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Offshore wind construction noise is measured underwater with calibrated hydrophones and reported as several sound-pressure and sound-exposure metrics, not as one decibel number. In the United States, the usual sequence is to model the project’s sound field before construction, then verify modeled ranges with simultaneous recordings during pile driving. A defensible report identifies the pile and hammer, range, hydrophone depth, frequency band, weighting, mitigation state, calibration and quality controls, and distributions such as median, mean, maximum and minimum. The details differ by jurisdiction: Germany applies received-level criteria at 750 m, Denmark evaluates weighted cumulative exposure for a fleeing animal, and BOEM’s offshore recommendations use modeled species thresholds followed by field verification.
The explanation below focuses on underwater pressure from impact and vibratory pile driving. Pressure measurements do not characterize particle motion, which is important to many fishes and invertebrates, and these protocols do not measure operational turbine or routine vessel noise.
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What exactly is measured during offshore wind construction?
Impact piling produces brief, broadband, intermittent impulses: pressure rises rapidly when a hydraulic hammer strikes the pile, then decays, often with a ringing tail. Vibratory piling shakes the pile into or out of the seabed and normally produces lower-level, continuous, non-impulsive sound over the operating interval. The distinction determines which metrics, exposure calculations and regulatory criteria apply.
The sensor is a hydrophone, not an airborne sound-level meter. A measurement chain normally contains one or more hydrophones, an amplifier and signal-conditioning stage, and a digital recorder, as described in BOEM’s offshore recommendations. The hydrophone measures pressure fluctuations in water. Levels use a reference pressure of 1 µPa; sound-exposure quantities use 1 µPa²s. The familiar dB(A) scale for community noise is therefore not the reporting basis here.
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These recommendations concern pressure only. The July 2026 NMFS data-collection memo says fishes and invertebrates are often primarily sensitive to particle motion and that NMFS does not currently provide particle-motion criteria for those taxa. A pressure-only hydrophone report should not be presented as a complete effects assessment for fish or invertebrates.
Which standards define the measurement language?
ISO 18406:2017 is the international method for measuring underwater sound radiated by percussive pile driving. Its approach combines fixed-range recordings with range-dependent hydrophone deployments. ISO 18405:2017 supplies terminology for sound-pressure and sound-exposure quantities. Agencies commonly specify an analysis band of at least 20 Hz to 20 kHz.
ISO 7605:2025 is a different standard. Released on 23 July 2025, it addresses ambient underwater sound recorded for a day or longer, including equipment performance, calibration, deployment, metadata and processing from raw data to statistics. It is useful for baseline and long-term soundscape monitoring around a wind farm, but it is not the pile-driving pulse standard and it excludes particle motion.
Keep three US documents separate. BOEM’s offshore recommendations describe pre-construction modeling and construction sound-field verification. The July 2026 NMFS memo gives detailed metrics and metadata for coastal impact and vibratory pile driving and expressly tells readers to consult NMFS for offshore pile driving. NMFS also says older 2012 acoustic criteria can be outdated; any threshold table reproduced from BOEM’s citation of NMFS 2018 must be labeled as such and checked against current technical guidance before being called current.
What do peak SPL, RMS SPL and SEL mean?
A report should identify each metric, its weighting and its integration window. A single broadband decibel value cannot show how a pulse was integrated or whether it was weighted for an animal hearing group.
- Peak sound pressure level (peak SPL, Lp,pk): the maximum absolute pressure in one pulse, referenced to 1 µPa. The German criterion uses the zero-to-peak form.
- Root-mean-square SPL (RMS SPL): an energy-averaged pressure level over a stated window. BOEM uses the window containing 90% of a pulse’s acoustic energy. Denmark also uses SPL125ms for behavioural evaluation; a pulse shorter than 125 ms can be converted from single-strike SEL under its guideline.
- Single-strike SEL: squared pressure integrated over one impact pulse, referenced to 1 µPa²s. The pulse duration normally follows the 90%-energy (T90) rule.
- Cumulative SEL (SELcum or SEL24h): the energy sum over an installation event or a 24-hour period, as the governing method specifies. BOEM’s offshore instruction sums strikes from the first to the last strike on one pile; NMFS includes ramp-up and power-down in cumulative SEL. Denmark evaluates each foundation, capped at 24 hours, with auditory weighting.
- T90 and repetition rate: T90 is the duration containing 90% of pulse energy. The report should also state hammer frequency and strikes per second.
If the 90%-energy window overlaps the interval between strikes, the Danish summary of ISO 18406 uses the interval between strikes for integration and allows a sequence mean SELss. If an offshore recording fails, BOEM permits a backup estimate of cumulative SEL as single-strike SEL + 10 log10(number of strikes), but discourages it because strike levels vary.
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Report unweighted broadband values and the required auditory weightings separately. BOEM’s US marine-mammal groups are low-, mid- and high-frequency cetaceans, phocid pinnipeds in water, other marine carnivores and sirenians. Denmark uses LF, HF, VHF and PCW weightings for species in Danish waters. A weighted SEL is not interchangeable with an unweighted SEL.
How is the frequency band selected?
For broadband reported levels, NMFS and BOEM generally use 20 Hz–20 kHz. BOEM asks that modeled source power spectral density extend from 10 Hz to at least 20 kHz. NMFS recommends measuring below 20 Hz when substantial low-frequency energy is expected, the frequencies matter to the species considered and accurate measurement is practicable. Many recording systems are not flat below 20 Hz, so hammer frequency and strike rate must be documented even when the low-frequency band cannot be measured reliably.
BOEM reporting also includes decidecade (one-tenth-octave) levels and power-spectral-density plots. Median and mean decidecade levels are requested for peak, RMS, single-strike SEL and cumulative SEL, together with single-strike SEL versus time and weighted cumulative SEL versus range.
How does the US offshore sound-field verification process work?
BOEM states the sequence plainly: model the noise impact before construction, then validate ensonified areas with construction measurements. The model estimates sound fields for the project design envelope and species exposures; an exposure above a threshold is not automatically a legal “take” under the Endangered Species Act or Marine Mammal Protection Act.
What the model must document
The acoustics appendix identifies numerical methods, environmental inputs, construction scenarios and species assumptions. It names the model classes used, such as finite-element source modeling and ray-trace, parabolic-equation or normal-mode propagation, and explains how frequency bands and ranges were joined. Inputs include pile type and diameter, water depth, monopile or jacket foundation, pre- or post-piled jacket piles, simultaneous driving, hammer energies and power levels. Modeled source spectra cover 10 Hz–20 kHz or more and should represent more than only maximum power.
Where offshore hydrophones are placed
BOEM’s offshore layout uses stationary recorders at approximately 750 m and 5 km on at least two radial directions; the close recorder is no more than 1 km from the pile. A third, intermediate range is recommended where available. Directions should reflect bathymetry, such as one radial toward deeper water and one along relatively constant depth; raked piles require the pile-tilt direction and an orthogonal direction.
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Hydrophones are placed in the lower half of the water column, between 2 m above the seabed and half the full water depth, where received levels are likely to be greatest. Measurements run continuously through all impact hammering of each monitored pile. Mobile vessel, autonomous-vehicle or drifting systems can add spatial detail but do not replace simultaneous fixed ranges.
What performance and calibration are required?
- Usable sensitivity and dynamic range at the expected ranges; clipping and a noise floor that masks the signal are both failures.
- At least 20 Hz–20 kHz response, with documented high-pass and anti-alias filtering.
- Omnidirectional response within 2 dB, apart from cable-direction effects.
- Every impact at least 10 dB above system noise.
- At least 12-bit analogue-to-digital resolution and lossless recording at a minimum 48 kHz sampling rate.
- End-to-end pistonphone calibration immediately before deployment and after retrieval; broadband calibration at least every five years and before fieldwork, with sensitivity flat to less than ±3 dB across 20 Hz–20 kHz.
Inspect time series and spectra, check algorithmically and visually for saturation, compare pre- and post-deployment calibration and listen to representative recordings. In high current, use slack water where possible, shorten exposed cable and use an acoustically transparent, gas-permeable flow shield.
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BOEM’s report is organized around environmental conditions, the actual pile and hammer, the sensor deployment and the results. Include dates and times; GPS positions and a map; water depth, temperature, salinity, substrate and tidal variation; vessel operating conditions; pile material, diameter, length and wall thickness; hammer model and energy per blow; penetration history; and every noise-abatement system, including bubble-curtain airflow, current and position relative to the pile and hydrophones.
For sensors, identify the recorder, bandwidth, filters, dynamic range, sampling rate, hydrophone model, sensitivity, directionality, suspension or bottom mount, hydrophone depths and calibration results. Results at close and far ranges include median, mean, maximum and minimum for broadband and relevant-band single-strike SEL, peak SPL, 90%-energy RMS SPL and T90; cumulative SEL for the pile; decidecade levels; power-spectral-density plots; weighted SEL-versus-time; and weighted cumulative SEL-versus-range with a transmission-loss fit. Each record is annotated with pile ID, distance, water depth and hydrophone depth. Raw acoustic files are normally retained and made available to BOEM or NMFS on request rather than appended to every report.
How does the July 2026 NMFS coastal method differ?
The NMFS memo is coastal, not an offshore-wind protocol, but it clarifies US measurement practice. For coastal validation, the near hydrophone is normally 10 m from the pile, or the greater of 10 m and three times water depth, and is placed at half depth. For transmission-loss work, it recommends at least three roughly logarithmic ranges (for example 10, 100 and 1,000 m), a far-field station and more than one direction. If hydrophones are scarce, prioritize distances over multiple depths.
NMFS recommends measuring at least 10% of events for each pile size, type and driving condition. Impact and vibratory operations on one pile count as separate samples. Record the entire event, but remove ramp-up, power-down and mitigation transitions when calculating representative single-strike metrics. Include those periods in cumulative SEL. When a bubble curtain starts or stops, exclude the transition until it is fully active or dissipated; bubbles can continue affecting measurements for about one minute after shutdown.
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Close coastal measurements can reach peak pressures as high as 220 dB re 1 µPa, an equipment dynamic-range expectation rather than an offshore compliance limit. Laboratory calibration should be no more than two years old, with in-situ checks before and after each deployment; drift greater than 0.75 dB triggers hardware replacement under the memo. Vibratory reporting uses 10-second RMS SPL plus event and daily cumulative SEL, while impact reporting includes strike count, peak, RMS, T90, single-strike SEL, cumulative SEL and total hammering duration.
What do Germany and Denmark require?
Germany uses a fixed received-level test. The BSH criterion, quoted in the 2020 itap/BSH experience report, is 160 dB SEL and 190 dB zero-to-peak SPL at 750 m. The quoted sentence does not state reference units beside those numbers, so do not silently add them. German monitoring emphasizes hydrophone height because lower-half water-column levels can be substantially higher than near-surface levels. BSH’s MarinEARS register has recorded impulsive events in the German exclusive economic zone since 2016. The report says technical abatement systems were used for all German percussive piling from 2011 and that reliable compliance was achieved from 2014.
Denmark requires a numerical or semi-empirical prognosis and on-site validation. Its 2022 guideline evaluates frequency-weighted cumulative SEL for a fleeing animal, normally moving at 1.5 m/s, and calculates a distance-to-threshold. For harbour porpoise, the impulsive permanent-threshold-shift criterion is 155 dB re 1 µPa²s (VHF), temporary threshold shift is 140, and the behavioural SPL125ms value is 103 dB re 1 µPa. Other listed species have different weighted criteria. The planned case must show that the permanent-threshold-shift radius does not exceed the justified safe radius.
Denmark generally requires an acoustic deterrent device at least 15 minutes before piling, subject to exceptions. It is not permitted when the piling threshold radius is below 200 m for all relevant hearing groups, and the device’s own behavioural range must be smaller than the piling range. These rules illustrate why mitigation state, weighting and animal movement must accompany every reported number.
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Bubble curtains, near-pile dampers and source-control measures change the measured field and must be treated as explicit conditions. Analyze fully active, fully inactive and transition periods separately; state hammer energy, pile identity and range for each. A Coastal Virginia Offshore Wind monitoring abstract reported that a bubble curtain reduced pile-driving noise above 200 Hz and that reduction varied by azimuth, demonstrating why a before/after spectrum and direction are more informative than one broadband average.
German experience identifies hammer control, blow rate, foundation design, seabed, depth, bathymetry and barrier performance as interacting factors. Big bubble curtains may be deployed at least 60 m from a pile; deeper water and large monopiles can require combined far-field and near-pile systems. Current can displace a curtain and reduce performance in the current direction, so current speed and curtain geometry belong in the report.
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What are the most common reporting mistakes?
- Publishing one unqualified dB value. State metric, weighting, reference, bandwidth, integration window, range and statistic.
- Mixing coastal and offshore layouts. NMFS’s 10 m coastal example is not BOEM’s approximately 750 m offshore close range.
- Calling a received level a source level. Source level is a modeled point-source equivalent; propagation loss and range must be shown.
- Hiding mitigation state. “Bubble curtain on” and “off” results are separate conditions, not interchangeable samples.
- Confusing exposure with legal take. BOEM distinguishes modeled threshold exposure from ESA or MMPA take determinations.
- Ignoring particle motion. Pressure-only data do not answer every fish or invertebrate question.
- Treating old thresholds as current. BOEM’s marine-mammal table is attributed to NMFS 2018; confirm later NMFS guidance before regulatory use.
What newer monitoring methods can add?
At Vineyard Wind in 2023, researchers combined conventional hydrophones with distributed acoustic sensing on a buried fibre-optic cable. The published abstract reports that buried-cable calibration is difficult; hydrophones, propagation modeling and a transfer function were used to estimate in-water levels. Distributed acoustic sensing may extend spatial coverage, but it is not a substitute for the calibrated hydrophone specified by the standards above.
Japanese professional guidance proposed long-term fixed recording before construction, measurements in three directions out to 6 km and a “safe distance” where background masks pile sound. The proposal remains guidance rather than a universal permit condition. In the United States, a 2025 conference abstract describes proposed received-level limits and long-term passive acoustic monitoring before, during and after construction, but the retrieved abstract does not provide numeric limits.
How can a reader tell whether two reports are comparable?
Put the reports on one axis only when they use the same metric, weighting, bandwidth, pulse-duration rule, hydrophone depth, range, hammer energy and mitigation state. A German 160 dB SEL criterion at 750 m, a Danish weighted cumulative SEL for a fleeing porpoise and a US unweighted peak SPL are different tests even if each is printed in decibels. A transparent report makes those differences visible instead of implying a league table of “strictest” rules.
Frequently Asked Questions
Why can’t a normal sound-level meter be used underwater?
Air microphones and dB(A) weighting are designed for atmospheric pressure and hearing, not water pressure. Pile-driving work requires a calibrated hydrophone, underwater reference units, suitable low-frequency response, and enough dynamic range to avoid clipping.
Does a higher SEL always mean a louder individual pile strike?
No. SEL integrates energy over time, so a longer or more numerous sequence can have a higher cumulative value even when its individual strikes have lower peaks. Compare single-strike and cumulative metrics separately.
Why are median and maximum both reported?
Pile strikes vary with hammer energy, penetration, seabed and changing water conditions. The median describes a typical recorded pulse, while the maximum reveals the worst observed sample; neither replaces the underlying distribution.
Can hydrophone data prove that marine mammals were harmed?
Hydrophone data establish received pressure levels and modeled exposure, not injury by themselves. Species presence, hearing weighting, movement, biological response and the applicable authorization determine how regulators interpret that exposure.
What should be retained if raw recordings are not published?
Keep lossless acoustic files, calibration signals, deployment logs, GPS and depth records, instrument settings, quality-control flags and mitigation timelines. BOEM expects raw files to be available to regulators on request even when they are not attached to the public report.
