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Section 4. Passive acoustic monitoring techniques
Passive Acoustic Monitoring (PAM) is a method used to measure, monitor, and study sounds in the ocean using acoustic recorders such as hydrophones. PAM can complement or, in some cases, replace traditional visual survey methods and is increasingly used in applied conservation to monitor marine species and their responses to human activities.
PAM data can provide valuable insight into:
- ocean soundscapes
- spatial and temporal distribution of marine mammals and vessels
- long-term noise trends
- Patterns of habitat use.
Using these data, scientists can infer important biological information, including:
- species presence and seasonal occurrence
- migration timing
- social structure and behaviour
- population density (in some cases)
- behavioural changes in the presence and absence of vessels.
Advantages of PAM
- Relatively non-invasive
- Increasingly affordable and scalable
- Allows detection of animals at night, in remote areas, and during poor weather
- Enables long-term monitoring with reduced field effort and cost
- Can cover large spatial areas
Limitations of PAM
- Animals can only be detected when they are producing sounds
- Estimating the number of individuals present is difficult for most species
- Numerous underwater sounds are still unclassified, limiting interpretation
What equipment is used to measure underwater sound?
PAM technologies have developed significantly over the past two decades, and are now used worldwide by scientists, governments, and conservation organizations, and private individuals participating in community-monitoring initiatives (e.g. coastal listening stations hosted by volunteers). These collaborative efforts not only expand the spatial and temporal coverage of acoustic data, but also deepen public engagement with soundscape science and stewardship.
Hydrophones
A Hydrophone is a specially designed underwater acoustic sensor used to listen to and measure sound underwater. In principle, hydrophones are similar to microphones, but they are designed to function reliably under water and withstand pressure, corrosion, and long deployment periods. Learn more
How does a hydrophone work?
Inside a hydrophone, a piezoelectric transducer detects pressure changes from sound waves and converts them into electrical signals. These signals can be recorded, amplified, played back, and analyzed to examine sound characteristics such as amplitude and frequency.
Hydrophones are used in many different configurations and can be deployed for periods ranging from hours to years, depending on the study goals.
Types of PAM systems
PAM technologies generally fall into two broad categories:
1. Real-time systems
These systems transmit acoustic data as it is collected, allowing researchers to monitor sounds as they occur.
Examples include:
- Moored buoys
- Autonomous gliders
- Towed hydrophone arrays
- Cabled or drop-down hydrophones
- Drifting buoys
Real-time systems are particularly useful for mitigation and management, such as vessel slowdowns or dynamic protection measures.
2. Archival systems
These instruments are deployed for long periods and retrieved later, after which the recorded acoustic data are analyzed.
Examples include:
- Bottom-mounted recorders
- Animal-borne acoustic tags
- Telemetry-enabled acoustic tags
Archival systems are especially valuable for studying long-term trends, seasonal patterns, and cumulative noise exposure.
While Passive Acoustic Monitoring allows scientists to collect vast amounts of underwater sound data, recording sound is only the first step. To understand what these sounds mean (how they vary over time, which frequencies they contain, and how they relate to marine life and human activity) researchers must analyze and visualize acoustic data using a range of tools and metrics. The next section introduces the key ways underwater sounds are represented, measured, and interpreted, providing the foundation for understanding how noise is assessed and managed in the marine environment.