Measurement of sound in the ocean
Dealing with uncertainty in the Marine Environment – assessing impact
of renewable developments
Friday 12 February 2016
Stephen Robinson, National Physical Laboratory
© Crown Copyright (NPL, 2016)
Presentation scope
 Units and quantities used for acoustics
 General measurement considerations
• instrumentation, performance, deployment…
 Uncertainty assessment
• uncertainty in measurement and prediction
 In-situ measurement in the ocean
• Ambient noise
• Radiated noise
 Specification Standards
• Work of ISO TC43 SC3, IEC TC87 and IEC TC114
• ISO DIS 18406 – marine pile driving noise
• IEC TC114
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Measures of sound
Measures of sound:
- peak pressure:
ppeak ≡ max p(t )
Peak to peak
pressure
- sound exposure:
T
E ≡ ∫ p 2 (t )dt
0
- Root mean square (RMS)
pressure:
pRMS
1
≡
T
T
∫
0
p 2 (t )dt =
E
T
all may be expressed in decibels
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Peak
Pressure ppeak
Decibels
 The decibel (dB) is a logarithmic unit of ratio
a difference in level of 10 dB implies a factor 10 in power or
energy
a difference in level of 20 dB implies a factor 10 in amplitude
 An absolute level expressed in dB must be
accompanied by
description of the physical quantity represented (eg RMS
pressure)
reference value of that quantity (eg 1 µPa)
any frequency weighting (eg A-weighting)
 A dimensionless ratio needs no reference unit
eg “Amplifier gain was 20 dB”
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Sound Exposure Level as a metric
.
•
Used in airborne acoustics
•
SEL is particularly useful for pulses as it
considers the energy in the signal
E90
=
t95
2
p
∫ (t ) dt
t5
 E90 
SEL = 10 log 

E
 0
where E0 is the reference value of 1 μPa2·s
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General measurement considerations
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Measurements of underwater noise
 Ambient noise
Long-term monitoring
Sound originates from many
directions
Levels may be low (but may have high
level events)
 Radiated noise from specific
sources
Sound arrives from specific direction
May be high amplitude
Typically shorter term deployment
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Drivers/motivation for measurements
• Regulation is a major driver
– Measurements often required for licensing requirements
– EU directives and national legislation
• Scientific research
– Eg: Characterisation of “soundscapes”
Example: Directive 2008/56/EC of the European Parliament
– Pollution*: “… the introduction of substances or energy, including
human-induced underwater noise, which results or is likely to result in
deleterious effects”
European Marine Strategy
Framework Directive
Good environmental status
Working Group 3, Descriptor 11:
“Introduction of energy,
including underwater noise,
is at levels that do not
adversely affect the marine
environment.”
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Technical Guidance
Good Practice Guide 133:
Underwater Noise measurement
Funded by:
Marine Scotland
The Crown Estate
Dept. Business Innovation and Skills (BIS)
(National Measurement Office)
Scope:
Definitions & Metrics
Instrumentation
Deployment
Ambient Noise
Radiated Noise
Propagation
Uncertainties
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EU MSFD TSG Noise
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MSFD Descriptor no. 11 covers underwater
noise (classed as pollution)
Two indicators:
11.1 LF impulsive noise
11.2 LF CW noise from shipping
Expert committee set up by Commission – TSG
Noise
Provides guidance to member states on
implementation of MSFD
Monitor both background levels and close to
traffic to determine levels of individual ships
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Reports published by Commission
Part I: Executive summary
Part II: Monitoring guidance specifications
Part III: Background Information and Annexes
http://publications.jrc.ec.europa.eu/repository/
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Choice of hydrophone / acquisition system
 Sensitivity
hydrophone and amplifiers
appropriate to signal level
avoid poor signal-to-noise
avoid saturation, nonlinearity and
clipping…
 Frequency response
high enough to faithfully record all
frequency components
Flat response desirable resonances will distort signals
 Trade off
High frequency hydrophones are
small and insensitive
• Dynamic range
– linearity of hydrophone and
system
– resolution of ADC (no. bits)
– multiple frequency bands or
filters sometimes used
• Noise floor
– limits lowest measured signal
– often ignored
• Directivity
– Omnidirectional desirable
– Care with reflections from
supports, recorder cases, etc
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Dynamic range
.m)
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Calibration and QA
 Hydrophones and acquisition system must be calibrated
laboratory calibration before and after sea-trial recommended
calibration over full frequency range is required
 In-situ calibration desirable
electrical calibration of system (insert voltage technique)
hydrophone calibrator (pistonphone) very useful
 In situ QA procedures
Best to check signals during the measurements
Visual display of measured data in real time is good
Playback of recorded signals desirable before end of trial
Listen to signals to check data is valid
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Data handling and storage
 Trade off between frequency bandwidth, dynamic range
(ADC resolution), record lengths and storage
requirements
high bandwidths produce large file sizes
high resolution produces large file sizes
 Store in lossless format
do not use data compression (loss of data quality)
 Data sampling required for long-duration
appropriate duty cycle needed (on – off schedule)
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Autonomous noise recorders
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Becoming more common
Commercially available
Good value, convenient…
Care needed when specifying key
performance metrics of commercial units
Self noise
Sensitivity (hydrophone/amplifier)
Frequency response
Directivity
2 kHz
18 kHz
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Summary: Measuring Instrumentation
Ensure measuring system performance is fit for purpose. Key performance parameters
include:
Sensitivity (3.1.1)
frequency response (3.1.2)
directivity (3.1.3)
system self-noise (3.1.4)
dynamic range(3.1.5).
The performance of any commercial off-the-shelf systems should be validated (3.1.6).
The measuring system should be calibrated over the full frequency range of interest (3.2.1).
Ensure appropriate quality assurance procedures are applied to the measurement (3.2.2 &
3.2.3).
Data storage should ideally be lossless format and include all necessary metadata and
calibration information (3.3).
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Instrument self-noise
 Need to minimise self noise
Noise due to sensors and instruments
Noise of platform/deployment
 Consider instrument self noise only :
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Self-noise & platform noise:
potential sources
 Surface wave motion (heave)
 Flow noise
 Strumming on cable to surface (hydrophone or support cable)
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Debris and/or sediment impacting the hydrophone
Deployment equipment or cables rubbing against each other
Biological abrasion noise
Loose mechanics in deployment system rattling
 Electrical noise in preamplifier
 Non-linearity or instability in the preamplifier
 Acoustic noise from deployment vessel
Engines, generators
 Wave slap on vessel hull
 Electrical interference from own ship (generators)
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 Electrical cross-talk in multi-core
cables
Noise from surface wave action
 Surface motion causes hydrostatic
pressure changes due to hydrophone
motion
 Low frequency but high amplitude
pressure = ρ.g.h
every 10 m depth is 1 atmosphere
1 kPa for a 10 cm (180 dB re 1 µPa)
can saturate ADC
 Need to decouple surface motion from
hydrophone
Use anti-heave suspension
Use high pass electronic filter (before ADC)
 Alternatively, mount hydrophones on
bottom or using sub-surface buoy
•
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Anti-heave suspension
–
–
anti-heave float from vessel
High pass filtering
Bottom mounted hydrophones
 Decoupled from surface
 Flow noise lower
 Good for long term deployment
Sub surface buoy with hydrophones
and recording pod
Bottom mounted frame/cage
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Flow noise
 Noise due to flow of medium
 Low frequency problem
Solutions
• Sonar-domes
 Produced in turbulent layer
around hydrophone
vortex shedding
 Not true acoustic signal
f v = 0.2U / d
•
U = velocity
•
D = cross section diameter
– like microphone “windshield”
•
•
•
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Spiral wrap around cables
Measure close to sea-bed
Measure at slack tide
Use drifting buoys
– drift with currents
– not long-term deployments
– not fixed position
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Drifting systems
From: Wilson B., Lepper P.A., Carter C., Robinson S.P.
“Rethinking underwater sound recording methods to work in
tidal-stream and wave energy sites”. In Marine Renewable
Energy and Environmental Interactions, Humanity and the Sea,
M. A. Shields, A. I. L. Payne (eds.), DOI 10.1007/978-94-0178002-5_9, p111-126, © Springer Science+Business Media,
Dordrecht, 2014.
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Other considerations
Cable strum
Mechanical noise
problem in high flow (tides)
decouple if possible
measure at slack tide
bottom mounted system
– avoid metal on metal
– avoid chains in mountings
– long-term deployments may
need servicing at intervals to
remove fouling
Vessel platform noise
Measure under quiet conditions
Engines OFF
Generator off (battery operation for
acquisition system)
Echosounder OFF
Wave slap
– Worse with certain hull types
– Stable vessel required
– Sea-bed mounting avoids
problem
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Summary: Deployments
Ensure deployment configuration is appropriate for measurement requirements with
hydrophones deployed at appropriate depths (4.1 & 4.2).
Ensure deployment related parasitic signals are minimised, including those originating from:
Flow noise (4.3.1)
Cable strum (4.3.2)
Surface heave (4.3.3)
Vessel/platform noise (4.3.4)
Mechanical noise (4.3.5)
Electrical noise (4.3.6)
Record all auxiliary data and metadata (4.4).
Ensure steps are taken to protect recorders and data from loss (4.5).
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More Technical Guidance
Speed of sound in sea water
Web-based interactive calculators
Sound absorption in sea water
Web-based interactive calculators
Other web-based guidance:
Hydrophone mounting, wetting, loading
corrections, uncertainties…
Good Practice Guide 133:
Underwater Noise measurement
Training courses in hydrophone and
transducer calibration
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Uncertainty assessment
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“Error” versus “uncertainty”
 “Error” is typically defined as:
error = measured value – true value
 implies a knowledge of the true value (unknowable)
 “Uncertainty” is an attempt to quantify measurement
accuracy without knowledge of the true value
 An uncertainty provides bounds around the
measured value within which it is believed that the
true value lies, with a specified level of confidence
• Only possible to state the probability that the value
lies within a given interval
• Probabilistic basis for uncertainty evaluation
 ISO Guidance document (ISO GUM)
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Where do uncertainties come from?
 The measuring instrument
Calibration, resolution, drift, ageing, …
 The item being measured
Stability, homogeneity, …
 Operator skill
Procedures and protocols
 The environment
Temperature, pressure …
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Why are uncertainties important?
 To quantify the quality of a measured value
 To compare different measured values
e.g. from different measuring systems
 To compare a measured value with theory
 To compare a measured value with a tolerance
e.g. in conformity assessment
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Uncertainty categories
Precision/repeatability
Bias/systematic accuracy
 measure of the variation in
repeated measurements
 measure of the systematic bias
or offset in the measurement
 Dispersion of values is a measure
of the quality of the measurement
 cannot be assessed by
repeating the measurement
 may be assessed using statistical
techniques
 separate assessment needed
variance or standard deviation
 causes may be uncontrolled
environmental factors, noise,
human factors ….
 causes may be instrument or
sensor calibration, interference,
bad assumptions…
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Uncertainties in acoustic measurement
 Hydrophone calibration
best ~4% (~ 0.4 dB)
typical system ~10% (~ 1 dB)
 Ocean acoustic measurement
typical ~15% (~ 1.6 dB)
 Source level of vessel
typical ~35% (~ 3 dB)
 Ranges and impact zones
Not well established but can
be large !
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Uncertainties in source level estimation
 Uncertainties in propagation model parameters add to uncertainties in
acoustic measurement
Seabed properties, bathymetry, sea surface conditions (roughness), sound
speed profile (temperature, salinity, depth)…
Ainslie et al. (2012) What is the Source Level of Pile Driving Noise in Water?,
Advances in Experimental Medicine and Biology, 1, Volume 730, The Effects
of Noise on Aquatic Life, Part VII, Pages 445-448
Transmission Loss and uncertainty (standard deviation) for
source level estimation of vessel source level measured at
100 m. Uncertainty estimated using Monte Carlo method.
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Uncertainties in mapping and impact zones
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Sound/noise maps often used to show sound field in region
Impact often shown by “drawing lines on the ocean”
These should really be “fuzzy bands”
Contributions from source and propagation effects
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More useful links…
 NPL uncertainty guides
www.npl.co.uk/publications/uncertainty-guide/
 Mathematics and Modelling for Metrology publications
www.npl.co.uk/mathematics-scientific-computing/software-support-formetrology/publications/
 Mathematics and Modelling for Metrology software
www.npl.co.uk/mathematics-scientific-computing/software-support-formetrology/software-downloads-(ssfm)
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Useful links
 JCGM: Joint Committee for Guides in Metrology
www.bipm.org/en/committees/jc/jcgm/
 GUM
www.bipm.org/en/publications/guides/gum.html
 Supplements to GUM
www.bipm.org/en/publications/guides/gum.html
 Bibliography on measurement uncertainty
www.bipm.org/en/committees/jc/jcgm/wg1_bibliography.html
 VIM: International Vocabulary of Metrology
www.bipm.org/en/publications/guides/vim.html
 UKAS document M3003
www.ukas.com/library/Technical-Information/Pubs-Technical-Articles/PubsList/M3003.pdf
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Summary: Uncertainties
All measurements require an estimate of uncertainty in order to be useful.
Uncertainties may be categorised into two classes (8.1):
Type A: a measure of the repeatability of the measurement (derived from the statistical
dispersion of repeated measurements);
Type B: a measure of uncertainty due to any the systematic bias.
There are a number of potential sources of uncertainty (8.2):
calibration of instrumentation;
position of source and receiver;
spurious signals introduced by the deployment
validity of any assumptions made;
environmental parameters (for use in a propagation model).
Uncertainties may be evaluated by following the international Guide to the Expression of
Uncertainty in Measurement (8.3)
Measurement uncertainties are an attempt to quantify how accurately we are able to
measure a quantity, and should not be confused with natural fluctuations and variability in
a quantity (for example, the potentially large variations in ambient noise due to changes in
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weather, etc).
Ambient noise
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Ambient noise (1)
 Often defined as all acoustic signals (other than a
specific local source under study), not including self
noise or platform noise
“classic” definition includes only “distant” shipping
 Sometimes measured for its own sake
all sources are then of interest
 Also measured to determine background levels
Eg when measuring a specific source
Done to determine “signal-to-noise ratio”
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Ambient noise (2)
 Depends on natural sources and man-made sources
Surface action (waves), rain, biological sounds …
Shipping and other anthropogenic sounds …
 Ambient noise varies temporally and spatially
Variation with seasons, weather, diurnally, with traffic
 Ambient noise measurement involves sampling
Short “snapshot” measurements cannot fully represent noise
Long-term recordings desirable (not often done)
Sampling regime must be chosen
Averaging must be undertaken
Statistical variation must be indicated with results
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Noise spectral units (1)
Noise spectral
density (in thirdoctaves)
“CPB”
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Noise spectral units (2)
Third-octave band
power
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Example of ambient noise configuration
ANEMOMETER
1.5 d
HYDROPHONE
1.5 d
d
CANISTER
ANCHOR
CLUMP
2d
Diagram from Cato, 2008
CLUMP
2d
CANISTER
HYDROPHONE
Auxiliary measurements during ambient noise
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Sea-state
Wind speed and associated measurement height
Rainfall and other precipitation, including snow
Water depth and tidal variations in water depth
Water temperature and air temperature
Hydrophone depth in the water column
GPS locations of hydrophones and recording systems
Sea-bed type
Current flow and associated measurement depth
Presence of shipping traffic and distance from hydrophone
Occasional events like lightning or passing aircraft
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Analysing and expressing ambient noise
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Summary: Ambient noise measurement
Ensure that the objectives of the measurements are clear and that the monitoring and
deployment configuration is appropriate for those objectives (5.1 & 5.3).
Ensure that the temporal sampling regime is appropriate for the objectives, and that the
duration and duty cycle are appropriately chosen (5.3).
Ensure that the spatial sampling regime is appropriate for the objectives, and that the
locations of monitoring stations are appropriately chosen (5.3).
Ensure that the instrumentation is correctly specified for the application (for example, in
terms of frequency range, dynamic range and self-noise) (5.3.2).
Ensure the deployment minimises measurement artefacts and parasitic signals (5.3.3).
Document and justify choice of data analysis methodology in terms of:
Metrics – arithmetic mean and median are recommended (5.4.3);
Averaging procedure – choice of snapshot time (5.4.4);
Statistical representation of data – representing dispersion of data by use of analysis such
as box-plots, and cumulative distributions (5.4.5).
Record all relevant auxiliary data and metadata including data which may correlate with
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acoustic data (ship traffic data, weather data, etc) (5.3.3)
Radiated noise
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Radiated noise
 Required to characterise a source in terms of:
Frequency (spectrum …)
Time variation (duration, pulse length, waveform…)
Amplitude (pressure, power, energy …)
 Source Level provides measure of source output
amplitude which characterises the source and not the
environment
may be function of frequency (source spectral density)
 Source Level may be used with propagation model to
“predict” the received level around source in any location
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Basic concepts:
Source Level
 Source level (SL) is a measure of
the source output amplitude
 SL is a “reduced” quantity –
obtained from measurements in
far-field projected back to 1 m
away from “acoustic centre”
 SL is a far-field parameter
 Related to radiated acoustic
power and directivity
 Units: “dB re 1 µPa at 1 m”
… or dB re 1 µPa.m
… or dB re 1 µPa².m²
Acoustic near-field/far-field
•
Near-field is region close to the
source where waves originating from
different parts of the source interfere
•
•
•
Pressure variation highly complex
Level;
Region within D2/λ of the source
(D is largest source dimension)
•
In far-field, waves from all parts of
the source are substantially in phase
 The SL value is NOT the actual
far-field, waves appear to spread
value of the pressure level 1© Crown
m Copyright• (NPL,In2016)
spherically from “acoustic centre”
from source
Acoustic near-field
Near-field of “piston” source transducer
Spherically spreading
field (pressure falls
inversely with range)
Near-field pressure variation
for seven element piston array
Actual pressure variation
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On axis response of circular plane piston of ka = 25. (After Kinsler & Frey, 1981)
Source Level determination
Source Level = Received Level + Transmission Loss
 Measure received level at range from source
 Transmission loss
Needed for estimation of source level and “outward” prediction of
acoustic field versus range (impact zones, etc)
 Choice of propagation model
Ideally need broadband model accounting for frequency
dependence, bathymetry, sea-bed transmission, etc
Time domain or frequency model
Simple spreading/absorption models often used
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Transmission Loss (Propagation Loss)
 Loss due to several mechanisms
Spreading
Absorption
Interaction with boundaries
Scattering
 In a free field (deep water):
spherical spreading: 20.log(R) (plus absorption)
variation in sound speed with depth complicates propagation
 In shallow water
propagation is compleicated (reverberant environment)
losses at surface and seabed
 Sophisticated models
raytrace, normal mode, parabolic equation, wavenumber
integration
standard models available – require expertise to use
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Sound propagation in shallow
water
Sound amplitude dies away at greater range because of Transmission
Loss due to:
• Spreading
• Absorption (frequency dependent)
• Interaction with boundaries (seafloor, seabed)
RL = SL – TL
“practical spreading laws”: RL = SL - 15 log(R) are sometimes used,
but cannot use close to source to derive source level (only between
two point in the distant far-field); can possibly use “mode stripping”
formula:
RL = SL - 15 log R + 5 log (η H/ π)
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Low frequency cut off
Radiated noise measurement:
shallow water configurations
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Radiated noise measurement:
deep water configurations
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Summary: Radiated noise measurement
Ensure that the objectives of the measurements are clear and that the measurement
configuration is appropriate for those objectives (6.1 & 6.2).
Ensure that the source output metrics are appropriate for the objectives, and that the
measurement configuration enables the chosen metrics to be derived (6.2).
If a source level is calculated, ensure that an appropriate propagation model is used which
accounts for the relevant physical propagation phenomena (6.2.3).
Ensure that the measurements satisfy the requirements of the objectives such that:
the instrumentation is correctly specified for the application in terms of frequency range,
dynamic range and self-noise (6.3.6, 6.3.7);
spatial sampling is appropriate to ensure far-field conditions (6.1.4) and (if required) to
provide an empirical check on propagation (6.3.2);
the temporal sampling captures any variation in acoustic output using a fixed (static)
recording position (6.3.1);
the deployment minimises measurement artefacts and parasitic signals (6.3.7);
contaminating noise sources are minimised (or eliminated) (6.3.8).
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Acoustic propagation
 Needed for:
Prediction, noise maps, derivation of source level…
 Factors affecting propagation:
•
The geometrical spreading of the sound away from the source;
•
•
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Absorption of the sound by the sea-water and the sea-bed;
The interaction with the sea-surface (reflection and scattering);
The interaction with (and transmission through) the sea-bed;
The refraction of the sound due to the sound speed gradient;
The bathymetry (water depth) between source and receiver positions;
Source and receiver depth
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Available models
 Ray tracing models
 Normal mode models
 Parabolic equation model
 Wavenumber integration models
 Energy flux models
 Semi-empirical models
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Acoustic propagation uncertainties
 Input parameters can be uncertain
Seabed, bathymetry, sea surface state…
 Model may not account for all physical phenomena
Range-dependence, 3-D propagation effects…
 Benchmarking provides some validation
Comparison with other models, or with experiment
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Summary: Propagation modelling
Ensure that the choice of model is appropriate for the application (7.3).
Ensure that the propagation model used accounts for the physical propagation
phenomena relevant to the scenario, including the following potential influencing
factors (7.4):
range-dependent bathymetry including dependence on varying water depth and the
frequency cut-off for the channel;
sound speed including the sound speed profile (especially for deeper water);
frequency dependence, including absorption in the water;
seabed properties, including propagation within the seabed;
interaction with the sea surface, including the effect of surface roughness.
Preferably, use a model that has been benchmarked against historical experimental
data or by comparison with other propagation models, or check consistency with
range-dependent measured data from current experimental work (for example, when
measuring radiated noise).
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International specification standards
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International Standards
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There are different levels in standardization: National, regional (e.g.
European), and International
The ISO level allows worldwide acknowledgement and are widespread -over
20,500 published International Standards
For more information: http://www.iso.org/iso/home/about.htm

Other international organizations are:
IEC (International ElectrotechnicalCommission)
ITU (International Telecommunication Union)
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Standards are prepared by committees formed with experts from different
countries particularly interested in the topic
ISO committee working on the topic
ISO/TC 43/SC 3 – Acoustics - Underwater Acoustics
(Also ISO/TC 8/SC 2 - Marine Technology – Protection of Environment)
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Note that IEC TC87 deals with calibration of electroacoustic transducers
(hydrophones and sonars)
Note that IEC TC114 deals with operational noise from marine renewables
(wave and tidal)
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ISO/TC 43/SC 3 working groups

Currently active working groups
WG1 – Measurement of underwater noise radiated from ships
Convenor: Mike Bahtiarian (USA)
WG2: Underwater acoustical terminology
Convenor: Michael Ainslie (Netherlands)
WG3: Underwater radiated sound from marine pile driving
Convenor: Stephen Robinson (UK)
WG4: Standard-target method of calibrating active sonars for
backscattering measurement
Convenor: Kenneth Foote (USA)
WG1: Joint working group with TC 8-2 on underwater noise radiated
from ships.
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Convenor: Koichi Yoshida (Japan)
ISO/TC 43/SC 3 – Current work plan
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Documents in final stage:
FDIS 17208-1 (WG1) - Quantities and procedures for description and measurement of
underwater sound from ships — Part 1: Requirements for precision measurements in
deep water used for comparison purposes
DIS 18405.2 (WG2) – Terminology
DIS 18406 (WG3) - Measurement of underwater radiated sound from percussive pile
driving
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CD 17208-2 (WG1) - Quantities and procedures for description and
measurement of underwater noise from ships — Part 2: Determination of
source levels (Project leader: Wenwei Wu, China)
AWI 20073 (WG4) - Standard-target method of calibrating active sonars for
imaging and measuring scattering
Preliminary work items:
Measurement of ambient sound
Measurement of sound pressure
Measurement of sound from offshore petroleum operations
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Measurement of underwater radiated sound from
percussive pile driving (ISO 18406)
66
Measurement of underwater radiated sound
from percussive pile driving
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Pile driving is a process frequently used for the installation of offshore
platforms or structures at sea
The rapid development of offshore wind farms at sea in many maritime areas
requires frequent pile driving operations, in relatively shallow waters. The
diameter of these piles can be large (several meters).
Underwater emissions by pile driving consists in series of high energy sound
pressure pulses, which can be harmful for marine life (e.g. exceed the limits
for auditory thresholds shifts)
Some countries have already enforced regulations:
Germany defined a regulation for limit values of peak levels and SEL for pile driving
underwater noise. The limit values are stringent, and as a consequence, industry
has developed devices for noise mitigation such as bubble curtains
Netherlands imposes the installation of wind farms only once a year in order to limit
the environmental impact
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Therefore, there is an urgent need for an international standard on the matter
67
Key features of ISO 18406 (1)
 For offshore measurements:
at least one stationary measurement location which measures the entire piling
sequence (if only one range used, it shall be 750 m from the pile)
additional measurement locations recommended -along specified transects
hydrophone depth: > 2 m above the seabed and >half the water depth
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Minimum measurement distance is three times the water depth
Measurement bandwidth must cover the frequency range 20 Hz to 20 kHz
Recommendations are provided for choice of hydrophone, instrumentation and
deployment configuration, including minimum requirements
Hydrophone/system must be calibrated over the frequency range
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Key features of ISO 18406 (2)
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Requirements and guidance for calculation of
acoustic metrics provided
Requirements for reporting of acoustic metrics
and auxiliary data
SEL (individual pulse/strike) –broadband and TOB
spectrum
peak sound pressure and peak sound pressure level
Other metrics: SPL, cumulative sound exposure
level, pulse repetition rate, pulse duration, peak
compressional/rarefactional, background noise
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Annexes provide discussion of source output
metrics and guidance on choice of
instrumentation
Guidance provided where mandatory
requirements are not practical/achievable
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ISO 18405 – Terminology for Underwater Acoustics
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CD stage with successful ballot in 2014, and
DIS prepared in 2015
Second DIS version in progress with
publication expected in 2016
Final version could be published in 2016 or
2017, depending on the need of a FDIS or
not
Contents of the document:
General terms
Levels used in underwater acoustics
Terms for properties of underwater sound
sources
Terms related to propagation and scattering of
underwater sound
Terms for properties of underwater sound
signals
Terms related to sonar equations
Terms related to underwater bioacoustics
70
IEC TC114 Acoustic Characterization of
Marine Energy Converters
 Working Group just started
 Leader: Brian Polagye (US)
 Aim: provide uniform methdologies to consistently characterize the sound
produced by marine and hydrokinetic energy converters, including wave,
current, and ocean thermal energy converters in operation. (20Hz – 20 kHz is
proposed range)
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Temporal characteristics
Spatial characteristics
Ambient noise
Variation in acoustic output with operational mode
UK contributors:
•
Paul Lepper (LU), Michael Butler (EMEC), S Robinson (NPL) + ???
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© Crown Copyright (NPL, 2016)
Air versus water
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Air versus water
 Comparisons with every day sounds can easily lead to
confusion
 “It is as if the whale is strapped to a Saturn 5 rocket …” etc
 For the same energy input into the media:
Different acoustic impedance: ~36 dB difference in SPL
Different reference levels (1µPa versus 20µPa): ~26 dB difference
So SPL values are ~62 dB greater in water for same acoustic power or
energy
 However, natural ambient noise levels in the ocean are
generally much greater than in air
 Marine creatures have evolved in this (noisier) environment and
have evolved appropriate hearing responses
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Air versus water: an analogy
 Who feels richer, a Canadian or an American ?
 Both are paid in dollars, units with the same name, but different
values. So you can correct for that using an exchange rate (analogous
to the use of different reference pressures: 20 µPa or 1 µPa).
 But the cost of living in Canada is higher than the US, so the same
amount of money does not go as a far in Canada as the US, which
again you can correct for using the retail price index (analogous to the
different acoustical impedances).
 Finally with the same “spending power” how rich you feel depends on
how much money people around you have, you need less money in
India to feel rich than in the US. The final factor is a subjective factor
and is probably much harder to correct for and is analogous to
perceived loudness.
Prof Paul White, ISVR
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Title of Presentation
Name of Speaker
Date
The National Measurement System delivers world-class
measurement science & technology through these organisations
The National Measurement System is the UK’s national infrastructure of measurement
Laboratories, which deliver world-class measurement science and technology through four
National Measurement Institutes (NMIs): LGC, NPL the National Physical Laboratory, TUV NEL
The former National Engineering Laboratory, and the National Measurement Office (NMO).
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