The Infrasonic Choir: Decoding Songs to Inform Decisions

Sarah McComas –

U.S. Army Engineer Research and Development Center, Vicksburg, MS, 39180, United States

Popular version of 1pPAb4 – The Infrasonic Choir: Decoding Songs to Inform Decisions
Presented at the 186th ASA Meeting
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–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

InfrasonicFigure 1. Infrasound is a low frequency, sub-audible sound propagated over long distances (10’s to 1000’s of kilometers) and typically below the threshold of human hearing. Image courtesy of author.

The world around us is continuously evolving due to the actions of Mother Nature and man-made activities, impacting how we interact with the environment. Many of these activities generate infrasound, which is sound below the frequency threshold for human hearing (Figure 1). These signals can travel for long distances, 10s to 100s km based on source strength, while maintaining key information about what generated the signal. The multitude of signals can be thought of as an infrasonic choir with voices from a wide variety of sources which include natural signals such as surf and volcanic activity and man-made including infrastructure or industrial activities. Listening to, and deciphering, the infrasonic choir around us allows us to better understand how the world around us is evolving.

The infrasonic choir is observed by placing groupings, called arrays, of specialized sensors around the environment we wish to understand. These sensors are microphones designed to capture very low frequency sounds. An array’s geometry enables us to identify the direction the signal is observed. Using multiple arrays around a region allow for identification of the source location.

One useful application of decoding infrasonic songs is listening to infrastructure, such as a bridge. Bridges vibrate at frequencies related to the engineering characteristics of the structure, such as mass and stiffness. Bridges are surrounded by the fluid atmosphere which allow the bridge vibrations to create waves that can be measured with infrasound sensor arrays. One can visualize this as waves generated after a rock is thrown into a pond. As the bridge’s overall health degrades, whether through time or other events, its engineering characteristics change causing a change in the vibrational frequency. Being able to identify a change from a healthy, “in-tune” structure to an “out-of-tune”, unhealthy structure without having to see or inspect the bridge would enable continuous monitoring of entire regional road networks. The ability to conduct this type of monitoring after a natural disaster, such as hurricane or earthquake, would enable quick identification of damaged structures for prioritization of limited structural assessment resources.

Understanding how to decode the infrasonic choir within the symphony of the environment to better understand the world around us is the focus of ongoing research at the U.S. Army Engineer Research and Development Center. This research effort focuses on moving monitoring into source rich urban environments, the design of lightweight and low-cost sensors and mobile arrays, and the development of automated processing methods for analysis. When successful, continuous monitoring of this largely untapped source of information will provide a method for understanding the environment to better inform decisions.

Permission to publish was granted by the Director, Geotechnical and Structures Laboratory, U.S. Army Engineer Research and Development Center.