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An automated method was developed to align underwater acoustic receptions at various depths and ranges to a single reference prediction of long range acoustic arrival structure as it evolves with range in order to determine source-receiver range. Acoustic receptions collected by four autonomous underwater vehicles deployed in the Philippine Sea as part of an ocean acoustic propagation experiment were used to demonstrate the method. The arrivals were measured in the upper 1000 m of the ocean at ranges up to 700 km from five moored, low frequency broadband acoustic tomography sources. Acoustic arrival time structure for pulse compressed signals at long ranges is relatively stable, yet real ocean variability presents challenges in acoustic arrival matching. The automated method takes advantage of simple projections of the measured structure onto the model space that represents all possible pairings of measured peaks to predicted eigenrays and minimizes the average travel-time offset across selected pairings. Compared to ranging results obtained by manual acoustic arrival matching, 93% of the automatically-obtained range estimates were within 75 m of the manually-obtained range estimates. Least squares residuals from positioning estimates using the automatically-obtained ranges with a fault detection scheme were 55 m root-mean-square.

In the early 1990s, as the Acoustic Thermometry of Ocean Climate (ATOC) program was starting, simple statistics of ocean ambient sound were unknown. For instance, what fraction of time does the sound level at some frequency exceed a certain value? When the Cold War ended, US Navy SOSUS arrays became available for “dual use” science. At the Applied Physics Laboratory, University of Washington, we began to collect ambient sound data in 1994 from Navy arrays in the Pacific. Initial data spanning 2 years were presented by Curtis et al. [JASA (1999)]. Data and corresponding analyses were separated according to process, i.e., shipping, marine mammals, and wind, as well as for the total. This monitoring effort continued for nearly two decades as reported by Andrew et al. [JASA (2011)], with other collaborators involved (Metzger and Mercer). Throughout, J. Nystuen was always providing guidance and advice in the data analysis and interpretation of the ambient sound data, in fact influencing the first author as graduate students together. We review these early results and connect them to present day understanding and data collection efforts that reflect the status of Ocean Sound as an Essential Ocean Variable of the Global Ocean Observing System.

This research investigated characteristics of the ocean, specifically in the Gulf of Mexico, to find the best wavelet to use for compression of the recorded data. Recorded underwater acoustic data as well as other types of oceanic data were studied. The power spectral density before and after wavelet decomposition, compression, and recomposition (decompression) are compared to assess the quality of the compression. This research describes a method to choose the best wavelet to analyze ocean acoustic data, with future use in the study of sea surface temperatures and heights (tides). [This material is based upon work supported by the National Science Foundation REU program under DMR-2049188, and partially funded by the Naval Research Laboratory-Stennis Space Center.]

The soundscape of the Northeast Pacific Ocean is studied with emphasis on frequencies in the range 63–125 Hz. A 34-year (1964–1998) increase and seasonal fluctuations (1994–2006) are investigated. This is achieved by developing a simple relationship between the total radiated power of all ocean sound sources and the spatially averaged mean-square sound pressure in terms of the average source factor, source depth, and sea surface temperature (SST). The formula so derived is used to predict fluctuations in the sound level in the range 63–125 Hz with an amplitude of 1.2 dB and a period of 1 year associated with seasonal variations in the SST, which controls the amount of sound energy trapped in the sound fixing and ranging (SOFAR) channel. Also investigated is an observed 5 dB increase in the same frequency range in the Northeast Pacific Ocean during the late 20th century [Andrew, Howe, Mercer, and Dzieciuch (2002). ARLO 3, 65–70]. The increase is explained by the increase in the total number of ocean-going ships and their average gross tonnage.

A machine learning model was developed to automatically align underwater acoustic measurements taken at various depths and ranges from a transmitting source in the Philippine Sea to a reference model of long range acoustic arrival structure, simultaneously determining source-receiver range and travel-time offsets associated with multipath arrivals. Ocean sound-speed variability complicates the task as the measured arrivals may exhibit scattering not present in range-independent predictions. Monte Carlo style broadband parabolic equation simulations through random internal wave fields consistent with the Garrett-Munk internal wave energy spectrum were used to generate a large data set of simulated acoustic receptions including scattered multipath arrivals with known source-receiver ranges and imposed travel time offsets. These simulated receptions were used to train and evaluate the machine learning model for arrival pattern matching to the reference model. The inclusion of various data dimensions, such as peak amplitude and width, and contextual information, such as range and depth, were also explored as input to the model. Ranging results for the machine learning model were compared to a programmatic solution engineered for the same task.

An automated method was developed to align underwater acoustic measurements taken at various depths and ranges to a reference model of acoustic arrival structure. Data used to demonstrate the method were collected by four autonomous underwater vehicles deployed in the Philippine Sea as part of an ocean acoustic tomography experiment. The arrivals were measured in the upper 1000 m of the ocean at ranges spanning several hundred kilometers from 5 moored acoustic tomography sources. The primary objective is to accomplish automatic source-receiver ranging by aligning measurements of long range acoustic arrivals to a single reference model. Acoustic arrival time structure for pulse compressed signals at long ranges is relatively stable, yet real ocean variability presents challenges in acoustic arrival matching. The presence of internal waves scatters the acoustic arrival structure and can introduce spurious arrivals in the measured data. This method takes advantage of simple projections of the measured structure onto the model space with constraints informed by scattering statistics consistent with the Garrett Munk internal wave energy spectrum. Compared to manual matching of the measured arrivals to eigenray models, more than 90% of the automatically obtained range estimates were within 150 m of the manually obtained range estimates.

The primary use of underwater gliders is to collect oceanographic data within the water column and periodically relay the data at the surface via a satellite connection. In summer 2006, a Seaglider equipped with an acoustic recording system received transmissions from a broadband acoustic source centered at 75 Hz deployed on the bottom off Kauai, Hawaii, while moving away from the source at ranges up to ∼200 km in deep water and diving up to 1000-m depth. The transmitted signal was an m-sequence that can be treated as a binary-phase shift-keying communication signal. In this letter multiple receptions are exploited (i.e., diversity combining) to demonstrate the feasibility of using the glider as a mobile communication gateway.

International audience; The generation of ultra-low frequency acoustic noise (0.1 to 1 Hz) by the nonlinear interaction of ocean surface gravity waves is well established. More controversial are the quantitative theories that attempt to predict the recorded noise levels and their variability. Here a single theoretical framework is used to predict the noise level associated with propagating pseudo-Rayleigh modes and evanescent acoustic-gravity modes. The latter are dominant only within 200 m from the sea surface, in shallow or deep water. At depths larger than 500 m, the comparison of a numerical noise model with hydrophone records from two open-ocean sites near Hawaii and the Kerguelen islands reveal: (a) Deep ocean acoustic noise at frequencies 0.1 to 1 Hz is consistent with the Rayleigh wave theory, in which the presence of the ocean bottom amplifies the noise by 10 to 20 dB; (b) in agreement with previous results, the local maxima in the noise spectrum support the theoretical prediction for the vertical structure of acoustic modes; and (c) noise level and variability are well predicted for frequencies up to 0.4 Hz. Above 0.6 Hz, the model results are less accurate, probably due to the poor estimation of the directional properties of wind-waves with frequencies higher than 0.3 Hz.

The article of record as published may be found at https://doi.org/10.1121/1.4820882 The propagation of weakly dispersive modal pulses is investigated using data collected during the 2004 long-range ocean acoustic propagation experiment (LOAPEX). Weakly dispersive modal pulses are characterized by weak dispersion- and scattering-induced pulse broadening; such modal pulses experience minimal propagation-induced distortion and are thus well suited to communications applications. In the LOAPEX environment modes 1, 2, and 3 are approximately weakly dispersive. Using LOAPEX observations it is shown that, by extracting the energy carried by a weakly dispersive modal pulse, a transmitted communications signal can be recovered without performing channel equalization at ranges as long as 500 km; at that range a majority of mode 1 receptions have bit error rates (BERs) less than 10%, and 6.5% of mode 1 receptions have no errors. BERs are estimated for low order modes and compared with measurements of signal-to-noise ratio (SNR) and modal pulse spread. Generally, it is observed that larger modal pulse spread and lower SNR result in larger BERs. This work was supported by the Office of Naval Research, Code 322, Grant Nos. N00014-06-1-0245, N00014-08-1-0195, and N00014-11-1-0194.

Four acoustic Seagliders were deployed in the Philippine Sea November 2010 to April 2011 in the vicinity of an acoustic tomography array. The gliders recorded over 2000 broadband transmissions at ranges up to 700 km from moored acoustic sources as they transited between mooring sites. The precision of glider positioning at the time of acoustic reception is important to resolve the fundamental ambiguity between position and sound speed. The Seagliders utilized GPS at the surface and a kinematic model below for positioning. The gliders were typically underwater for about 6.4 h, diving to depths of 1000 m and traveling on average 3.6 km during a dive. Measured acoustic arrival peaks were unambiguously associated with predicted ray arrivals. Statistics of travel-time offsets between received arrivals and acoustic predictions were used to estimate range uncertainty. Range (travel time) uncertainty between the source and the glider position from the kinematic model is estimated to be 639 m (426 ms) rms. Least-squares solutions for glider position estimated from acoustically derived ranges from 5 sources differed by 914 m rms from modeled positions, with estimated uncertainty of 106 m rms in horizontal position. Error analysis included 70 ms rms of uncertainty due to oceanic sound-speed variability.

Ocean current profiling using ocean acoustic tomography (OAT) was conducted in the Kuroshio Current southeast of Taiwan from August 20 to September 15, 2009. Sound pulses were transmitted reciprocally between two acoustic stations placed near the underwater sound channel axis and separated by 48 km. Based on the result of ray simulation, the received signals are divided into multiple ray groups because it is difficult to resolve the ray arrivals for individual rays. The average differential travel times from these ray groups are used to reconstruct the vertical profiles of currents. The currents are estimated with respect to the deepest water layer via two methods: An explicit solution and an inversion with regularization. The strong currents were confined to the upper 200 m and rapidly weakened toward 500 m in depth. Both methods give similar results and are consistent with shipboard acoustic Doppler current profiler results in the upper 150 m. The observed temporal variation demonstrates a similar trend to the prediction from the Hybrid Coordinate Ocean Model.

An investigation is made of the influence of subglottal boundary conditions on the prediction of voiced sounds. It is generally assumed in mathematical models of voicing that vibrations of the vocal folds are maintained by a constant subglottal mean pressure pI, whereas voicing is actually initiated by contraction of the chest cavity until the subglottal pressure becomes large enough to separate the vocal folds. The problem is reformulated to determine voicing characteristics in terms of a prescribed volumetric flow rate Qo of air from the lungs-the evolution of the resulting time-dependent subglottal mean pressure p[overline]-(t) is then governed by glottal mechanics, the aeroacoustics of the vocal tract, and the influence of continued contraction of the lungs. The new problem is analyzed in detail for an idealized mechanical vocal system that permits precise specification of all boundary conditions. Predictions of the glottal volume velocity pulse shape are found to be in good general agreement with the traditional constant-pI theory when pI is set equal to the time averaged value of p[overline]-(t). But, in all cases examined the constant-pI approximation yields values of the mean flow rates Qo and sound pressure levels that are smaller by as much as 10%.

Data collected during the 2004 Long-range Ocean Acoustic Propagation Experiment provide absolute intensities and travel times of acoustic pulses at ranges varying from 50 to 3200 km. In this paper a subset of these data is analyzed, focusing on the effects of seafloor reflections at the shortest transmission range of approximately 50 km. At this range bottom-reflected (BR) and surface-reflected, bottom-reflected energy interferes with refracted arrivals. For a finite vertical receiving array spanning the sound channel axis, a high mode number energy in the BR arrivals aliases into low mode numbers because of the vertical spacing between hydrophones. Therefore, knowledge of the BR paths is necessary to fully understand even low mode number processes. Acoustic modeling using the parabolic equation method shows that inclusion of range-dependent bathymetry is necessary to get an acceptable model-data fit. The bottom is modeled as a fluid layer without rigidity, without three dimensional effects, and without scattering from wavelength-scale features. Nonetheless, a good model-data fit is obtained for sub-bottom properties estimated from the data.

An equation describing the time-evolution of glottal volume velocity with specified vocal fold motion is derived when the sub- and supra-glottal vocal tracts are present. The derivation of this Fant equation employs a property explicated in Howe and McGowan [(2011) J. Fluid Mech. 672, 428–450] that the Fant equation is the adjoint to the equation characterizing the matching conditions of sub- and supra-glottal Green’s functions segments with the glottal segment. The present aeroacoustic development shows that measurable quantities such as input impedances at the glottis, provide the coefficients for the Fant equation when source-tract interaction is included in the development. Explicit expressions for the Green’s function are not required. With the poles and zeros of the input impedance functions specified, the Fant equation can be solved. After the general derivation of the Fant equation, the specific cases where plane wave acoustic propagation is described either by a Sturm-Liouville problem or concatenated cylindrical tubes is considered. Simulations show the expected skewing of the glottal volume velocity pulses depending on whether the fundamental frequency is below or above a sub- or supra-glottal formant. More complex glottal wave forms result when both the first supra-glottal fundamental frequencies are high and close to the first sub-glottal formant.

The voice source is dominated by aeroacoustic sources downstream of the glottis. In this paper an investigation is made of the contribution to voiced speech of secondary sources within the glottis. The acoustic waveform is ultimately determined by the volume velocity of air at the glottis, which is controlled by vocal fold vibration, pressure forcing from the lungs, and unsteady backreactions from the sound and from the supraglottal air jet. The theory of aerodynamic sound is applied to study the influence on the fine details of the acoustic waveform of "potential flow" added-mass-type glottal sources, glottis friction, and vorticity either in the glottis-wall boundary layer or in the portion of the free jet shear layer within the glottis. These sources govern predominantly the high frequency content of the sound when the glottis is near closure. A detailed analysis performed for a canonical, cylindrical glottis of rectangular cross section indicates that glottis-interior boundary/shear layer vortex sources and the surface frictional source are of comparable importance; the influence of the potential flow source is about an order of magnitude smaller.

Nearly two decades of low-frequency (20–500 Hz) ambient noise measurements at seven open-ocean sites in the North Pacific Ocean basin have revealed a complex pattern of long-term trends. The trends in the Northeastern Pacific Ocean show a significant decrease of almost 2 dB/decade. Along the Aleutian archipelago, the levels are either slightly increasing or remaining flat. Levels in two north central Pacific Ocean sites are essentially flat. Comparisons with very sparse measurements made over the last 5 decades suggest that the mid-latitude noise levels may have peaked in the 1990s. These measurements also show, however, that the noise level is still rising elsewhere. The mechanisms driving these trends appear to be more subtle than simply the number of merchant ships or the local wind speed. Climatically-influenced basin-scale acoustic propagation conditions may have an important role.

The unsteady drag on the vocal folds is the major source of sound during voiced speech. The drag force is caused by vortex shedding from the vocal folds. The influence of the ventricular folds (i.e., the “false” vocal folds that protrude into the vocal tract a short distance downstream of the glottis) on the drag and the voice source are examined in this paper by means of a theoretical model involving vortex sheets in a two-dimensional geometry. The effect of the ventricular folds on the output acoustic pressure is found to be small when the movement of the vocal folds is prescribed. It is argued that the effect remains small when fluid-structure interactions account for vocal fold movement. These conclusions can be justified mathematically when the characteristic time scale for change in the velocity of the glottal jet is large compared to the time it takes for a vortex disturbance to be convected through the vocal fold and ventricular fold region.

Observations of scattering of low-frequency sound in the ocean have focused largely on effects at long ranges, involving multiple scattering events. Fluctuations due to one and two scattering events are analyzed here, using 75-Hz broadband signals transmitted in the eastern North Pacific Ocean. The experimental geometry gives two purely refracted arrivals. The temporal and vertical scales of phase and intensity fluctuations for these two ray paths are compared with predictions based on the weak fluctuation theory of Munk and Zachariasen, which assumes internal-wave-induced sound-speed perturbations [J. Acoust. Soc. Am. 59, 818-838 (1976)]. The comparisons show that weak fluctuation theory describes the frequency and vertical-wave-number spectra of phase and intensity for the two paths reasonably well. The comparisons also show that a resonance condition exists between the local acoustic ray and the internal-wave field, as predicted by Munk and Zachariasen, such that only internal waves whose crests are parallel to the local ray path contribute to acoustic scattering. This effect leads to filtering of the acoustic spectra relative to the internal-wave spectra, such that steep rays do not acquire scattering contributions due to low-frequency internal waves.

Receptions, from a ship-suspended source (in the band 50-100 Hz) to an ocean bottom seismometer (about 5000 m depth) and the deepest element on a vertical hydrophone array (about 750 m above the seafloor) that were acquired on the 2004 Long-Range Ocean Acoustic Propagation Experiment in the North Pacific Ocean, are described. The ranges varied from 50 to 3200 km. In addition to predicted ocean acoustic arrivals and deep shadow zone arrivals (leaking below turning points), "deep seafloor arrivals," that are dominant on the seafloor geophone but are absent or very weak on the hydrophone array, are observed. These deep seafloor arrivals are an unexplained set of arrivals in ocean acoustics possibly associated with seafloor interface waves.

Propagation of energy along the sound channel axis cannot be formally described in terms of geometrical acoustics due to repeated cusped caustics along the axis. In neighborhoods of these cusped caustics, a very complicated interference pattern is observed. Neighborhoods of interference grow with range and overlap at long ranges. This results in the formation of a complex interference wave--the axial wave--that propagates along the sound channel axis like a wave belonging to a crescendo of near-axial arrivals. The principal properties of this wave are calculated for the actual space-time configuration realized during a 2004 long-range propagation experiment conducted in the North Pacific. The experiment used M-sequences at 68.2 and 75 Hz, transmitter depths from 350 to 800 m, and ranges from 50 to 3200 km. Calculations show that the axial wave would be detectable for an optimal geometry-both transmitter and receiver at the sound channel axis--for a "smooth" range-dependent sound speed field. The addition of sound speed perturbations--induced here by simulated internal waves--randomizes the acoustic field to the extent that the axial wave becomes undetectable. These results should be typical for mid-latitude oceans with similar curvatures about the sound speed minimum.

An analytical and experimental investigation is made of the effect of a 2-D ventilated gas cavity on the spectrum of turbulent boundary layer wall pressure fluctuations upstream of a gas cavity on a plane rigid surface. The analytical model predicts the ratio of the wall pressure spectrum in the presence of the cavity to the blocked wall pressure spectrum that would exist if the cavity were absent. The ratio is found to oscillate in amplitude with upstream distance (−x) from the edge of the cavity. It approaches unity as −ωx∕Uc→∞, where ω is the radian frequency and Uc is the upstream turbulence convection velocity. To validate these predictions an experiment was performed in a water tunnel over a range of mean flow velocities. Dynamic wall pressure sensors were flush mounted to a flat plate at various distances upstream from a backward facing step. The cavity was formed downstream of the step by injecting carbon dioxide gas. The water tunnel measurements confirm the predicted oscillatory behavior of the ...

Acoustic signals transmitted from a 75-Hz broadband source near Kauai as part of the North Pacific Acoustic Laboratory (NPAL) experiment were recorded on an array of receivers near California at a range of 3890 km, and on a vertical line array at a range of 3336 km in the Gulf of Alaska. Because the source is approximately 2 m above the seafloor, and the bottom depth at the receivers near California is approximately 1800 m, acoustic interaction with the bathymetry complicates the identification of the recorded arrivals with those present in numerical simulations of the experiment. Ray methods were used to categorize acoustic energy according to interactions with the sea bottom and surface and to examine the significance of seafloor geometry. A modal decomposition was also used to examine the role of range-dependent bathymetry and to associate the effects on the acoustic field with seafloor features at specific ranges. Parabolic-equation simulations were performed in order to investigate the sensitivity of the received signal to geoacoustic parameters; shear excitations within the seafloor were modeled using a complex-density, equivalent-fluid technique. Incorporation of bottom interaction into models of the propagation enables an identification between experimental and simulated arrivals.

This article analyzes the coherence of low-mode signals at ranges of 3515 and 5171 km using data from the Acoustic Thermometry of Ocean Climate (ATOC) and Alternate Source Test (AST) experiments. Vertical line arrays at Hawaii and Kiritimati received M-sequences transmitted from two sources: the 75-Hz bottom-mounted ATOC source on Pioneer Seamount and the near-axial dual-frequency (28/84 Hz) AST source deployed nearby. This study demonstrates that the characteristics of the mode signals at 5171-km range are quite similar to those at 3515-km range. At 75 Hz the mode time spreads are on the order of 1.5 s, implying a coherence bandwidth of 0.67 Hz. The time spread of the 28-Hz signals is somewhat lower, but these signals show significantly less frequency-selective fading than the 75-Hz signals, suggesting that at the lower frequency the multipaths are temporally resolvable. Coherence times for mode 1 at 75 Hz are on the order of 8 min for the 3515-km range and 6 min for 5171-km range. At 28 Hz mode 1 is much more stable, with a magnitude-squared coherence of greater than 0.6 for the 20-min transmission period.

We examine statistical and directional properties of the ambient noise in the 10-100 Hz frequency band from the NPAL array. Marginal probability densities are estimated as well as mean square levels, skewness and kurtoses in third octave bands. The kurotoses are markedly different from Gaussian except when only distant shipping is present. Extremal levels reached approximately 150 dB re 1 micro Pa, suggesting levels 60dB greater than the mean ambient were common in the NPAL data sets. Generally, these were passing ships. We select four examples: i) quiescent noise, ii) nearby shipping, iii) whale vocalizations and iv) a micro earthquake for the vertical directional properties of the NPAL noise since they are representative of the phenomena encountered. We find there is modest broadband coherence for most of these cases in their occupancy band across the NPAL aperture. Narrowband coherence analysis from VLA to VLA was not successful due to ambiguities. Examples of localizing sources based upon this coherence are included. kw diagrams allow us to use data above the vertical aliasing frequency. Ducted propagation for both the quiescent and micro earthquake (T phase) are identified and the arrival angles of nearby shipping and whale vocalizations. MFP localizations were modestly successful for nearby sources, but long range ones could not be identified, most likely because of signal mismatch in the MFP replica.

A method to obtain coherent acoustic wave fronts by measuring the space–time correlation function of ocean noise between two hydrophones is experimentally demonstrated. Though the sources of ocean noise are uncorrelated, the time-averaged noise correlation function exhibits deterministic waveguide arrival structure embedded in the time-domain Green’s function. A theoretical approach is derived for both volume and surface noise sources. Shipping noise is also investigated and simulated results are presented in deep or shallow water configurations. The data of opportunity used to demonstrate the extraction of wave fronts from ocean noise were taken from the synchronized vertical receive arrays used in the frame of the North Pacific Laboratory (NPAL) during time intervals when no source was transmitting.

The Arctic Ocean is undergoing dramatic changes, the most apparent being the rapidly reducing extent and thickness of the summer ice cover. As has been well established over prior decades, the environmental acoustics of the ice-covered Arctic is dominated by two major effects: the highly inhomogeneous ice cover, and the monotonically upward refracting sound speed profile, the combination of which forces all sound paths to be exposed to strong scattering loss and the associated loss of coherence. In some portions of the Arctic Ocean, however, inflow of warm Pacific water has created the so-called “Beaufort Lens,” a neutrally buoyant high sound velocity layer at 70-80 meter depth, which has dramatically altered the acoustic environment, creating a strong acoustic duct between approximately 100 and 300 m depth. This duct has the potential of trapping sound out to significant ranges (80-100 km) without interacting with the ice cover, resulting in much higher coherence and signal preservation. Acoustic noise measurement results collected with a vertically suspended array during ICEX 2016 illustrate the spatial and temporal noise properties in the presence of this acoustic duct at different depths. Comparisons of the ICEX 2016 data are also made with modeled Arctic noise data. [Work supported by ONR and DARPA.]

The article of record as published may be found at https://doi.org/10.1121/1.4818887 A series of experiments conducted in the Philippine Sea during 2009–2011 investigated deep-water acoustic propagation and ambient noise in this oceanographically and geologically complex region: (i) the 2009 North Pacific Acoustic Laboratory (NPAL) Pilot Study/Engineering Test, (ii) the 2010–2011 NPAL Philippine Sea Experiment, and (iii) the Ocean Bottom Seismometer Augmentation of the 2010–2011 NPAL Philippine Sea Experiment. The experimental goals included (a) understanding the impacts of fronts, eddies, and internal tides on acoustic propagation, (b) determining whether acoustic methods, together with other measurements and ocean modeling, can yield estimates of the time-evolving ocean state useful for making improved acoustic predictions, (c) improving our understanding of the physics of scattering by internal waves and spice, (d) characterizing the depth dependence and temporal variability of ambient noise, and (e) understanding the relationship between the acoustic field in the water column and the seismic field in the seafloor. In these experiments, moored and ship-suspended low-frequency acoustic sources transmitted to a newly developed distributed vertical line array receiver capable of spanning the water column in the deep ocean. The acoustic transmissions and ambient noise were also recorded by a towed hydrophone array, by acoustic Seagliders, and by ocean bottom seismometers.

The article of record as published may be found at http://dx.doi.org/10.1121/1.4818847 Mode travel time estimation in the presence of internal waves (IWs) is a challenging problem. IWs perturb the sound speed, which results in travel time wander and mode scattering. A standard approach to travel time estimation is to pulse compress the broadband signal, pick the peak of the compressed time series, and average the peak time over multiple receptions to reduce variance. The peak-picking approach implicitly assumes there is a single strong arrival and does not perform well when there are multiple arrivals due to scattering. This article presents a statistical model for the scattered mode arrivals and uses the model to design improved travel time estimators. The model is based on an Empirical Orthogonal Function (EOF) analysis of the mode time series. Range-dependent simulations and data from the Long-range Ocean Acoustic Propagation Experiment (LOAPEX) indicate that the modes are represented by a small number of EOFs. The reduced-rank EOF model is used to construct a travel time estimator based on the Matched Subspace Detector (MSD). Analysis of simulation and experimental data show that the MSDs are more robust to IW scattering than peak picking. The simulation analysis also highlights how IWs affect the mode excitation by the source. Office of Naval Research (ONR) grants N00014-03-1-0181 and N00014-03-1-0182 ONR Graduate Traineeship Award N00014-06-1-0223 National Research Council Research Associateship Award Office of Naval Research (ONR) Award N00014-12-1-0412 Office of Naval Research (ONR) grants N00014-03-1-0181 and N00014-03-1-0182 ONR Graduate Traineeship Award N00014-06-1-0223 National Research Council Research Associateship Award Office of Naval Research (ONR) Award N00014-12-1-0412

The article of record as published may be found at https://doi.org/10.1121/1.4818845 Ocean bottom seismometer observations at 5000 m depth during the long-range ocean acoustic propagation experiment in the North Pacific in 2004 show robust, coherent, late arrivals that are not readily explained by ocean acoustic propagation models. These “deep seafloor” arrivals are the largest amplitude arrivals on the vertical particle velocity channel for ranges from 500 to 3200 km. The travel times for six (of 16 observed) deep seafloor arrivals correspond to the sea surface reflection of an out-of-plane diffraction from a seamount that protrudes to about 4100 m depth and is about 18 km from the receivers. This out-of-plane bottom-diffracted surface-reflected energy is observed on the deep vertical line array about 35 dB below the peak amplitude arrivals and was previously misinterpreted as in-plane bottom-reflected surface-reflected energy. The structure of these arrivals from 500 to 3200 km range is remarkably robust. The bottom-diffracted surface-reflected mechanism provides a means for acoustic signals and noise from distant sources to appear with significant strength on the deep seafloor. The OBS/Hs used in the LOAPEX field program were provided by Scripps Institution of Oceanography under the U.S. National Ocean Bottom Seismic Instrumentation Pool (SIO-OBSIP, http:// www.obsip.org). The OBS/H deployments themselves were co-funded through direct funding to SIO-OBSIP by the National Science Foundation and by Woods Hole Oceanographic Institution under a grant from the WHOI Deep Ocean Exploration Institute. The LOAPEX source deployments and the moored DVLA receiver deployments were funded by the Office of Naval Research under Award Nos. N00014-03-1-0181 and N00014-03-1-0182. The data reduction and analysis in this paper were funded by the Office of Naval Research under Award Nos. N00014-06-1-0222 and N00014-10-1-0510. Additional post-cruise analysis support was provided to RAS through the Edward W. and Betty J. Scripps Chair for Excellence in Oceanography.

Long-term statistics of ambient sound in an ocean basin have been derived from 2 years of data collected on 13 widely distributed receivers in the North Pacific. The data consist of single hydrophone spectra (1–500 Hz in 1-Hz bands) averaged over 170 s and recorded at 5-min intervals. Cumulative probability distributions of the ambient sound level show that for the open-ocean arrays at 75 Hz, sound levels are 3 dB higher than the median level 10% of the time and 6 dB higher 1% of the time. For the coastal arrays, sound levels are 7 dB higher than the median level 10% of the time and 15 dB higher 1% of the time. The clearest feature in many of the spectrograms is a strong annual cycle in the 15–22 Hz band with peak signal levels up to 25 dB above the sound floor; this cycle is attributed to the presence and migration of blue and fin whales. On average, whales are detected 43% of the time. Ships are heard 31%–85% of the time on the coastal receivers and 19%–87% of the time on the open-ocean receivers, depending on the receiver. On average, ships are detected 55% of the time. The correlation coefficient between the sound level in the 200–400 Hz band and wind speed, determined from satellite and global meteorological analysis, is on average 0.56 for the coastal receivers and 0.79 for the open-ocean receivers. For some receivers, the sound level in the 12–15 Hz band is correlated with the sound level in the 200–400 Hz band, with a correlation coefficient of 0.5.

Noise from aircraft ground operations often reaches residences in the vicinity of airports via grazing incidence paths that attenuate high-frequency noise more than air-to-ground propagation paths, thus increasing the relative low-frequency content of such noise with respect to overflight noise. Outdoor A-weighted noise measurements may not appropriately reflect low-frequency noise levels that can induce potentially annoying secondary emissions inside residences near runways. Contours of low-frequency noise levels were estimated in a residential area adjacent to a busy runway from multi-site measurements of composite maximum spectra of runway sideline noise in the one-third octave bands between 25 and 80 Hz, inclusive. Neighborhood residents were interviewed to determine the prevalence of annoyance attributable to runway sideline noise at frequencies below 100 Hz, and of its audible manifestations inside homes. Survey respondents highly annoyed by rattle and vibration were concentrated in areas with low-frequency sound levels due to aircraft operations in excess of 75 to 80 dB.