Using sound to observe the global ocean

Peter Tyack – plt@st-andrews.ac.uk

School of Biology, University of St Andrews, St Andrews, Fife, KY16 8LB, United Kingdom

Popular version of 3pAO2 – Long-term global ocean observing using sound
Presented at the 189th ASA Meeting
Read the abstract at https://eppro02.ativ.me/appinfo.php?page=Inthtml&project=ASAASJ25&id=3983499%20#ASAASJ25

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

Of all the ways to sense objects in the ocean, sound reaches the furthest. Light seldom travels more than hundreds of meters in the ocean, but sound can travel thousands of kilometers. For example, the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) created a network of just 6 hydrophones (underwater microphones) that can detect a nuclear explosion anywhere in the global oceans (Figure 1).

Figure 1. Map of 6 underwater acoustic sensing stations of the Comprehensive Test-Ban Treaty Organization.
Reproduced with permission from Ainslie, Michael A., et al. “Ocean soundscapes and trends from 2003 to 2021: 10–100 Hz.” The Journal of the Acoustical Society of America 157.6 (2025):4358-4384, Figure 1.

Sound can travel so far that listening has long been selected for detecting threats such as submarines, but the ocean carries acoustic signals of wind, waves, rain and sea ice that are important for monitoring climate change, and the sounds of animals vocalizing underwater tell us about their distribution and about the health of marine ecosystems.

Using sound to understand changes in the ocean requires many years of data to sort out changes on daily, seasonal, and longer-term time scales. The CTBTO hydrophone array is one of few sources of such long time series of ocean sound.

Figure 2. Measured and modeled Sound Pressure Level in the 10-40 Hz band from Wake Island in the Pacific Ocean. Reproduced with permission from Robinson, Stephen, et al. “Impact of the COVID-19 pandemic on levels of deep-ocean acoustic noise.” Scientific Reports13.1 (2023): 4631, Figure 2. Creative Commons Attribution 4.0 International License

Figure 2 shows 12 years of low frequency acoustic data from the Pacific. The strongest pattern in sound level is annual variation, but you can also see a longer-term pattern of increasing variation from 2010 to 2020.

Figure 3. Expanded view of sound level data from figure 2, here from 2018-2020. Reproduced with permission from Robinson, Stephen, et al. “Impact of the COVID-19 pandemic on levels of deep-ocean acoustic noise.” Scientific Reports13.1 (2023): 4631, Figure 4. Creative Commons Attribution 4.0 International License

Analyzing data from before COVID hit in 2020, shown with blue dots in figure 3, makes it possible to estimate what sound levels were expected in 2020 based on earlier years. The data points change color from blue to red once COVID started to affect seagoing activities. You can see that many of the red dots from January to July 2020 are lower than the gray area predicted from pre-COVID data, demonstrating that the reduction in human activities during COVID reduced sound levels around Wake Island.

Excessive sound can also harm wildlife by damaging hearing and by causing stress or disturbance. Recordings from the west coast of the US have shown that increased shipping caused underwater sound energy to more than double each decade from the 1960s to the 1990s. This increase in human-generated ocean noise stimulated concerns about increasing effects of noise on marine life. In June 2025, a coalition of 37 countries declared “ocean noise is intensifying, driven by the ongoing expansion of global shipping activities and development of ocean industries” and formed a high ambition coalition for a quiet ocean (https://www.foraquietocean.org).

Figure 4. Trends in annual mean sound pressure level (SPL) from 2006-2021 at Ascension Island at 5 frequency bands: VLF=9-14 Hz; LF=14-28 Hz, MF=28-56 Hz; HF=56-112 Hz; ADEON B = 9-89 Hz. Reproduced with permission from Ainslie, Michael A., et al. “Ocean soundscapes and trends from 2003 to 2021: 10–100 Hz.” The Journal of the Acoustical Society of America 157.6 (2025):4358-4384, Figure 18.

Analysis of CTBTO data from sites around the world document changes in ocean sound after 2000. This century has not seen significant increases in sound pressure level at any of the CTBTO sites. Figure 4 shows the long term decline in sound level in 4 frequency bands from 2006-2021 at Ascension Island in the Atlantic Ocean. A close look at the acoustic record (Figure 5A) shows that airguns used to prospect for offshore oil and gas along the African and South American coasts were a primary source. Figure 5B shows that the best predictor of sound pressure level there in the MF band is the price of Brent Crude Oil. When the price of oil is low, there are fewer surveys. The major sources of sound energy varied between sites and included natural sounds of earthquakes and whales and sounds of human activities such as shipping.

Figure 5. 5A shows a waveform on the top and spectrogram (plot of frequency against time) of a series of pulses from an airgun source recorded from the CTBTO station at Ascension Island. Figure 5B shows how the sound level recorded there correlates with the price of oil. Reproduced with permission from Ainslie, Michael A., et al. “Ocean soundscapes and trends from 2003 to 2021: 10–100 Hz.” The Journal of the Acoustical Society of America 157.6 (2025):4358-4384, 5A: Figure 16; 5B Figure 20.

Access to long time series of ocean sound data is critical for understanding how changes in sound sources cause trends in ocean sound. This information is not only critical for estimating indicators of climate change and ocean health, but also for managing the effects of sound. Noise from human activities may not have been increasing ocean sound levels globally in this century, but sound produced by more localized activities and the harm it causes to wildlife needs to be measured and managed.

Over the last 50 years, the integration of global weather data with improved computer models has drastically improved our ability to forecast weather. The applications of ocean sound to measure threats, climate change and ecosystem health are so important that we need similar expansion and improvement of our global ocean sound observation networks so that we can forecast and manage these important applications.