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Underwater Sound and the Marine Mammal Acoustic Environment

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Underwater Sound and the Marine Mammal Acoustic Environment Side Lobe (a) A A Main Lobe Lininne Transducer Array (b) y A Acoustic Axis eCirrculculccuuullalarar PlaPlPlaannee ArrA ayy ource SoS EqEquuaa nne LoLoudd UnderwaterA AND x Sound THE MARINE MAMMAL ACOUSTIC ENVIRONMENT A Guide to Fundamental Principles Prepared for the U. S. Marine Mammal Commission by David L. Bradley, Ph.D. T Richard Stern, Ph.D. July 2008 w (t) UNDERWATER SOUND AND THE MARINE MAMMAL ACOUSTIC ENVIRONMENT A Guide to Fundamental Principles by David L. Bradley, Ph.D. Richard Stern, Ph.D. Prepared for the U. S. Marine Mammal Commission July 2008 Cover and text design by Raj Patel, North Potomac, Maryland. Printing by Spectrum Printing & Graphics, Rockville, Maryland. Contents A NOTE FROM THE COMMISSIONERS .............................................................................. iii PREFACE ....................................................................................................................................v Chapter 1 FUNDAMENTAL PRINCIPLES AND DEFINITIONS .........................................1 What is sound? .........................................................................................................1 Acoustical parameters ..............................................................................................2 Pressure ............................................................................................................3 Particle velocity ...............................................................................................3 Density .............................................................................................................3 Intensity ............................................................................................................3 Ambient (background) conditions ...........................................................................4 How does sound travel? ...........................................................................................4 Time, space, and information ...................................................................................4 The sine wave ..................................................................................................5 Period and frequency .......................................................................................7 Wavelength.......................................................................................................7 Speed of sound .................................................................................................7 Phase ................................................................................................................8 Dimensional units and the decibel ...........................................................................8 Sound pressure and intensity levels .......................................................................10 Sound levels in air versus water .............................................................................11 Points to remember ................................................................................................13 Chapter 2 THE OCEAN ENVIRONMENT ...........................................................................15 Geographical and physical properties ....................................................................15 The ocean’s boundaries ..........................................................................................16 Expected values and variation—a caveat ..............................................................16 Variation of the speed of sound in the ocean .........................................................17 Sound velocity profi le ....................................................................................17 Frequency spectrum ...............................................................................................19 Noise spectrum in the ocean environment .............................................................19 Points to remember ................................................................................................21 Chapter 3 SOURCES OF SOUND .........................................................................................23 Generating sound ...................................................................................................23 Scale .......................................................................................................................23 Long and short times ......................................................................................24 Large and small sizes .....................................................................................25 Near and far fi elds ..........................................................................................25 Sources of sound found in the sea ..........................................................................25 Natural, biotic sources ...................................................................................25 Marine mammals ...............................................................................26 Fish .....................................................................................................26 Invertebrates .......................................................................................27 ~~ i ~~ Natural, abiotic sources ..................................................................................27 Seismic disturbances ..........................................................................27 Anthropogenic sources ...................................................................................27 Large vessels and ports ......................................................................28 Air traffi c ............................................................................................30 Fishing industry .................................................................................30 Petroleum exploration ........................................................................31 Military equipment and operations ....................................................32 Other signifi cant sources ....................................................................32 Points to remember ................................................................................................32 Chapter 4 SOUND GENERATION, PROPAGATION, AND RECEPTION .........................35 The spatial characteristics of sound sources ..........................................................36 Simple sources ...............................................................................................36 Complex sources (arrays) ..............................................................................36 The unbounded and ideal ocean .............................................................................38 Geometric spreading ......................................................................................38 The bounded and real ocean ..................................................................................38 Absorption......................................................................................................40 Viscosity .............................................................................................40 Relaxation processes ..........................................................................40 Refl ection .......................................................................................................41 Refraction .......................................................................................................42 Scattering .......................................................................................................43 Reverberation .................................................................................................43 Deep water (blue water) and shallow water ...........................................................43 Spatial characteristics of sound receivers ..............................................................44 Points to remember ................................................................................................45 Chapter 5 HEARING SOUND: HUMANS AND MARINE MAMMALS ...........................47 Transduction ...........................................................................................................47 Humans ..................................................................................................................49 Marine mammals ...................................................................................................50 Sound levels ...................................................................................................50 Points to remember ................................................................................................50 Chapter 6 SOUND AND MARINE MAMMALS—FINAL THOUGHTS ...........................53 Hearing-centric sensing .........................................................................................53 Potential threats to marine mammal survival .........................................................54 Measurements and observations ............................................................................55 Points to remember ................................................................................................56 ACKNOWLEDGEMENTS ...................................................................................57 FURTHER READING ...........................................................................................59
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