## Appendix B Glossary of terms

Absorption - loss of acoustic energy due to conversion to heat.

Accreditation - the official certification that a model or simulation is acceptable for a specific purpose.

Acoustic daylight - imaging of underwater objects using the ambient noise field.

Acoustic impedance - characteristic acoustic impedance is quantitatively equal to the product of the density and sound speed of the medium.

Acoustic tomography - inverse technique that uses acoustic signals to sample the interior of a water body.

Adiabatic process - thermodynamic change in which there is no transfer of heat or mass across the system boundaries.

Advection - movement of oceanic properties by currents.

Ambient noise - background level of unwanted sound in the sea, exclusive of occasional (transient) noise sources.

Ambient noise models - mathematical models that predict the levels and directionality of noise in the ocean due to surface weather and shipping sources.

Analog models - controlled acoustic experiments in water tanks using appropriate scaling factors.

Analytical models - same as physical (physics-based) models.

Anticyclonic - gyral pattern of motion, induced by Earth's rotation, that is clockwise in the northern hemisphere but counterclockwise in the southern hemisphere.

Archipelagic apron - a gentle slope with a smooth surface on the sea floor.

Attenuation - loss of acoustic energy due to the combined effects of absorption and scattering.

Backscattering - scattering of sound in the direction of the source.

Bank - an elevation of the sea floor located on a shelf.

Baroclinic - state of stratification in the ocean in which surfaces of constant pressure intersect surfaces of constant density.

Barotropic - state of stratification in the ocean in which surfaces of constant pressure and surfaces of constant density are parallel.

Basic acoustic models - category of models containing underwater acoustic propagation, noise and reverberation models.

Basin - a depression of variable extent, generally in a circular or oval form.

Bathythermograph - instrument used to measure water temperature versus depth.

Beam displacement - lateral displacement of an acoustic beam, of finite width, undergoing reflection at a water-sediment interface.

Beam noise statistics models - mathematical models that predict the levels and directionality of low-frequency shipping noise for application to large-aperture, narrow-beam passive sonar systems.

Bistatic - geometry in which the acoustic source and receiver are not at the same position.

Borderland - a region adjacent to a continent that is highly irregular with depths in excess of those typical of a shelf.

Bottom bounce - ray paths that have reflected off the sea floor.

Bottom limited - ocean environment characterized by a water-depth and sound-speed profile that will not support long-range refracted paths.

Bottom loss - reflection loss at the sea floor.

Boundary conditions - constraints imposed on possible solutions of the wave equation by adjacent surfaces.

Buoyancy frequency - frequency at which a water parcel displaced from equilibrium will oscillate.

Canonical sound-speed profile - standard sound-speed profile applicable to ocean areas having a deep sound channel; the profile normally assumes an exponential form in the region of the sound channel axis.

Canyon - a narrow, deep depression with steep slopes.

Caustics - envelopes formed by sets of tangential rays.

Cell scattering models - mathematical models of reverberation based on the assumption that the scatterers are uniformly distributed throughout the ocean.

Chaotic - a deterministic process for which certain ranges of parameters are unpredictable.

Composite roughness model - model that partitions treatment of boundary scattering into two regimes according to large-scale and small-scale surface roughness.

Conjugate depth - depth below the sound channel axis at which the value of sound speed equals that at a shallower depth above the axis.

Constructive simulation - models and simulations involving simulated people operating simulated systems.

Continental margin - a zone separating the continent from the deeper sea bottom, generally consisting of the rise, slope and shelf.

Continental rise - a gentle slope rising toward the foot of the continental slope.

Continental shelf -zone adjacent to a continent or island from the waterline to the depth at which there is usually a marked increase of slope to greater depth (shelf break).

Continental slope - zone between the continental shelf and the continental rise.

Convergence zone - ray paths formed by long-range refraction in deep-water environments having both a critical depth and sufficient depth excess.

Cordillera - an entire underwater mountain system including all the subordinate ranges, interior plateaus and basins.

Coriolis force - an apparent force acting on moving particles resulting from Earth's rotation; the force is proportional to the speed and latitude of the particle, and results in a deflection to the right of motion in the northern hemisphere, or to the left of motion in the southern hemisphere.

Crank-Nicolson method - method for numerically solving partial differential equations of the parabolic type (can be considered integration by the trapezoidal rule).

Critical angle - angle in the vertical plane separating the angular region of total reflection from that of partial reflection and partial transmission at a boundary.

Critical depth - depth at which the value of sound speed equals that of the near-surface maximum sound speed.

Cusped caustic - intersection of two (smooth) caustics.

Cyclonic - gyral pattern of motion, induced by Earth's rotation, that is counterclockwise in the northern hemisphere but clockwise in the southern hemisphere.

Cylindrical spreading - form of geometrical spreading that is confined between two parallel planes.

Decibel - unit of measure of acoustic intensity based on a logarithmic scale.

Deep scattering layer - layer of biological organisms associated with an increased scattering of sound.

Deep sound channel axis - location of the deep-sound speed minimum in the ocean.

Deterministic - state of being completely predictable (as distinguished from stochastic or chaotic).

Diagnostic information - information used to analyze the past or present state of a system, particularly with reference to identifying sonar system pathologies.

Diel - characterized by a 24-h period.

Diffraction - frequency-dependent interference effects.

Direct path - ray paths directly connecting source and receiver over short ranges without boundary interaction.

Dispersion - separation of sound into component frequencies; condition in which the phase velocity is frequency dependent.

### Diurnal - having a daily cycle.

Domains of applicability - the spatial, temporal or spectral ranges over which a model's output can be considered valid; in general, such limitations are imposed by the model's physical or mathematical basis.

Doppler - frequency shift resulting from relative motion of source and receiver.

Downwelling - downward-directed water motion in the ocean.

Ducted precursors - acoustic energy that leaks out of a surface duct, travels via a convergence zone or bottom bounce path or both, before coupling back into the surface duct down range.

Earth curvature correction - correction applied to the sound-speed profile to adjust for propagation of sound over the curved Earth surface rather than over a flat surface.

Eddies - isolated patterns of gyral motion in the ocean.

Eigenray - ray that connects the source and the receiver.

Eigenvalue - solution to the normal-mode equation.

Eikonal - acoustic path length as a function of the endpoints.

Ekman drift current - surface current produced by the wind, but directed perpendicular to the wind direction.

Empirical models - mathematical models based on observations.

Environmental models - empirical algorithms used to quantify the boundary conditions and volumetric effects of the ocean environment.

Escarpment - an elongated and comparatively steep slope of the sea floor separating flat or gently sloping areas.

Fan - a gently sloping, fan-shaped feature normally located near the lower termination of a canyon.

Fast-field models - mathematical models of acoustic propagation based on a variation of the normal-mode technique.

Fathometer returns - multiply reflected echoes arriving from the sea surface directly above or from the sea floor directly below a monostatic sonar.

Feature model - statistical representation of a common synoptic structure in the ocean such as a front or an eddy.

Fidelity - the accuracy of a simulation representation when compared to the real world.

Figure of merit - quantitative measure of sonar performance; value equals allowable one-way transmission loss in passive sonars or two-way transmission loss in noise-limited active sonars.

Finite difference - mathematical technique for solving differential equations.

Finite element - mathematical technique for solving complex problems; in underwater acoustics, this method is implemented by dividing the range-depth plane into a gridded pattern of small elements.

Fracture zone - an extensive linear zone of unusually irregular topography of the sea floor characterized by large seamounts, steep-sided ridges, troughs or escarpments.

Front - boundary between two different water masses.

Galerkin's method - a member of the class of so-called weighted-residual methods and a variational formulation that has been the generally accepted basis of finite-element discretizations.

Gap - a depression cutting transversely across a ridge or rise.

Gaussian beam tracing - method that associates each acoustic ray with a beam having a Gaussian intensity profile normal to the ray.

Geodesic path - shortest distance between two points on the surface of an elliptical Earth; the shortest distance between two points on the surface of a spherical Earth is referred to as a great-circle path.

Grazing angle - angle in the range-depth plane measured from the horizontal.

Half channel - ducted environment defined by the equivalent sound-speed profile either above or below the deep-sound channel axis (the deep-sound channel is considered to be a full channel).

Harmonic solution - solution to the wave equation based on a single frequency.

Hidden depths - concept that regards as unimportant the deep ocean sediment structure below the ray turning point.

Hill - a small elevation rising less than about 200 m above the sea floor.

Hindcast - estimate of a previous state of a system.

Hole - a small depression on the sea floor.

Hybrid models - models based on multiple physical or mathematical approaches in order to broaden the domains of applicability.

Incidence angle - angle in the range-depth plane measured from the vertical.

Inertial motion - periodic, quasi-circular oceanic motions influenced by Earth's rotation.

Isotherm - line connecting values of equal temperature.

Knoll - an elevation rising less than about 1 km above the sea floor and of limited extent across the summit.

Levee - an embankment bordering either one of both sides of a sea channel, or the low-gradient seaward part of a canyon or valley.

Limiting angle - angle of ray measured relative to a horizontal surface that leaves the source tangent to the horizontal plane.

Littoral zone - region between the shore and water depths of approximately 200 m. (The usage and interpretation of this term can vary widely.)

Live simulation - simulation involving real people operating real systems.

Lloyd mirror effect - near-field interference patterns associated with a shallow acoustic source and receiver.

Marginal ice zone - area of ice cover separating pack ice from the open ocean.

Mathematical models - category of models containing empirical models and numerical models.

Mesoscale features - dynamic features of the ocean with characteristic length scales on the order of 100 m to 100 km; examples include eddies and internal waves.

Mixed layer - surface layer of uniform temperature that is well mixed by wind or wave action or by thermohaline convection.

Moat - a depression located at the base of many seamounts or islands.

Modified data banks - data banks containing modified or extrapolated data derived from a primary data bank.

Monostatic - geometry in which the acoustic source and receiver are at the same position.

Mountains - a well-delineated subdivision of a large and complex feature, generally part of a cordillera.

Multipath - propagation conditions characterized by a combination of different types of paths.

Multipath expansion models - mathematical models of acoustic propagation based on a variation of the normal mode technique.

Noise models - mathematical models of noise levels and directionality in the ocean consisting of two categories: ambient-noise and beam-noise statistics models.

Normal-mode models - mathematical models of acoustic propagation based on the normal-mode technique.

Normal modes - natural frequencies at which a medium vibrates in response to an excitation.

Nowcast - estimate of the present state of a system.

Numerical models - mathematical models based on the governing physics.

Ocean impulse response function - time-variant and space-variant Green's function that describes the response of the ocean medium to a unit impulse.

Parabolic equation models - mathematical models of acoustic propagation based on a solution of the parabolic versus the elliptic-reduced wave equation.

Pathological test case -specification of inputs to a model that prove to be particularly troublesome; a test case that produces disorders in an otherwise well-functioning model.

Physical models - theoretical or conceptual (physics-based) representations of the physical and acoustical processes occurring within the ocean.

Plain - a flat, gently sloping or nearly level region of the sea floor.

Plateau - a comparatively flat-topped elevation of the sea floor of considerable extent across the summit and usually rising more than 200 m above the sea floor.

Point-scattering models - mathematical models of reverberation based on the assumption that the scatterers are randomly distributed throughout the ocean.

Primary data banks - data banks containing original or modestly processed environmental acoustic data.

Prognostic information - information used to forecast the future state of a system, particularly with reference to predicting sonar system performance.

Propagation - transmission of acoustic energy through the ocean medium.

Propagation models - mathematical models of underwater acoustic transmission loss consisting of five categories: ray-theory, normal-mode, multipath expansion, fast-field and parabolic equation models.

Province - a region composed of a group of similar bathymetric features whose characteristics are markedly in contrast with the surrounding areas.

Range - a series of generally parallel ridges or seamounts.

Rayleigh parameter - measure of the acoustic roughness of a surface.

Ray-theory models - mathematical models of acoustic propagation based on ray-tracing techniques.

Reef - an offshore consolidation with a depth below the sea surface of less than about 20 m.

Reflection - return of a portion of the incident energy in the forward direction after an encounter with a boundary.

Refraction - directional changes in acoustic propagation caused by density discontinuities.

Reliable acoustic path - ray paths formed when a source or receiver is located at the critical depth.

### Reverberation - backscattered sound.

Reverberation models - mathematical models of reverberation based on boundary and volumetric scattering processes consisting of two categories: cell- and point-scattering models.

Ridge - a long, narrow elevation of the sea floor with steep sides.

Saddle - a low part on a ridge or between seamounts.

Salinity - a measure of the quantity of dissolved salts in sea water.

Scattering - random dispersal of sound after encounters with boundaries or with volumetric inhomogeneities.

Seachannel - a long, narrow, shallow depression of the sea floor, usually occurring on a gently sloping plain or fan, with either a U-shaped or V-shaped cross-section.

Seamount - an elevation rising 1 km or more above the sea floor, and of limited extent across the summit.

Self-noise - noise background originating from own ship or sonar structure.

Shadow zone - region of the range-depth plane into which little, if any, acoustic energy penetrates owing to the refractive properties of the water column.

Shoal - an offshore hazard to navigation with a depth below the sea surface of 20 m or less, usually composed of unconsolidated material.

Signal processing models - mathematical models of signal detection processes.

Significant wave height - the average height of the one-third highest waves of a given wave group.

Sill - the low part of a ridge or rise separating ocean basins from one another or from the adjacent sea floor.

Simulation - dynamic execution of models to generate prognostic or diagnostic information.

Skip distance - distance traveled by a ray that just grazes the sea surface, the sea floor or some intermediate layer in the ocean such as the sonic layer depth.

Smart systems - simulated people operating real systems.

Soliton - Internal solitary waves in the ocean, often generated by the nonlinear deformation of internal tides.

Sonar performance models - mathematical models organized to solve specific sonar applications problems; contain environmental models, basic acoustic models and signal processing models.

Sonic layer - acoustic equivalent of mixed layer; surface layer characterized by near isovelocity conditions.

Spherical spreading - form of geometrical spreading that is unbounded.

Spur - a subordinate elevation, ridge or rise projecting from a larger feature.

Stochastic - state of being random; describable in probabilistic terms.

Surface duct - propagation channel bounded by the sea surface and the base of the sonic layer.

Surface loss - reflection loss at the sea surface.

Tablemount - a seamount having a comparatively smooth, flat top (also called a bench).

Thermocline - region of the water column characterized by a strong negative temperature gradient.

Thermohaline - of or relating to the effects of water temperature (thermo) and salinity (haline).

Transmission anomaly - difference between the observed transmission loss and the loss expected from spherical spreading alone.

Trench - a long, narrow and deep depression of the sea floor, with relatively steep sides.

Trough - a long depression of the sea floor, normally wider and shallower than a trench.

Turning point - position in the range-depth plane at which an upgoing or a downgoing ray reverses direction.

### Upwelling - upward-directed motion in the ocean.

Validation - the process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended uses of the model.

Valley - a relatively shallow, wide depression with gentle slopes, the bottom of which grades continually downward (as opposed to a canyon).

Verification - The process of determining that a model implementation accurately represents the developer's conceptual description and specifications.

Virtual mode - mode associated with acoustic propagation in the surface duct.

Virtual simulation - simulation involving real people operating simulated systems (human-in-the-loop).

Water mass - body of water characterized by a unique temperature-salinity relationship.

Wave equation - equation describing motion of acoustic waves in the ocean.

Whispering gallery - phenomenon relating to the propagation of sound along the curved walls of a gallery, especially those of a hemispherical dome.

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