Deep sediment heterogeneity inferred using very low-frequency features from merchant shipsa).

The very low-frequency noise from merchant ships provides a good broadband sound source to study the deep layers of the seabed. The nested striations that characterize ship time-frequency spectrograms contain unique acoustic features corresponding to where the waveguide invariant β becomes infinite. In this dataset, these features occur at frequencies between 20 and 80 Hz, where pairs of modal group velocities become equal.

Out-of-plane arrivals recorded by drifting hydrophones during the Northern Ocean Rapid Surface Evolution Experiment.

This paper reports on an observation of three-dimensional (3D) arrivals for which the change in the direction of horizontally refracted sound is nearly 180°. The experimental site is Jan Mayen Channel (JMCh), which connects the Greenland and Norwegian Seas. During the experiment, signals from a moored source transmitting a 500-1500 Hz sweep every 4 h were recorded by three surface drifters equipped with hydrophone arrays.

Evaluating machine learning architectures for sound event detection for signals with variable signal-to-noise-ratios in the Beaufort Sea.

This paper explores the challenging polyphonic sound event detection problem using machine learning architectures applied to data recorded in the Beaufort Sea during the Canada Basin Acoustic Propagation Experiment. Four candidate architectures were investigated and evaluated on nine classes of signals broadcast from moored sources that were recorded on a vertical line array of hydrophones over the course of the yearlong experiment. These signals represent a high degree of variability with respect to time-frequency characteristics, changes in signal-to-noise ratio (SNR) associated with varying signal levels as well as fluctuating ambient sound levels, and variable distributions, which resulted in class imbalances.

Clustering analysis of a yearlong record of ambient sound on the Chukchi Shelf in the 40 Hz to 4 kHz frequency range.

Changes in the Arctic environment with regard to declining sea ice are expected to alter the ambient sound field, affecting both the sound generating processes and the sound propagation. This paper presents acoustic recordings collected on the 150-m isobath on the Chukchi Shelf over a yearlong period spanning October 2016 to October 2017. The analysis uses sections of recordings approximately 12 min long collected six times daily.

Temporal and spatial dependence of a yearlong record of sound propagation from the Canada Basin to the Chukchi Shelf.

The Pacific Arctic Region has experienced decadal changes in atmospheric conditions, seasonal sea-ice coverage, and thermohaline structure that have consequences for underwater sound propagation. To better understand Arctic acoustics, a set of experiments known as the deep-water Canada Basin acoustic propagation experiment and the shallow-water Canada Basin acoustic propagation experiment was conducted in the Canada Basin and on the Chukchi Shelf from summer 2016 to summer 2017. During the experiments, low-frequency signals from five tomographic sources located in the deep basin were recorded by an array of hydrophones located on the shelf.

Application of acoustical remote sensing techniques for ecosystem monitoring of a seagrass meadow.

Seagrasses provide a multitude of ecosystem services and serve as important organic carbon stores. However, seagrass habitats are declining worldwide, threatened by global climate change and regional shifts in water quality. Acoustical methods have been applied to assess changes in oxygen production of seagrass meadows since sound propagation is sensitive to the presence of bubbles, which exist both within the plant tissue and freely floating the water as byproducts of photosynthesis.

Broadband sound propagation in a seagrass meadow throughout a diurnal cycle.

Acoustic propagation measurements were conducted in a Thalassia testudinum meadow in the Lower Laguna Madre, a shallow bay on the Texas Gulf of Mexico coast. A piezoelectric source transmitted frequency-modulated chirps (0.1 to 100 kHz) over a 24-h period during which oceanographic probes measured environmental parameters including dissolved oxygen and solar irradiance.

Measurements and modeling of acoustic propagation in a scale model canyon.

Two scale-model acoustic propagation experiments were conducted in a laboratory tank to investigate three-dimensional (3D) propagation effects induced by range-dependent bathymetry. The model bathymetry, patterned after measured bathymetric data, represents a portion of the Hudson Canyon at 1:7500 scale. The bottom condition in the scale model is nearly pressure release, and as a result, the bottom reflection and backscattering are stronger than that of the real ocean environment.

Scale model observations of coupled vertical modes in a translationally invariant wedge.

Scale-model tank experiments offer a controlled environment in which to make underwater acoustic propagation measurements that can provide high-quality data for comparison with numerical models. This paper presents results from a scale model experiment for a translationally invariant wedge with a 10° slope fabricated from closed-cell polyurethane foam to investigate three-dimensional (3-D) propagation effects. A computer controlled positioning system accurately located a receiving hydrophone in 3-D space to create a dense field of synthetic vertical line arrays, which are subsequently used to mode filter the measured pressure field.

Observation of out-of-plane ambient noise on two vector sensor moorings in Lake Travis.

Two Autonomous Underwater Multi-Dimensional Acoustic Recorders (AUMDAR) were deployed in the southeastern part of Lake Travis during the summer of 2018. Each AUMDAR system possessed a three-axis acoustic vector sensor capable of estimating the azimuthal and vertical arrival angles from discrete sound sources. A unique and complicated propagation environment existed during the experiment due to the mooring locations and the range-dependent lake bathymetry.

A comparison between directly measured and inferred wave speeds from an acoustic propagation experiment in Currituck Sound.

An acoustic propagation experiment was conducted in Currituck Sound to characterize low-frequency propagation in a very-shallow-water estuarine environment. The water column properties were homogeneous over the study area, and the emphasis of this work is on understanding the propagation effects induced by the estuarine bed. During the experiment, low-frequency sound propagation measurements of waterborne sound and interface waves were acquired, and direct measurements of the compressional and shear wave properties were obtained at high frequencies.

Development of a standing wave apparatus for calibrating acoustic vector sensors and hydrophones.

An apparatus was developed to calibrate acoustic hydrophones and vector sensors between 25 and 2000 Hz. A standing wave field is established inside a vertically oriented, water-filled, elastic-walled waveguide by a piston velocity source at the bottom and a pressure-release boundary condition at the air/water interface. A computer-controlled linear positioning system allows a device under test to be precisely located in the water column while the acoustic response is measured.

Testing and verification of a scale-model acoustic propagation system.

This paper discusses the design and operation of a measurement apparatus used to conduct scale-model underwater acoustic propagation experiments, presents experimental results for an idealized waveguide, and compares the measured results to data generated by two-dimensional (2D) and three-dimensional (3D) numerical models. The main objective of this paper is to demonstrate the capability of the apparatus for a simple waveguide that primarily exhibits 2D acoustic propagation. The apparatus contains a computer-controlled positioning system that accurately moves a receiving transducer in the water layer above a scale-model bathymetry while a stationary source transducer emits broadband pulsed waveforms.

Numerical analysis of three-dimensional acoustic propagation in the Catoche Tongue.

Analysis of modeled time series data is presented to provide insight into propagation physics of horizontally refracted sound in the Catoche Tongue region of the Gulf of Mexico. The analysis is motivated by the observation of out-of-plane arrivals in measured time series data. In particular, the extended duration of the refracted arrivals is shown to be caused by interaction with multiple locations along the steep sides of the Tongue.

Evidence of three-dimensional acoustic propagation in the Catoche Tongue.

This paper presents observations of two classes of acoustic arrivals recorded on a sparsely populated vertical line array (VLA) moored in the center of the Catoche Tongue, a major reentrant in the Campeche Bank in the southeastern Gulf of Mexico. The acoustic signals were generated by signals underwater sound (SUS) located 50-80 km from the VLA. The first class of arrivals was identified as resulting from a direct (non-horizontally refracted) path.

An impulsive source with variable output and stable bandwidth for underwater acoustic experiments.

The Combustive Sound Source (CSS) is being developed as an environmentally friendly source to be used in ocean acoustics research and surveys. It has the ability to maintain the same wide bandwidth signal over a 20 dB drop in source level. The CSS consists of a submersible combustion chamber filled with a fuel/oxidizer mixture.

Statistical inference of seabed sound-speed structure in the Gulf of Oman Basin.

Addressed is the statistical inference of the sound-speed depth profile of a thick soft seabed from broadband sound propagation data recorded in the Gulf of Oman Basin in 1977. The acoustic data are in the form of time series signals recorded on a sparse vertical line array and generated by explosive sources deployed along a 280 km track. The acoustic data offer a unique opportunity to study a deep-water bottom-limited thickly sedimented environment because of the large number of time series measurements, very low seabed attenuation, and auxiliary measurements.

An extended lumped-element model and parameter estimation technique to predict loudspeaker responses with possible surround-dip effects.

Lumped-element models have long been used to estimate the basic vibration and radiation characteristics of moving-coil loudspeakers. The classical low-frequency model combines and simplifies several important driver elements, predicting only a single mechanical resonance wherein the diaphragm (e.g.

Ultrasonic measurements of the reflection coefficient at a water/polyurethane foam interface.

Measured ultrasonic reflection coefficients as a function of normal incidence angle are reported for several samples of polyurethane foam submerged in a water bath. Three reflection coefficient models are employed as needed in this analysis to approximate the measured data: (1) an infinite plane wave impinging on an elastic halfspace, (2) an infinite plane wave impinging on a single fluid layer overlying a fluid halfspace, and (3) a finite acoustic beam impinging on an elastic halfspace. The compressional wave speed in each sample is calculated by minimizing the sum of squared error (SSE) between the measured and modeled data.

Analysis of a homemade Edison tinfoil phonograph.

Thomas Edison's phonograph was a landmark acoustic invention. In this paper, the phonograph is presented as a tool for education in acoustics. A brief history of the phonograph is outlined and an analogous circuit model that describes its dynamic response is discussed.

Active sound transmission control of a double-panel module using decoupled analog feedback control: experimental results.

Low-frequency sound transmission through passive lightweight partitions often renders them ineffective as means of sound isolation. As a result, researchers have investigated actively controlled lightweight partitions in an effort to remedy this problem. One promising approach involves active segmented partitions (ASPs), in which partitions are segmented into several distinctly controlled modules.

A double-panel active segmented partition module using decoupled analog feedback controllers: numerical model.

Low-frequency sound transmission has long plagued the sound isolation performance of lightweight partitions. Over the past 2 decades, researchers have investigated actively controlled structures to prevent sound transmission from a source space into a receiving space. An approach using active segmented partitions (ASPs) seeks to improve low-frequency sound isolation capabilities.