that helicopters produce infrasound? This sound energy is too low frequency for humans to hear but is in most cases the strongest sound that helicopters generate.

The main rotor of a helicopter produces sound into principal ways: The first is called “blade slap” which is what we hear. That sound is produced when an advancing blade strikes the air wake produced by the preceding blade, but it is not the strongest sound. The strongest sound is produced as so-called “thickness noise,” which arises due to pressure pulses emitted from the front and trailing edges of a rotor blade, and through constructive interference of these pulses as they reach a target. This sound energy is strongly directional, emitted in the plane of the main rotor.

The frequency of infrasound is controlled by the number of blades on the main rotor and the main rotor revolution rate, and the lowest infrasound tones are between ~10 and 30 Hz. At left is a plot of the sound spectrum generated by a two-blade Bell 206 helicopter. In it you see the first frequency peak at 13 Hz with a series of overtones at integer multiples. During flight these frequencies may also be Doppler shifted.

Helicopter infrasound is known to generate resonant vibrations in objects (e.g. an arch above), and past studies have shown that vibration amplitudes can reach potentially damaging levels in ancient structures, for example adobe walls, in addition to rock pinnacles (King 1996, King 2001). To learn more about helicopter induced resonance see: Moore (2018) Rainbow Bridge Vibration Risk Assessment and information below.

For in-depth technical study, see our recent papers:

  • Vibration of Natural Rock Arches and Towers Excited by Helicopter-Sourced Infrasound

    • Riley Finnegan, Jeffrey R. Moore, and Paul R. Geimer
    Department of Geology and Geophysics, University of Utah

    Citation: Finnegan, R., Moore, J. R., and Geimer, P. R.: Vibration of natural rock arches and towers excited by helicopter-sourced infrasound, Earth Surface Dynamics, 9, 1459–1479, doi.org/10.5194/esurf-9-1459-2021, 2021.

     

  • Contribution of Anthropogenic Vibration Sources to Crack Growth in Natural Rock Arches

    • Riley Finnegan, Jeffrey R. Moore, and Paul R. Geimer
    Department of Geology and Geophysics, University of Utah

    Citation: Finnegan, R., J.R. Moore, and P.R. Geimer. Contribution of Anthropogenic Vibration Sources to Crack Growth in Natural Rock Arches, Frontiers in Earth Science, 10:1035652. doi.org/10.3389/feart.2022.1035652, 2022.

     

    Related technical study on helicopter induced vibration of ancient structures:

  • Investigation of Helicopter Induced Vibrations at Balcony House, Mesa Verde

    • Ken King, 1996

     

  • Media assocated with AGU Fall Meeting presentation S23D-0679 by Finnegan, Moore, & Geimer:

    The following data were generated and analyzed by Finnegan et al. (2019): Vibration Response of Rock Arches and Towers to Helicopter-Sourced Infrasound, AGU Fall Meeting, S23D-0679

    All figures and images copyright: Geohazards Research Group, University of Utah

  • Photos/Video: Arsenic Arch 1, Arsenic Arch 2, Arsenic Arch 3, Delicate Arch, Rainbow Bridge, Squint Arch, Little Egypt 1, Little Egypt 2, Little Egypt 3

  • Animations: Arsenic Arch, Little Egypt, Two Bridge

  • Vibration recordings: (video) Arsenic Arch, Two Bridge, Squint Arch; (sound only) Arsenic Arch, Two Bridge, Squint Arch

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    Copyright 2022 - for information or contact: jeff.moore@utah.edu

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