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Underwater Acoustics for Biologists and Conservation ManagersA comprehensive tutorial designed for environmental professionals

InstructorsDr. William T. Ellison is president of Marine Acoustics,

Inc., Middletown, RI. Dr. Ellison has over45 years of field and laboratory experiencein underwater acoustics spanning sonardesign, ASW tactics, software models andbiological field studies. He is a graduate ofthe Naval Academy and holds the degreesof MSME and Ph.D. from MIT. He has

published numerous papers in the field of acoustics and isa co-author of the 2007 monograph Marine MammalNoise Exposure Criteria: Initial ScientificRecommendations, as well as a member of the ASATechnical Working Group on the impact of noise on Fishand Turtles. He is a Fellow of the Acoustical Society ofAmerica and a Fellow of the Explorers Club.Dr. Orest Diachok is a Marine Biophysicist at the JohnsHopkins University, Applied Physics Laboratory. Dr.Diachok has over 40 years experience in acoustical

oceanography, and has publishednumerous scientific papers. His career hasincluded tours with the NavalOceanographic Office, Naval ResearchLaboratory and NATO Undersea ResearchCentre, where he served as ChiefScientist. During the past 16 years his work

has focused on estimation of biological parameters fromacoustic measurements in the ocean. During this periodhe also wrote the required Environmental Assessments forhis experiments. Dr. Diachok is a Fellow of the AcousticalSociety of America.

What You Will Learn• What are the key characteristics of man-made

sound sources and usage of correct metrics.• How to evaluate the resultant sound field from

impulsive, coherent and continuous sources.• How are system characteristics measured and

calibrated.• What animal characteristics are important for

assessing both impact and requirements formonitoring/and mitigation.

• Capabilities of passive and active monitoring andmitigation systems.

From this course you will obtain the knowledge toperform basic assessments of the impact ofanthropogenic sources on marine life in specific oceanenvironments, and to understand the uncertainties inyour assessments.

SummaryThis three-day course is designed for biologists, and

conservation managers, who wish to enhance theirunderstanding of the underlying principles ofunderwater and engineering acoustics needed toevaluate the impact of anthropogenic noise on marinelife. This course provides a framework for makingobjective assessments of the impact of various types ofsound sources. Critical topics are introduced throughclear and readily understandable heuristic models andgraphics.

Course Outline1. Introduction. Review of the ocean

anthropogenic noise issue (public opinion, legalfindings and regulatory approach), current stateof knowledge, and key references summarizingscientific findings to date.

2. Acoustics of the Ocean Environment.Sound Propagation, Ambient NoiseCharacteristics.

3. Characteristics of Anthropogenic SoundSources. Impulsive (airguns, pile drivers,explosives), Coherent (sonars, acoustic modems,depth sounder. profilers), Continuous (shipping,offshore industrial activities).

4. Overview of Issues Related to Impact ofSound on Marine Wildlife. Marine Wildlife ofInterest (mammals, turtles and fish), BehavioralDisturbance and Potential for Injury, AcousticMasking, Biological Significance, and CumulativeEffects. Seasonal Distribution and BehavioralDatabases for Marine Wildlife.

5. Assessment of the Impact ofAnthropogenic Sound. Source characteristics(spectrum, level, movement, duty cycle),Propagation characteristics (site specificcharacter of water column and bathymetrymeasurements and database), Ambient Noise,Determining sound as received by the wildlife,absolute level and signal to noise, multipathpropagation and spectral spread. Appropriatemetrics and how to model, measure andevaluate. Issues for laboratory studies.

6. Bioacoustics of Marine Wildlife. HearingThreshold, TTS and PTS, Vocalizations andMasking, Target Strength, Volume Scattering andClutter.

7. Monitoring and Mitigation Requirements.Passive Devices (fixed and towed systems),Active Devices, Matching Device Capabilities toEnvironmental Requirements (examples ofpassive and active localization, long termmonitoring, fish exposure testing).

8. Outstanding Research Issues in MarineAcoustics.

June 15-17, 2010Silver Spring, Maryland

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Introduction• Student Introduction• Identify key Interests of Students• Course Objectives

– Introduction to Marine Mammals from an Acoustic Viewpoint• their sounds & hearing and • how they are affected by and respond to anthropogenic sounds

– Methods and Tools for Bioacoustic Issues• Metrics• Examples of past/present research (may do last!)

– Bowhead Whales in the Arctic (1980’s)– SOCAL SRP Tagged Fin Whale (1990’s)– Stellwagen Bank NOPP (Today)

– Tools and Concepts for Evaluating Impacts on the Marine Environment

• Life Cycle Approach to Environmental Compliance (EC)• The Utility of Modeling as an EC Tool• Assessment Techniques

W

Key Reference Material• Southall, et al. 2007, Marine Mammal Noise Exposure Criteria: Initial

Scientific Recommendations• Richardson, et al.1995, Marine Mammals and Noise• Urick, (any ed.) Principles of Underwater Sound for Engineers• Harris (ASA Reprint) Handbook of Acoustical Measurements and

Noise Control• Crocker (ASA Pub), Encyclopedia of Acoustics• Kryter (any ed.) The Effects of Noise on Man• Bregman, Acoustic Scene Analysis, MIT Press• ANSI STD’s

– ANSI S12.7 – Methods for measurement of impulse noise– ANSI S1.1 – Acoustical Terminology– ANSI S1.42 – Acoustic Weighting Networks

• NRC Reports– 2000 Marine Mammals and Low Frequency sound– 2003 Ocean Noise and Marine Mammals– 2005 Marine Mammal Populations and Ocean Noise: Determining when

Noise causes Biologically Significant Effects

Part I - Introduction to Marine Mammals from an Acoustic Viewpoint*

*Primary Reference is Southall, et al. 2007*Primary Reference is Southall, et al. 2007

Mystery Sound

Whale Sounds&

Videos{Separate Media}

Marine Mammal Hearingo One of the major accomplishments in [Southall, 2007] was the derivation of recommended frequency-weighting functions for use in assessing the effects of relatively intense sounds on hearing in some marine mammal groups. It is abundantly clear from:

o measurements of hearing in the laboratory, o sound output characteristics made in the field and in the laboratory, and o auditory morphology

o that there are major differences in auditory capabilities across marine mammal species (e.g., Wartzok & Ketten, 1999).

o Most previous assessments of acoustic effects failed to account for differences in functional hearing bandwidth among marine mammal groups and did not recognize that the ‘nominal’ audiogram might be a relatively poor predictor of how the auditory system responds to relatively strong exposures.

Marine Mammal Hearing• [Southall, 2007] delineated five groups of functional

hearing in marine mammals and developed a generalized frequency-weighting (called “M-weighting”) function for each.

• The five groups and the associated designators are: – (1) mysticetes (baleen whales), designated as “low-

frequency” cetaceans (Mlf); – (2) some odontocetes (toothed whales) designated as “mid-

frequency” cetaceans (Mmf); – (3) odontocetes specialized for using high frequencies, i.e.,

porpoises, river dolphins, Kogia, and the genus Cephalorhynchus (Mhf);

– (4) pinnipeds, (seals, sea lions and walruses) listening in water (Mpw); and

– (5) pinnipeds listening in air (Mpa).

Frequency Weighting“In assessing the effects of noise on humans, either an A- or C-weighted curve is applied to correct the sound level

measurement for the frequency-dependent hearing function of humans. Early on, the panel recognized that similar, frequency-weighted hearing curves were needed for marine mammals; otherwise, extremely low- and high-frequency sound sources that are detected poorly, if at all, might be subject to unrealistic criteria.” Southall et al. (2007).

Figure 3.1a below illustrates the A-, B- and C-weighting curves for human hearing (Harris, 1998, Figure 5.17).

Weighting Curves for Human Hearing

Metrics. C-Filter is used as

Functional Basis for the M-Weighting Filter for Marine

Mammals

Weighting Curves for Human Hearing

Metrics. C-Filter is used as

Functional Basis for the M-Weighting Filter for Marine

Mammals

M-Weighting

For Marine Mammal Hearing Metrics: same mathematical structure

as the C-weighting used in human hearing,

For Marine Mammal Hearing Metrics: same mathematical structure

as the C-weighting used in human hearing,

Odontocetes

Mysticetes

Southall, 2007 - For injury assessment, behavior not addressed. Issue!

Southall, 2007 - For injury assessment, behavior not addressed. Issue!

M-WeightingThe M-weighting Southall, 2007 developed for the five functional marine

mammal hearing groups has the same mathematical structure as the C-weighting used

in human hearing, which reflects the fact that sounds must be more intense at high and

low frequencies for them to be perceived by a listener as equally loud. This weighting is

most appropriate determining the effects of intense sounds, i.e., those with equal

loudness to a tone 100 dB above threshold at 1000 Hz. The M-weighting was designed

to do much the same for the different marine mammal groups with the only difference

being the low- and high-frequency cutoffs. The M-weighting for marine mammals, like

the C-weighting used in humans, rolls off at a rate of 12-dB per octave.

The general expression for M-weighting [M(f)], using estimated frequency cut-

offs for each functional marine mammal hearing group, is given as:

})(max{)(log20)( 10 fRfRfM (7) eq.

))(()( 2222

22

lowhigh

high

ffffff

fR

(8) eq.

The estimated lower and upper “functional” hearing limits are designated (flow and

fhigh) for each of the five functional marine mammal hearing groups

M-Weighting (Application)The application of M-Weighting is most easily conceived of as a simple filter. For example, if a Hi-Freq Cetacean was exposed to a sound at 100Hz, the effective level for assessment purposes could be reduced by 9dB.

-9dB

100 Hz

Part II - Methods and Tools for Bioacoustic Issues

& Analysis

Bioacoustic metrics and field workSound source characterization– Sound Types

• Pulsed• Non-Pulsed• Continuous

– Issues include:• Effective SL as most are not point sources

(SL=RL+TL)• Energy (Time integration), Peak, RMS???• Band measurements (M-Filter, 1/3 Octave….)

Sound source characterization• Sound Types need to be broken down in categories:

– Pulsed – Non-Pulsed– Continuous

• Why?– Experience has shown that these sound types result in different

effects for both injury and behavior– Need different metrics like:

• SEL, • Peak Pressure or RMS, • Freq. Weighting, • Barotrauma (Acoustic impulse Pa-Sec)

Pulse vs. Non-Pulse*•The term PULSE is used here to describe brief, broadband, atonal, transients (ANSI 12.7, 1986; Harris, Ch. 12, 1998), which are characterized by a relatively rapid rise time to maximum pressure followed by a decay that may include a period of diminishing and oscillating maximal and minimal pressures. Examples of pulses are explosions, gunshots, sonic booms, seismic airgun pulses, and pile driving strikes. •NON-PULSE (intermittent or continuous) sounds can be tonal, broadband, or both. They may be of short duration, but without the essential properties of pulses (e.g., rapid rise-time). Examples of anthropogenic, oceanic sources producing such sounds include vessels, aircraft, machinery operations such as drilling or wind turbines, and many active sonar systems. As a result of propagation, sounds with the characteristics of a pulse at the source may lose pulse-like characteristics at some (variable) distance and can be characterized as a non-pulse by certain receivers. (This last is a key issue to be analyzed)

*As defined in Southall, 2007 Criteria Paper

Metrics Peak sound pressure is the maximum absolute value of the instantaneous sound pressure during a specified time interval and is denoted as Pmax in units of Pascals (Pa). It is not an averaged pressure. Peak pressure is a useful metric for either pulses or non-pulse sounds, but it is particularly important for characterizing pulses (ANSI 12.7, 1986; Harris, Ch. 12, 1998). Because of the rapid rise-time of such sounds, it is imperative to use an adequate sampling rate, especially when measuring peak pressure levels (Harris, Ch. 18, 1998). mean-squared pressure (rms) is the average of the squared pressure over some duration. For non-pulse sounds, the averaging time is any convenient period sufficiently long to permit averaging the variability inherent in the type of sound. To be applied with care to pulse soundsSPL - Sound pressure levels are given as the decibel (dB) measures of the pressure metrics defined above. The root-mean-square (rms) sound pressure level (SPL) is given as dB re: 1 µPa for underwater sound and dB re: 20 µPa for aerial sound. Peak sound pressure levels (hereafter “peak”) are given as dBpeakre: 1 µPa in water and dBpeak re: 20 µPa in air. Peak-to-peak sound pressure levels (hereafter “peak-peak”) are dBp-p re: 1 µPa in water and dBp-p re: 20 µPa in air.

Metrics Sound exposure level (SEL) is the decibel level of the cumulative sum-of-square pressures over the duration of a sound (e.g., dB re: 1 μPa2-s) for sustained non-pulse sounds where the exposure is of a constant nature (i.e., source and animal positions are held roughly constant), . For pulses and transient non-pulse sounds, it is extremely useful because it enables sounds of differing duration to be related in terms of total energy for purposes of assessing exposure risk. The SEL metric also enables integrating sound energy across multiple exposures from sources such as seismic airguns and most sonar signals.

ref

N

n

T

n

p

dttpSEL 2

1 0

2

10

)(log10

Source Characterization (SL)

• Distributed sources (arrays) require special consideration– Major issue in understanding near field

exposure for large aperture arrays such as LFA and seismic (early point of contention!)

– Modeling requires near/far field analysis– Particle velocity considerations (seismic

example)

A Tool that engineers can bring to the table!A Tool that engineers can bring to the table!

HN

RN = [RC2+HN

2]1/2

RC

Far Field Criteria for aVertical Line Array of Sources:

RFF = RCwhen [RN-RC ]< /4

[RN-RC ]< /4

SL in the Near field/Far field Regions

SL=SLE+20Log(NFF)where:

NFF = # of elements in the Far Field

SLE = SL of ea element

2. Near Field Receive Level Analysis - The analysis required to evaluate the near field of a VLA source can be easily accomplished by replacing each nth element of the N element array with an equivalent point source,1

Pn[R] = {PE/|R-Rn|}{cos(k|R-Rn|) + i sin(k|R-Rn|)} (3) where,

PE = 10exp[SLE/20] (4) The resultant pressure, P[R] at the field point R is given by:

P[R] = Pn[R], n=1,N (5) Note that this is a complex term, and the resultant receive level value, RL in dB, can be arrived at by taking:

RL=20Log(|P[R]|) (6)

The difference, RL, between that value and that approximated by simple spherical spreading from the center of the array using the far field SL is given by:

RL= RL-[SL-20Log(|R|)] (7)

The geometry used to evaluate the VLA and relevant coordinate system is shown in Figure 1 along with an example for an array of 4 elements. R = xiX + yiY + ziZ (8)

1 M.C. Junger, D.L. Feit, Sound, Structures, and Their Interaction, MIT Press, Cambridge, 1972, Section 3, Applications of the Elementary Acoustic Solutions, et seq.

Z

Y

Xr

z

R

R=xiX+yiY+ziZR= zniZx=rcos()y=rsin()z=Rcos(r=Rsin(

iZ

iY

iX

Fig 1: CartesianCoordinate System

With example showing an Nelement VLA with spacing=d

d

nth

element

R-Rn

zn

The near field value can also be evaluated in an approximate way by determining the far field range of each of the embedded subapertures in the array. For example, the far field range for array subapertures from 4 elements to 18 is shown in Table 2-1:

Table 2-1 Subaperture Far Field Effects

No. Elements Rff 20Log(N/Rff)4 6 -46 18 -108 35 -13

10 58 -1512 87 -1714 122 -1916 162 -2018 208 -2120 260 -22

In Table 2-1, RFF was calculated from Eqn 1 for a typical LFAA VLA. The third column in Table 2-1 demonstrates the difference between the element source level and the on-axis receive level calculated by using the subaperture method: RL[RFF(NS)] = SLE + 20Log(NS) - 20Log(RFF) [Column 3 of Table 2-1]

Subaperture Shortcut to Array Near-Field Effects

Farfield Region•Focused beam•RL=SLE+20Log(NE)-TL•Can Measure ‘Effective SL’ of the array•RL equals SL-TL

Near field Region•Diffuse unfocused beam•Receive Level near HLA = SLE•Cannot Measure Effective SL of the array•RL not equal to Far-Field SL-TL•Velocity component 3 dimensional & computed by dP/dx, dP/dy, dP/dz

Effective SL in the Near field & Fairfield Regions

Horizontal Line Array (HLA) Source, Example shows 4 elements

Range

RFF

150

100

50

0

ArrayHorizontal

Axis

Main Response Axis

0 100 200 300Vertical Range in meters

late

ral D

ista

nce

in m

eter

s

Receive Level relative to the SL of an individual element, SLE

0 -20 -40 -60 -80

Transmitted Near Field Pressure Sound Levels from a Low Frequency Multi-Element HLA

Fig 2-2: Comparing Actual Coherent Array Levels on Axis with the Far Field Approximation & a SubAperture Approximation

(Element SL=0dB, 20 Elements, Narrowband Signal)

-60

-50

-40

-30

-20

-10

0

10

20

30

1.0 10.0 100.0 1000.0

Range in meters

Rec

eive

Lev

el in

dB

20*log(|Coherent sum|)

20log(N)-20Log(R)

Sub Aperture Approx

Particle velocity considerations (single element seismic example)

Particle velocity normal to the radial direction for the 50Hz source at 7m depth, log scale in cm/sec, i.e. @ color scale = -1, Ut = 1x10-1 cm/sec

Particle velocity in the radial direction for the 50Hz source at 7m depth, log scale in cm/sec, i.e. @ color scale = -1, uR = 1x10-1 cm/sec

Based on same analytical technique used for line array with MATLAB GraphicsBased on same analytical technique used for line array with MATLAB Graphics

Examples of Bioacoustic Research

(Past & Present)–Bowhead Whales in the Arctic

(1980’s)–SOCAL SRP Tagged Fin Whale–Stellwagen Bank NOPP (Today)