The surf’s roar is more than just what we perceive with our ears. While the audible crashing of waves is familiar, much of the acoustic energy produced by breaking waves exists at frequencies below 20 hertz (Hz), known as infrasound. These low-frequency pressure waves, along with associated seismic vibrations passing through the ocean floor, provide a rich but hidden acoustic landscape. The research team at UCSB employed sophisticated arrays of infrasound sensors coupled with seismometers to probe these subtle signals. Their findings were recently published in Geophysical Journal International, detailing the unique acoustic and seismic footprints left by the surf and demonstrating the feasibility of pinpointing wave-breaking locations along the coastline through this approach.

The genesis of these inaudible waves lies in the physical mechanics of wave breaking. When waves collide with the rocky shore or seabed, air pockets get entrained, forming and collapsing bubbles that oscillate collectively due to pressure instabilities. Jeremy Francoeur, lead author and former UCSB graduate student, describes this phenomenon as a synchronized expansion and contraction of bubble clouds, generating persistent pressure oscillations. These oscillations translate into infrasound waves that travel upward through the atmosphere and downward as seismic waves through the Earth’s crust. Though these pressure variations lie beneath the human auditory range, their amplitude reaches levels comparable to everyday urban noise, making them significant natural acoustic sources.

Infrasound below 20 Hz includes ordinary acoustic waves, but their low pitch means that they go unnoticed by humans. Senior author and geophysicist Robin Matoza emphasizes that these “hidden sounds” originate from a broad spectrum of natural and anthropogenic activities worldwide. They include major geological and atmospheric events such as volcanic eruptions, earthquakes, landslides, hurricanes, and even atmospheric phenomena like auroras and wind flowing over mountainous terrain. Each source produces distinct low-frequency acoustic signatures, which scientists can harness to better understand and monitor Earth’s dynamic processes.

Motivated by UCSB’s coastal location, Matoza’s research group turned their attention to the acoustic mysteries of surf noise. Deploying an array of sensors at Coal Oil Point Reserve—a protected site within the UC Natural Reserve System—the researchers recorded infrasound and seismic data synchronized with high-definition video of wave activity. This multi-modal data set allowed them to correlate specific acoustic pulses with precise moments of wave breaking, greatly enhancing signal identification compared to previous single-sensor studies. By aligning sound signals with video “snapshots,” the team was able to discern a robust acoustic fingerprint unique to breaking waves.

The infrasound signals identified arrived in repetitive bursts between 1 and 5 Hz frequency, distinctly marking the crashing surf’s rhythmic energy. Though “loudness” is a subjective notion tied to human hearing perceptions, the acoustic wave amplitudes measured reached remarkable levels. Typical surf-generated infrasound ranged between 0.1 and 0.5 pascals, comparable to the sound pressure of busy traffic, while stronger swells produced waves as intense as 1 to 2 pascals—akin to the noise of a factory floor. This quantitative analysis reveals that the ocean’s low-frequency acoustic emissions rival common urban soundscapes in energy, a startling discovery since these sounds remain inaudible.

A key insight emerged from the team’s exploration of how these infrasound signals relate to actual sea conditions. The researchers found a correlation between infrasound amplitude and significant wave height, a critical parameter measuring the vertical scale of open ocean swells. However, they noted that the relationship between acoustic data and observed wave behavior was more complex than initially hypothesized. The interplay of factors such as tides, wind patterns, and bathymetry introduced nonlinearities that challenged simplistic models, underscoring the need for further investigation into environmental influences on surf-generated acoustics.

The array’s capability extended beyond mere detection; by measuring minuscule variations in arrival times of infrasound waves at multiple sensors, the team applied reverse-time migration techniques to triangulate the exact origins of breaking waves. Remarkably, the analysis localized the acoustic source consistently to the rock shelf at Coal Oil Point. This led to the hypothesis that specific underwater topography concentrates wave impact zones, triggering synchronized bubble oscillations that amplify the infrasound output. Understanding these spatial patterns opens exciting avenues for mapping coastal processes through sound.

Looking ahead, the researchers aim to explore whether individual beach segments universally serve as primary infrasound emitters or if such patterns vary with geography and environmental conditions. Questions linger about how wave infrasound signatures might differ between globally diverse shorelines like Santa Barbara and Tahiti, or how dynamic factors like changing tides and fluctuating wind fields modulate these acoustic emissions. Unraveling these complexities will extend the applicability of surf acoustic monitoring as a powerful natural observatory tool.

Matoza’s lab enjoys unique advantages owing to the proximity of Coal Oil Point Reserve, only 2.5 miles from UCSB’s main campus. This closeness enables rapid deployment and iterative refinement of sensor arrays, facilitating extensive field experimentation and hypothesis testing. Moreover, students actively engaged in this project gain hands-on experience across the entire scientific workflow, from data collection and instrument installation to advanced signal analysis and scholarly writing. This immersion cultivates the next generation of geophysicists skilled in cutting-edge Earth science methodologies.

The ultimate goal is the development of an autonomous system capable of characterizing nearshore surf conditions solely from infrasound and seismic data, independent of visual observations. Current video monitoring technologies suffer from limitations imposed by darkness, fog, and adverse weather, which reduce visibility and reliability. Acoustic and ground-motion sensing could thus become indispensable complements, offering continuous, all-weather surveillance possibilities vital for coastal management, hazard preparedness, and environmental research.

This pioneering research sets a precedent in marine geophysics, unveiling a novel sensory interface through which the Earth’s atmospheric and oceanic interactions reveal themselves. By “listening” below the human audible spectrum, scientists can access a previously hidden domain of natural signals rich with information on coastal wave dynamics. As this technology matures and integrates with existing oceanographic tools, it promises significant advances in our understanding of ocean processes, coastal environments, and their responses to climate and anthropogenic changes.

Subject of Research: Acoustic and seismic signatures of breaking ocean waves

Article Title: Researchers characterize infrasound and seismic signals from surf to monitor coastal wave dynamics

News Publication Date: Not specified

Web References: https://academic.oup.com/gji/advance-article/doi/10.1093/gji/ggaf317/8236357

References: Study published in Geophysical Journal International

Image Credits: Elena Zhukova

Keywords: Space sciences; Seismology; Oceanography; Coastal processes

Since its debut in 2010, the Falcon 9 rocket—a two-stage rocket designed for a variety of space missions including satellite deployments and crewed flights—has completed over 400 launches. The Vandenberg Space Force Base serves as a pivotal west coast launch site, particularly for missions requiring polar orbits and trajectories over the Pacific Ocean. Amid the surge in frequency and diversity of these launches, with 46 launches recorded in 2024 alone, the ripple effects of sonic booms have become more pronounced in coastal communities. These sonic disturbances have elevated acoustic concerns among residents, prompting detailed scientific measurement and analysis to unravel how these booms vary with environmental and operational factors.

Sonic booms arise when an object travels through the atmosphere faster than the speed of sound, creating shock waves—essentially powerful bursts of compressed air—that propagate outward. While often associated with supersonic aircraft, rocket ascent sonic booms possess distinct acoustical signatures. The researchers emphasize that the typical sonic boom from a rocket differs considerably from that generated by aircraft perforating the sound barrier. Specifically, rocket booms register peak energy at frequencies below 1 hertz, a range under human auditory perception. This ultralow frequency energy translates not to loud bangs but to vibrating sensations that can rattle buildings and induce a sense akin to seismic tremors indoors.

During the summer of 2024, the collaborative research team amassed 132 separate sonic boom measurements across an approximately 200-square-mile area encompassing Ventura County. Utilizing a network of precision acoustic sensors, their dataset includes variations in boom intensity, temporal profiles, and frequency characteristics correlated with differing weather patterns, rocket trajectories, launch times, and seasonal atmospheric conditions. This broad spectrum of data allows for granular insights into how sonic booms propagate through the complex coastal environment, influenced by factors such as wind shear, temperature gradients, and oceanic weather systems.

Current observations indicate that Ventura County residents might experience between zero to five audible rocket sonic booms monthly, highlighting substantial variability in occurrence and intensity. Not every Falcon 9 launch produces a boom strong enough to be detected on land, and those that do vary remarkably in their perceptual and physical impact. The interplay between the rocket’s ascent path, how high its supersonic speeds persist, and atmospheric fluctuations critically shapes the sonic footprint on the ground. This dynamic variability challenges base operations and local authorities attempting to manage community noise concerns while supporting an increasing cadence of space missions.

Beyond qualitative descriptions, the physics behind these rocket sonic booms involves complex aeroacoustic and atmospheric propagation principles. The low-frequency shock waves lose energy differently than higher-frequency noises, often penetrating structures more efficiently and eliciting vibroacoustic responses in homes and buildings. The unique nature of rocket sonic booms underscores the need for tailored mitigation strategies distinct from those designed for aircraft or standard community noise regulatory frameworks. By characterizing the pressure waveforms, rise times, and resultant structural responses, the research aims to inform practical actions that could alleviate homeowner impacts without constraining critical aerospace operations.

Furthermore, the implications of this research resonate beyond Ventura County. As commercial spaceflight accelerates and launch footprints widen, populations along coastal and inland corridors globally may experience increased acoustic exposure. Understanding the physics of rocket sonic booms, particularly at supersonic ascent stages over populated areas, becomes crucial for sustainable aerospace development. These insights hold the promise of guiding launch trajectory planning, notification protocols, and engineering improvements in vehicle design to reduce the sonic impact while maintaining safety and mission success.

The team’s presentation at the 188th Meeting of the Acoustical Society of America, co-hosted with the 25th International Congress on Acoustics, offers the wider scientific community access to these critical findings. Presenters Makayle Kellison and Professor Kent Gee emphasize the broader goal of integrating acoustic science with aerospace operations to forge pathways toward quieter, community-compatible space launches. In-depth discussions of sonic boom wave mechanics, environmental dependencies, and human perceptual responses illuminate the complex challenge facing both researchers and policymakers.

Communication of these findings to the public and industry stakeholders is equally important. The research highlights that while sonic booms can feel dramatic inside homes—much like a mild earthquake—they do not cause eardrum damage due to their primarily infrasonic character. This clarification helps demystify community concerns and fosters informed dialogue about balancing technological progress with local quality of life. As rocket launches increase in frequency, such transparent communication rooted in rigorous science will be vital to maintaining public trust and facilitating future aerospace growth.

In conclusion, the evolving story of Falcon 9 rocket launches and their associated sonic booms exemplifies the intersection of cutting-edge aerospace technology and environmental acoustic research. Efforts to characterize and mitigate these sonic effects advance not only our scientific understanding but also enable responsible stewardship of the surrounding communities. As humanity ventures deeper into space exploration, the lessons learned from these acoustic investigations at Vandenberg set a precedent for harmonizing technological ambition with the lived experience of people on Earth.

Subject of Research: Acoustic characterization and impact assessment of Falcon 9 rocket ascent sonic booms in Ventura County, California.

Article Title: SpaceX Falcon 9 Launches Generate Audible Sonic Booms Felt Miles Away: A New Frontier in Acoustic Impact Research

News Publication Date: May 19, 2025

Web References:

• Acoustical Society of America Press Room: https://acoustics.org/asa-press-room/
• Lay Language Papers on Acoustics: https://acoustics.org/lay-language-papers/
• Acoustical Society of America: https://acousticalsociety.org/
• International Commission for Acoustics: https://www.icacommission.org/

Image Credits: Kent Gee

Keywords

Acoustics, Applied acoustics, Sound, Rocket engines