Section 5. Underwater acoustics and noise metrics

How are sounds analyzed and visualized?

Sounds in the real world are rarely simple pure tones. Instead, they usually contain many frequencies at once, with intensities that change over time. To study and understand these complex sounds, scientists convert acoustic recordings into visual representations that allow us to “see” sound patterns, structure, and energy.

Pure vs. complex tone

Common visual tools include waveforms, frequency spectrum graphs, and spectrograms. Each highlights different aspects of sound and together they form the foundation of underwater acoustic analysis.

Waveforms

Waveform graphs show how sound pressure changes over time. Pressure is displayed as positive values (compression of the medium) and negative values (expansion of the medium) around a central baseline.

Waveform and Spectrogram

Figure with waveform (amplitude) and spectrogram (Frequency) displays over time.

Waveforms are useful for:

Identifying the timing and duration of sounds
Visualizing sudden or impulsive events
Understanding overall signal structure

However, waveforms do not show which frequencies are present in a sound. For that, we need frequency-based views.

Frequency spectrum graphs

A frequency spectrum graph shows how much acoustic energy is present at each frequency at a specific moment in time. It is essentially a snapshot of a sound’s frequency composition.

Spectrogram and Frequency Spectrum

A pure tone appears as a single sharp peak at one frequency.
A complex sound shows multiple peaks across a range of frequencies.

Frequency spectra are useful for identifying dominant frequency bands and distinguishing between different sound sources, but they do not show how sounds change over time.

Spectrograms

A frequency spectrum graph provides a snapshot of a sound’s frequency content at a given moment, whereas a spectrogram displays how frequency and intensity vary over time.

To create a spectrogram:
The waveform is divided into short, overlapping time windows
A frequency spectrum is calculated for each window

These spectra are displayed side by side along the time axis

Colour or shading represents sound intensity, with brighter or more intense colours indicating stronger sound energy.

Spectrograms are one of the most powerful tools in underwater acoustics because they reveal patterns, rhythms, and overlaps that are not visible in waveforms or spectra alone.

How to read a spectrogram

A spectrogram can be read like a map of sound:

Vertical axis: Frequency (Hz)
Horizontal axis: Time (seconds or minutes)
Colour or intensity: Sound level (louder sounds appear brighter or more intense)

Unlike musical notes, most natural and human-made sounds are not single frequencies. Instead, they span ranges of frequencies, forming bands, contours, or bursts that can be linked to specific animals, vessels, or environmental processes.

Understanding sound energy across frequencies

Most underwater sounds distribute energy across a range of frequencies rather than concentrating it at one frequency. This is often referred to as broadband sound.

Understanding how sound energy is distributed across frequencies is essential because:

Different species hear best in different frequency ranges
Noise impacts depend on overlap between sound energy and an animal’s hearing sensitivity
Management and mitigation strategies often target specific frequency bands

This frequency-based perspective provides a bridge between sound visualization and quantitative noise metrics.

Octave bands and third-octave bands

To investigate sounds, the frequency spectrum is often divided into standardized bands called octaves.

An octave band is a frequency band where the upper frequency is twice the lower frequency.
Third-octave bands divide each octave into three narrower bands, allowing finer resolution.

These bands are widely used in environmental noise analysis to:

Compare sound levels across frequencies
Identify which frequency ranges contribute most to overall noise
Align noise measurements with the hearing ranges of marine species

Listen

Octave band highlights.

You may come across the term decidecade band when learning about underwater noise analysis. This means 1/10th of a decade, which is approximately equal to one-third of an octave (1 ddec ≈ 0.3322 oct, to be precise!). For that reason, decidecade and one-third octave bands are terms used interchangeably.

Noise metrics: Measuring and comparing sounds

There are many ways to measure and interpret underwater sound, each designed to capture different aspects of acoustic energy. Over the decades, acousticians have developed a widely accepted set of metrics that are useful for understanding underwater noise and marine mammal communication. It is important to note, however, that new metrics continue to be developed as the field evolves.

Below are some of the most commonly used metrics in underwater noise studies.

Sound Pressure Levels (SPLs)

Sound pressure level (SPLs) describes the intensity (amplitude) of a sound at a given moment. It is expressed in decibels (dB) relative to a reference pressure.

SPLs are useful for:

Comparing sound levels across locations or times
Describing background noise conditions
Assessing instantaneous sound intensity

It is important to note that SPL represents a snapshot in time and does not account for how long a sound lasts.

Decibels and reference pressures

Humans and other animals can detect sound across an extremely wide range of amplitudes. Quiet (low sound pressure) sounds are often of great importance to us, and as a result, our hearing system has evolved to discriminate small differences in the intensity of quiet sounds more accurately than differences in very loud (high sound pressure) sounds.

Because of this, sound levels are usually measured in decibels (db) rather than in linear units of sound energy. The decibel scale is logarithmic and provides a numerical framework that reflects how sound intensity is perceived.

Decibel values are expressed relative to a reference sound pressure level. Because sound propagates differently in air and water, different reference sound pressure levels are used in the two media. In air the reference pressure is 20 micropascals (20 uPa). In water, the reference pressure is 1 micropascal (1 uPa).

As a result, decibel values measured in air and water are not directly comparable, even if the numbers appear similar, as illustrated below.

Illustrated figure comparing decibel scales in air and underwater, with different reference pressures. — Comparison of decibel scales in air and underwater. Because different reference pressures are used in each medium, 20 μPa in air and 1 μPa underwater, the two scales are offset by 62 dB. Sounds at the same vertical position on both scales have equal acoustic intensity, despite showing different dB values. Figure from Reckendorf et al. 2023.

Caption: Comparison of decibel scales in air and underwater. Because different reference pressures are used in each medium, 20 μPa in air and 1 μPa underwater, the two scales are offset by 62 dB. Sounds at the same vertical position on both scales have equal acoustic intensity, despite showing different dB values. Figure from Reckendorf et al. 2023.

Sound Exposure Levels (SELs)

Sound Exposure Level (SEL) is a measure of acoustic energy that accounts for both the received sound level andtheduration of exposure. It is commonly used to help assess potential effects of sounds on marine life, particularly for repeated or prolonged exposures.

Example: If an underwater sound source, such as marine construction, is present for one second, the resulting Sound Exposure Level (SEL) will be lower than if the same sound persists for ten seconds. Likewise, a quieter sound lasting a longer period can result in a similar SEL to a louder sound lasting for a shorter period.

Other commonly used sound metrics

Additional metrics are often used alongside SPL and SEL, including:

Root Mean Square (RMS): Represents average sound pressure over a time window and is commonly used for continuous noise.
Peak sound pressure: Captures the maximum instantaneous pressure of a sound and is particularly relevant for impulsive noises such as pile driving or explosions.

Each metric highlights a different acoustic property and should be interpreted in context.

Power Spectral Density (PSD)

Power Spectral Density (PSD) describes how sound energy (or power) is distributed across frequencies, typically expressed per unit bandwidth (e.g., per Hz).

PSD plots are useful for:

Comparing noise levels across frequency ranges
Examining long-term soundscape characteristics
Identifying frequency-specific contributions from different sources

PSD plots are especially valuable for understanding chronic background noise and cumulative sound exposure.

Choosing metrics for underwater noise analysis

Underwater noise analysis does not rely on a single ‘one size fits all’ metric. Instead, scientists select metrics based on:

The type of sound source
The duration and variability of noise
The hearing sensitivity and behaviour of affected species
The ecological or management question being addressed

International standards, such as those developed by the International Organization for Standardization (ISO), provide guidelines for selecting appropriate metrics under a range of conditions.

A note on interpretation

Sound measurements are only meaningful when interpreted carefully. Factors such as equipment sensitivity, frequency weighting, and environmental conditions can influence results.

Most importantly, understanding underwater noise impacts requires linking acoustic measurements to biological relevance, including hearing ranges, communication needs, and behavioural responses of marine species.