bluuebunny/crossref_metadata_2025 huggingface dataset:
A new multichannel spatial diversity technique for long-range acoustic communications in shallow water
Authors: Beaujean Pierre-Philippe J. , Bernault Emmanuel P.
Published: October 2013

A new multichannel spatial diversity technique has been developed for underwater acoustic communications in very shallow waters. This technique combines a novel synchronization method with maximum-likelihood symbol estimation. It was tested with a multichannel Mills-Cross receiver using various numbers of elements. The FAU general purpose acoustic modem source transmitted messages using four types of frequency-hopped multiple-frequency-chirp-keying (FH-MFSK) modulation: 4 hops at 221 coded bits per second (cps), 2 hops at 442 cps, or no hopping at 1172 cps. These types of modulation allowed for robust data transmission in adverse environment. The modem operated between 16 and 32 kHz at 192-dB source level, at ranges from 1 to 5 km in 40 ft of water. Using only four channels of the Mills-Cross receiver array, messages coded at 1172 cps were received with a frame error rate (FER) of 0% at a range of 3 km. In the same four-channel configuration, messages coded at 221 cps were received with no frame error at 5 km. The experimental results matched the performance predictions from the Crepeaus model. This reliable and computation-efficient method can be implemented on new generations of embedded acoustic modems, such as the four-channel FAU acoustic modem, and can provide significant improvements in communication performance.
Coherent equalization for underwater acoustic communications: A performance evaluation of adaptive MMSE and time-reversal techniques
Authors: Preisig James C.
Published: October 2013

A simple analytical expression is presented for evaluating and comparing the performance of channel-estimate-based coherent equalizers. A channel-estimate-based equalizer is one whose filter coefficients are calculated as a function of an explicit estimate of the acoustic channel impulse response. This is in contrast to a direct adaptation equalizer whose coefficients are calculated directly as the solution to a data estimation error minimization problem. The equalizer performance expression is different from other commonly used expressions because it is formulated from the viewpoint of the estimate of the channel impulse response rather than the actual channel impulse response. The expression allows the dependence of the performance of coherent equalizers on channel complexity and the quality of the channel estimate to be separately evaluated. The expression is used to evaluate and compare the performance of adaptive MMSE and time-reversal equalizers. [Work supported by ONR Ocean Acoustics.]
Estimation of spatial coherence in shallow water waveguides
Authors: Rozenfeld Ilya , Carey William M. , Cable Peter G. , Siegmann William L.
Published: October 2013

For various underwater acoustic applications such as beamforming and matched field processing, it is important to understand the spatial characteristics of shallow water acoustic fields. Oceanic and geoacoustic variabilities introduce random fluctuations into the fields, for which an important metric is the spatial coherence that is the correlation function of the field. The theory developed here is based on a perturbation approach to the modal solution of the Helmholtz equation and treats scattering from sound speed fluctuations as well as from rough interfaces. The results here extend previous work to media where fluctuations can have arbitrary correlation functions and are not necessarily horizontally isotropic or homogeneous. This allows for modeling of coherence degradation due to physically realistic mechanisms. The behavior of the coherence function with respect to various parameters is illustrated using a Pekeris waveguide with superimposed randomness. The effects of several different scattering mechanisms on the coherence will be compared and rank ordered for a range of frequencies. [Work partially supported by ONR.]
Toward a practical cross section for rough surface scattering
Authors: Broschat Shira L.
Published: October 2013

The lowest-order small slope approximation (SSA) provides a practical means of obtaining the scattering strength for rough surfaces. While results are accurate over a wide range of scattering angles, as surface slopes increase and for forward scattering beyond the specular angle, even higher-order SSA results become inaccurate. This is due, in part, to the lack of nonlocal interactions in the SSA. Voronovich introduced the nonlocal small slope approximation (NLSSA) as a generalization of the SSA to explicitly include nonlocal interactions. It has been shown that the NLSSA is more accurate than the SSA both for larger surface slopes and at low forward grazing angles. However, the computational cost precludes use of the NLSSA in practice. The NLSSA expression for the scattering strength can be separated into two terms. The first term is identical to the lowest-order SSA and, hence, the second term of the NLSSA can be considered a correction to the lowest-order SSA. In this paper, we make an approximation to the second term at forward scattering angles that reduces it to a simple algebraic expression. We present numerical results and discuss the region of validity for the reduced form of the equation. [Work supported by ONR.]
Temporal coherence of acoustic signals propagating through the near surface bubble layer
Authors: Culver R. Lee , Bradley David , Reeves Jon
Published: October 2013

The influence of the near surface bubble layer on the temporal coherence of high frequency acoustic signals over intervals ranging from a few seconds to tens of hours have been investigated experimentally. Acoustic data were collected during the broadband coherence experiment conducted in July 2002 off the coast of San Diego in a water depth of 1000 m. Signals transmitted include short pure tone pulses and broadband (up to 22 kHz) linear frequency modulated (LFM) sweeps centered at 20, 40, and 60 kHz. The projector and receiver hydrophone were suspended from FLIP. Environmental measurements included a towed CTD string, water temperature at a fixed point, near surface bubble layer using a multibeam sonar, current speed and direction, directional wave height and wind speed. [Work supported by ONR Code 321US under the ARL Program.]
Sound speed and attenuation in unconsolidated sand with viscous pore fluids
Authors: Hefner Brian T. , Williams Kevin L.
Published: October 2013

During the Sediment Acoustics Experiment 1999 (SAX99) several researchers measured sound speed and attenuation in the sediment over a broad range of frequencies (125 Hz to 400 kHz). While the variation of sound speed with frequency was well modeled by Biot theory, the variation of attenuation with frequency deviates from Biot theory at high frequencies. In an effort to understand this deviation, attenuation and sound speed measurements were made in unconsolidated glass beads saturated with various high-viscosity fluids using the ‘‘attenuation array,’’ which was used during SAX99. According to Biot theory, changing the viscosity of the pore fluid alters the frequency range where the most significant dispersion occurs. Thus by increasing the viscosity of the fluid, it was possible to observe significant dispersion in the frequency range of the attenuation array (80 to 300 kHz). The results of these measurements were compared to both Biot theory and Buckingham’s grain-to-grain shearing model. [Work supported by ONR.]
Investigation of the relative significance of individual environmental parameters to sonar performance prediction uncertainty
Authors: Wang Ding , Xu Wen , Schmidt Henrik
Published: October 2013

A large part of sonar performance prediction uncertainty is associated with the uncertain ocean acoustic environment. Optimal in situ measurement strategy, i.e. adaptively capturing the most critical uncertain environmental parameters within operational constrains can minimize the sonar performance prediction uncertainty. Understanding the relative significance of individual environmental parameters to sonar performance prediction uncertainty is fundamental to the heuristics to determine the most critical environmental parameters. Based on this understanding, the optimal parametrization of ocean acoustic environments can be defined, which will significantly simplify the adaptively sampling pattern. As an example, the matched-field processing is used to localize an unknown sound source position in a realistic ocean environment. Typical shallow water environmental models are used with some of the properties being stochastic variables. The ratio of the main lobe peak to the maximum side lobe peak of the ambiguity function and the main lobe peak displacement due to mismatch are chosen as performance metrics, respectively, in two different scenarios. The relative significance of some environmental parameters such as sediment thickness, weights of empirical orthogonal functions (EOFs) has been computed. Some preliminary results are discussed.
Low-frequency models to compare acoustical performance of ducts of gradual area change
Authors: Arenas Jorge P. , Poblete Victor
Published: October 2013

When two ducts of different circular cross-sectional area are joined together to form an acoustical transmission system, an additional impedance is introduced. This particular problem is found commonly in air conditioning systems among others. Simple models are used to solve the sound wave equation, and they assume that the plane wave mode is responsible for the most part of the transmission/insertion loss calculation. From previous works, it appears that, although higher-order modes can propagate at certain frequencies, their transmission efficiency is low and they do not cause a critical deviation from the assumption of one-dimensional propagation. The models are based on numerical/semi-analytical and asymptotic approximations to transform the sound pressure wave equation into an acoustic impedance equation with proper boundary conditions. The final aim is to calculate the transfer impedance of the duct section which can be used to calculate the sound field radiated. The model should be extended to ducts with incompressible mean flow for low flow Mach numbers, and further work is currently under progress on this subject. However, for low mean flow, the effect on the transmission loss has been found to be only marginal for some particular geometries. [Work supported by Fondecyt, Grant No. 1020196.]
Acoustical performance testing of duct silencers
Authors: Lilly Jerry G.
Published: October 2013

The test method used in North America to evaluate the acoustical performance of duct silencers is ASTM E477. The ASTM standard provides an approved method for measuring the aerodynamic pressure drop, dynamic insertion loss, and self-generated noise of duct silencers. Unfortunately, restrictions on the construction of the test duct in the current standard cause erroneous results in the measurement of insertion loss at low frequencies. These errors are due to acoustic resonances that occur naturally in the empty test duct. It is recommended that the standard be modified to require anechoic terminations at both ends of the test duct. It is also recommended that more specific design criteria for the source chamber be established, and that a new section be added to provide for the measurement of the radiated noise from the casing of the test specimen. Specific recommendations relating to all of these issues will be provided.
Silencing using flexible plate in a duct
Authors: Ramamoorthy Sripriya , Grosh Karl , Nawar Tony G.
Published: October 2013

A flexible plate interacting with air in a duct can provide passive means for low frequency broadband transmission loss. The sensitivity of the system to various parameters including unintentionally applied tension, effect of external fluid loading, lateral plate cross modes, structural and acoustic boundary condition are analyzed through experimental measurements and theoretical predictions. In order to avoid breakout noise, a backing cavity can be introduced below the plate. This introduces differences in filtering characteristics. Compared to plate in a single duct, the two-duct system will have higher plate resonance frequencies due to cavity loading on the plate. Means to achieve low frequency broadband transmission loss using two-duct silencers will be discussed. Significance of three dimensionality of the problem will be brought out by comparing the results of three-dimensional finite-element analysis with experimental data. Successful designs and experiments for low frequency attenuation will be presented. [Work supported by NSF and ONR.]
Parametric analysis of open plan offices
Authors: Nogueira Flavia F. , Viveiros Elvira B.
Published: October 2013

The workspace has been undergoing many changes. Open plan offices are being favored instead of ones of traditional design. In such offices, workstations are separated by partial height barriers, which allow a certain degree of visual privacy and some sound insulation. The challenge in these offices is to provide acoustic privacy for the workstations. Computer simulation was used as a tool for this investigation. Two simple models were generated and their results compared to experimental data measured in two real offices. After validating the approach, models with increasing complexity were generated. Lastly, an ideal office with 64 workstations was created and a parametric survey performed. Nine design parameters were taken as variables and the results are discussed in terms of sound pressure level, in octave bands, and intelligibility index.
Acheiving speech intelligibility at Paddington Station, London, UK
Authors: Goddard Helen M.
Published: October 2013

Paddington Station in London, UK is a large rail terminus for long distance electric and diesel powered trains. This magnificent train shed has four arched spans and is one of the remaining structural testaments to the architect Brunel. Given the current British and European legislative requirements for intelligible speech in public buildings AMS Acoustics were engaged to design an electroacoustic solution. In this paper we will outline how the significant problems of lively natural acoustics, the high operational noise levels and the strict aesthetic constraints were addressed. The resultant design is radical, using the most recent dsp controlled line array loudspeakers. In the paper we detail the acoustic modeling undertaken to predict both even direct sound pressure level coverage and STI. Further it presents the speech intelligibility measured upon handover of the new system. The design has proved to be successful and given the nature of the space, outstanding speech intelligibility is achieved.
Assessment of training noise impacts on the Red-cockaded woodpecker
Authors: Pater Larry , Delaney David
Published: October 2013

The Endangered Species Act requires that federal agencies carry out programs to conserve Threatened and Endangered Species (TES) and to evaluate the impacts of agency activities on listed species. Few scientific studies have investigated the effects of military training activity, including noise, on TES. Regulatory agencies, when faced with a lack of definitive impact information, tend to be conservative in their findings. This can lead to land use restrictions that can affect the military’s ability to train to standard and maintain operational readiness. The primary objectives for the multiyear study reported here were to determine the effects of military training noise on the reproductive fitness of the endangered Red-cockaded Woodpecker (RCW) and to provide noise dose-response data to guide species conservation decisions. The research was conducted at Fort Stewart, an Army installation in SE Georgia. Overall results from this three year field study are presented. [Support for this research was provided by the Strategic Environmental Research and Development Program (SERDP).]
System to measure heart performance
Authors: Andrade Armando , Rios Heriberto , Lizana Pablo R. , Puente Ernestina , Mendoza Diego
Published: October 2013

Systems to measure heart condition are applied to patients with early or chronic cardiac problems with the aim of diagnosing and exactly locat- ing the problem. Two very important factors exist that are taken into account in order to obtain a reliable diagnosis and to be able to give suitable medical treatment. One of them is the volume of blood that the heart pumps, the other is the temperature gradient. In our system we measure both parameters at the same time with the purpose of determining how the heart is working from the amount of blood pumped per unit time. (To be presented in Spanish.)
How to measure soundscapes. A theoretical and practical approach
Authors: Schulte-Fortkamp Brigitte
Published: October 2013

Noise sources interact with the specific acoustic and environmental makeup, topography, meteorology, land use pattern, and lifestyle. The evaluation of soundscapes needs subject-related methodological procedures. With such suitable measurements a way has to be found that allows us to rely on different dimensions on reaction to noise. Improving the soundscape of an urban environment imposes to account for the qualitative appreciation as a cognitive judgment given by listeners and, particularly, for the interaction between acoustic dimensions and other sensory modalities in qualitative judgments of an urban environment (Maffiolo). The structure of the residential area that is, the combination of noise sources are important for the judgment of a soundscope and are also important as subjective parameters which are relevant in people’s point of view. Moreover, the relationship of both define the background for assessments. Studies are needed on the subject and its capability in perception and interpretation; studies on the subject inside the society, studies on the social and cultural context, and field studies including physical measurements. Soundscapes may be defined in its effects on man and vice versa and probably acoustical ecology will serve to understand the function of soundscapes.
A human-hearing-related prediction tool for soundscapes and community noise
Authors: Genuit Klaus
Published: October 2013

There are several methods of calculation available for the prediction of the A-weighted sound-pressure level of environmental noise, which are, however, not suitable for a qualified prediction of the residents’ annoyance and physiological strain. The subjectively felt noise quality does not only depend on the A-weighted sound-pressure level, but also on other psychoacoustical parameters, such as loudness, roughness, sharpness, etc. In addition to these physical and psychoacoustical aspects of noise, the so-called psychological or cognitive aspects have to be considered, too, which means that the listeners’ expectations, their mental attitude, as well as the information content of the noise finally influence the noise quality perceived by the individual persons. Within the scope of a research project SVEN (Sound Quality of Vehicle Exterior Noise), which is promoted by the EC, a new tool has been developed which allows a binaural simulation and prediction of the environmental noise to evaluate the influence of different contributions by the sound events with respect to the psychoacoustical parameters, the spatial distribution, movement, and frequency. By means of this tool it is now possible to consider completely new aspects regarding the audible perception of noise when establishing a soundscape or when planning community noise.
The neighborhood soundscape and the residents’ perceptions of its quality
Authors: Klaeboe Ronny , Fyhri Aslak , Solberg Sigurd
Published: October 2013

In autumn of 2001 a small socio-acoustic survey was undertaken in an affluent Oslo city area. A telephone survey provided data on neighborhood quality and environmental annoyances from 400 respondents after a response rate of 38%. The Nordic calculation method was employed to estimate 24h LAeq noise exposure values in front of the most exposed facade of each resident’s apartment, and for various neighborhood locations. The quality of the neighborhood soundscape of each dwelling (and respondent) was thereafter characterized by a simple indicator of the maximum of these exposure values within a radius of 75 meters. Ordinal logit models were used to model people’s noise annoyance when at home as a function of the noise level outside the most exposed facade and the modifying impact of the neighborhood soundscape. The analyses using data from the Oslo West city area thus replicate previous analyses undertaken in Oslo East and Drammen. However, the Oslo West questionnaire also included simple questions about the residents’ perceptions of their neighborhood soundscape, and their use of their neighborhood area. The inclusion of these questions made it possible to analyze the correspondence between the estimated neighborhood soundscape quality and people’s assessment of their noise neighborhood.
Dancing the aerobics ‘‘hearing loss’’ choreography
Authors: Pinto Beatriz M. , Carvalho Antonio P. O. , Gallagher Sergio
Published: October 2013

This paper presents an overview of gymnasiums’ acoustic problems when used for aerobics exercises classes (and similar) with loud noise levels of amplified music. This type of gymnasium is usually a highly reverberant space, which is a consequence of a large volume surrounded by hard surfaces. A sample of five schools in Portugal was chosen for this survey. Noise levels in each room were measured using a precision sound level meter, and analyzed to calculate the standardized daily personal noise exposure levels (LEP,d). LEP,d values from 79 to 91 dB(A) were found to be typical values in this type of room, inducing a health risk for its occupants. The reverberation time (RT) values were also measured and compared with some European legal requirements (Portugal, France, and Belgium) for nearly similar situations. RT values (1 kHz) from 0.9 s to 2.8 s were found. These reverberation time values clearly differentiate between good and acoustically inadequate rooms. Some noise level and RT limits for this type of environment are given and suggestions for the improvement of the acoustical environment are shown. Significant reductions in reverberation time values and noise levels can be obtained by simple measures.
Noise levels in the learning–teaching activities in a dental medicine school
Authors: Matos Andreia , Carvalho Antonio P. O. , Fernandes Joao C. S.
Published: October 2013

The noise levels made by different clinical handpieces and laboratory engines are considered to be the main descriptors of acoustical comfort in learning spaces in a dental medicine school. Sound levels were measured in five types of classrooms and teaching laboratories at the University of Porto Dental Medicine School. Handpiece noise measurements were made while instruments were running free and during operations with cutting tools (tooth, metal, and acrylic). Noise levels were determined using a precision sound level meter, which was positioned at ear level and also at one-meter distance from the operator. Some of the handpieces were brand new and the others had a few years of use. The sound levels encountered were between 60 and 99 dB(A) and were compared with the noise limits in A-weighted sound pressure level for mechanical equipments installed in educational buildings included in the Portuguese Noise Code and in other European countries codes. The daily personal noise exposure levels (LEP,d) of the students and professors were calculated to be between 85 and 90 dB(A) and were compared with the European legal limits. Some noise limits for this type of environment are proposed and suggestions for the improvement of the acoustical environment are given.
Acoustic effects of speaking rate changes in articulatory models
Authors: Berry Jeff
Published: October 2013

Speaking rate effects on articulatory phasing, velocity, and magnitude have been reported in the speech-kinematic literature. Yet little is known about the acoustic consequences of manipulating these dimensions. Using an articulatory modeling method developed by Milenkovic, simulations of the acoustic effects of changes in articulatory phasing, velocity, and magnitude have been explored. Articulatory models have been optimized for speakers from the University of Wisconsin X-Ray Microbeam Speech Production Database. Preliminary work has revealed that models of rate change which use only a single dimension (e.g., phasing) may reveal less plausible acoustic solutions than models involving multiple dimensions. Using sample productions of ‘‘a boy’’ to determine articulatory parameter time histories, the acoustic consequences of rate change simulations have been described by the F2 value at onset (F2on), the maximum F2 value (F2tar), and the shape of the F2 trajectory throughout the diphthong. Speaking rate models that combine velocity and phase changes to articulatory parameters result in a reduction in the magnitude of the F2on effect, near elimination of any effect on F2tar, and a relative increase in the homogeneity of the F2 trajectory shapes across speaking rates when compared with phase-only models. [Work supported by NIH (NIDCD) P60 DC1409-10.]
Prosodic evaluation of speech
Authors: Hernandez Juan A. , Arechiga Rene O.
Published: October 2013

This work presents English prosodic evaluation advances: specifically intonation and accent as spoken by hispanics. A widely used algorithm Hirst (Momel) [D. J. Hirst and R. Espesser, Travaux de l’Institut de Phontique d’Aix 15, 71–85 (1993)] to determine the Fundamental Frequency contour (F0), based on quadratic regression and splines is reproduced, and an alternative method, with comparable results, based on signal processing is proposed. We are presently experimenting with Hidden Markov Modes for accent evaluation. Intonation is the melody of speech and helps in transmitting emotions and intentions. The feature most commonly associated with intonation is the Fundamental Frequency of the speech signal. The accent refers to the emphasis given to a particular syllable within a word. The features associated with accent are: duration, pitch, and intensity of vocalic nuclei. The Hirst (Momel) algorithm is divided into five steps: preprocessing of Fundamental Frequency, estimation, partition and reduction of target candidates, and interpolation of the curve. The alternative method consists of: Preprocessing of Fundamental Frequency; Detection of voiceless regions; Fill-up of voiceless regions; Digital filtering, downsampling and upsampling. (To be presented in Spanish.)
Estimating glottal voicing source characteristics by measuring and modeling the acceleration of the skin on the neck
Authors: Cheyne Harold A.
Published: October 2013

Quantifying vocal function is becoming increasingly important both for corroborating clinicians’ subjective impressions during a voice evaluation and for assessing the effectiveness of surgery or voice therapy. This work investigated the extent to which measures of the acceleration of the skin on the neck (near the larynx) could be used to estimate important vocal function parameters. Simultaneous acceleration, acoustic and aerodynamic recordings from one male subject with a normal voice were made to examine relationships between the acceleration signal and the acoustic or aerodynamic signals. A vocal system model was also developed to provide insights into these relationships. The model’s acceleration-derived measures of Maximum Flow Declination Rate (MFDR) and Sound Pressure Level (SPL) were compared to the same measures obtained from the airflow (MFDR) and acoustic (SPL) signals at the lips. Results showed high correlations between the two MFDR (r2=0.89) and SPL (r2=0.75) measures, suggesting that acceleration-based measures have the potential to provide quantitative data on vocal function. [Funding for this research was provided by funding from NIDCD Grant No. R21 DC05397 and SBIR Grant No. R44 DC02917.]
Jitter, shimmer, and noise in pathological voice quality perception
Authors: Kreiman Jody , Gerratt Bruce R. , Gabelman Brian
Published: October 2013

Although jitter, shimmer, and noise characterize all voice signals, their perceptual importance has not been established psychoacoustically. To determine which of these acoustic attributes is important in listeners’ perceptions of the inharmonic component in pathologic voices, copies of natural pathological voices were synthesized parametrically using seven different models of the inharmonic part of the voice source: jitter only, shimmer only, noise only, jitter plus shimmer, jitter plus noise, shimmer plus noise, and jitter plus shimmer plus noise. Listeners then compared pairs of these stimuli and judged whether they were the same or different. Listeners also compared synthetic and natural stimuli, to determine the extent to which different aspects of the inharmonic part of the source improved or worsened the quality of the match to the original natural voice sample. Preliminary results suggest that jitter and shimmer may not be perceptually important features of most pathological voices, despite their long history as acoustic measures of voice. In contrast, it appears that listeners perceive spectrally shaped additive noise as the critical inharmonic acoustic element contributing to pathologic voice quality. [Research supported by NIDCD.]
Detection improvement using multiple time-reversal guide-sources in shallow water
Authors: Gaumond Charles F. , Fromm David M.
Published: October 2013

Detection in a monostatic broadband, active sonar system in shallow water is degraded by propagation induced spreading. The method of multiple guide sources has been presented as a method to mitigate this degradation. This paper shows the detection improvement from the use of the separable kernel receiver (SKR) that is based on a statistical behavior of the ensemble of multiple guide-source signals, which provide several one-way propagation signals from guide sources at various ranges at a single depth. A measure of information is derived to quantify the variability of these guide-source signals. The improvement of detection is shown as a function of range and depth for several sets of system and environmental parameters. In particular, two environments are shown. One is the case of farther ranges and lower frequencies in shallow water where the propagation causes dispersion. The other is the case of shorter ranges and higher frequencies in shallow water where the propagation effects are predominantly multipath. [Work supported by ONR.]
Multiple focusing with adaptive time-reversal mirror
Authors: Kim J. S. , Shin K. C.
Published: October 2013

Recently, adaptivity was introduced to time-reversal mirror (TRM) to steer the nulls, and referred to as an adaptive time-reversal mirror (ATRM) [J. S. Kim, H. C. Song, and W. A. Kuperman, J. Acoust. Soc. Am. 109, 1817–1825 (2001)]. In this study, ATRM is extended to simultaneous multiple focusing in a free space as well as in a wave guide. The multiple focusing is achieved by imposing a set of constraints in the formulation for finding the weight vectors. The algorithm is applied to the long-range underwater acoustic communication to show that the self-equalizations at multiple receiving locations are achieved. Also the size of resolution cell, where the distortionless response (or, self-equalization) is approximately achieved, is discussed. [Work supported by Grant No. R05-2002-000-01097-0 from the basic Research Program of the Korea Science and Engineering Foundation.]
Nonlinear internal waves in the South China Sea during ASIAEX
Authors: Liu Antony , Tang David , Ramp Steve
Published: October 2013

Internal wave distribution maps have been compiled from more than 100 ERS-1/2, RADARSAT and Space Shuttle SAR images in the South China Sea (SCS) from 1993 to 2000. Based on these distribution maps, most of the internal waves in the northeast part of the SCS were propagating westward. The wave crest can be as long as 200 km with an amplitude of 100 m, due to strong current from the Kuroshio branching out into the SCS. In the recent Asian Seas International Acoustics Experiment (ASIAEX), moorings have been deployed in April 2000 and May 2001. Simultaneous RADARSAT ScanSAR images have been collected during the field test to integrate with the model and the in-situ measurements in the SCS. During ASIAEX in May 2001, many large internal waves were observed at the test area and were the major features for acoustic volume interaction. The evolution and dissipation of huge internal waves on the shelf break, mode-two waves, and wave–wave interaction are very important issues for acoustic propagation. Numerical simulations have been performed to produce the wave evolution on the continental shelf and compared with the field measurements. The implication of internal wave effects on acoustic propagation will also be discussed. [Work supported by ONR.]
Tracking the generation sites and packet variability of internal solitons in the South China Sea
Authors: Ramp Steven R. , Chiu Ching Sang , Bahr Frederick L. , Lynch James , Duda Tim , Tang David , Liu Anthony K.
Published: October 2013

An array of seven moorings were deployed across the Chinese continental shelf and slope near Dongsha Island as part of the ONR-sponsored Asian Seas International Acoustics Experiment (ASIAEX). The moorings spanned 82- to 350-m depth and sampled temperature, salinity, and velocity at 1- or 2-min intervals from 21 April to 19 May 2001. The most energetic features observed in the data set were the highly nonlinear internal solitary waves. This paper attempts to identify the generation sites for these waves and determine their predictability. A statistical analysis was performed on the wave orbital velocities to determine their direction of propagation, and inversely, their sources. The bimodal directional histograms point to two generation sites: The largest waves came from around the Batan Islands in the Luzon Strait, and the smaller ones from near Dong-sha Island. The large wave arrivals were semidiurnal with a very strong diurnal inequality, such that the strongest waves arrived every 24 h. The wave packets arrived in two groups separated by a 5-day period in between when weak or no waves arrived. The wave groups had an unusual and counterintuitive relationship to the barotropic tide, and two hypotheses will be advanced to explain this phenomenon.
Environmental correlate of acoustic impulse response and arrivals with solibore propagation in ASIAEX SCS experiment
Authors: Chen Chi-Fang , Lin Ying-Tsong , Lin Wen-Fei , Tang David , Yang Yiing-Jang , Lynch James , Chiu Chin-Sang , Ramp Steven
Published: October 2013

An ASIAEX South China Sea experiment was conducted in April–May 2001. The experiment site is characterized as an active internal tide/solibore propagation region along the northwestern shelf break of the South China Sea. On May 4–5, one package of solibore propagated through the sources and receiver arrays as clearly seen from the satellite image and the temperature time series recorded in the environmental moorings. The propagation path of the solibore is reconstructed using the data from the environmental moorings. In this paper we also present results of the acoustic impulse responses and the arrivals of the same time as the solibore passing through. Correlations of the acoustic impulse responses and the arrivals with the solibore propagation show the solibore’s effect on the acoustic propagation when the solibore enters the acoustic propagation paths. [Work supported by the National Science Council of Republic of China for Taiwanese authors. Work supported by ONR, for the U.S. authors.]
Soliton-induced matched-field array-gain variability in the South China Sea
Authors: Mignerey Peter C. , Orr Marshall H. , Pasewark Bruce H. , Duda Timothy F. , Lynch James F. , Chiu Ching-Sang
Published: October 2013

Large amplitude nonlinear internal-wave packets propagated across the acoustic signal propagation path at the ASIAEX experimental site. The wave packets perturbed the sound-speed field and caused temporal variability in the phase of narrow-band (224, 300, 400, and 500 Hz) acoustic signals propagating along upslope and cross slope paths. The acoustic signals were received on a vertical hydrophone array. The signal phase variability caused degradation and temporal variability in the signal gain of a matched-field processor applied to the data. The temporal variability of the sound-speed field along the signal propagation paths has been simulated using field CTD and flow visualization measurements to initialize the sound-speed field. A matched field processor has been applied to the modeled acoustic field. The calculated gain degradation will be compared to field data and inferences will be drawn concerning our ability to simulate the sound speed field variability. [Work supported by ONR.]
Remote estimation of seabed attenuation using a chirp sonar during ASIAEX
Authors: Schock Steven G.
Published: October 2013

Acoustic remote sensing of the seabed using a chirp sonar generates estimates of the attenuation coefficient of sediments in the South and East China Seas. The chirp sonar transmits FM pulses over the band of 1 to 15 kHz at normal incidence to the seabed. After cross correlating the acoustic returns with the transmission wave form, a wideband reflection profile is generated showing sediment layering. The time series data is filtered into 11 pass bands to generate 11 images with center frequencies of 1 kHz spacing starting at 2 kHz and ending at 12 kHz. The attenuation coefficient of the surficial layer is estimated by dividing the average reflection amplitude of the sediment layer interface echo from the base of the surficial layer by the average amplitude of the sediment-water interface echo for each bandpass image. The amplitude ratios are plotted as a function of frequency. The slope of a line fitted to the frequency function in a least squares sense is called the estimated attenuation coefficient. The estimates are useful for sediment classification and aocustic propagation modeling.
Statistical analysis of low-frequency noise recorded in ASIAEX by a PANDA system
Authors: Potter John R. , Beng Koay T. , Pallayil Venugopalan
Published: October 2013

As part of the ASIAEX experiment, the Acoustic Research Laboratory in the Tropical Marine Science Institute at the National University of Singapore deployed a Pop-up Ambient Noise Data Acquisition (PANDA) system. The PANDA was recovered 18 days later with over 9 days of continuous data recorded from a single hydrophone at 2 kSa/s. The data show the various sources that were deployed as part of the experiment, but also provide interesting statistical information on low-frequency ambient noise in the region and the passage of numerous ships. This deployment was in a heavy shipping traffic area, hostile both in terms of potential snagging by fishing activity and in terms of the high levels of noise encountered, both of which are of interest for the deployment and successful use of autonomous acoustic systems in busy littoral waters. We present some statistical results from the 3+ GByte of data. [Work supported by the Defence Science and Technology Agency, Singapore and the US ONR.]
Applications of source localization by waveguide invariant theory during the ASIAEX sea tests
Authors: Turgut Altan , Wolf Stephen , Pasewark Bruce , Orr Marshall , Lynch James
Published: October 2013

Applications of source localization in range and azimuth with minimum environmental information have been investigated by using a sample data set from the ASIAEX vertical and horizontal arrays. Source localization by waveguide invariant theory has been successfully demonstrated at the ASIAEX experimental site by using broadband (50–200 Hz and 240–260 Hz) signals transmitted from a towed J-15 source. Range and bearing of both J-15 transducer and other surface ships in the vicinity have been successfully identified using a reference signal from the J-15 source at a known range. Accuracy of the localization has been also investigated for both adiabatic and strongly range-dependent environments. It has been shown that, when nonlinear internal waves are not present, the assumption of adiabatic propagation is sufficient for the accurate localization of sources from 0–10 km ranges at the experimental site. [Work supported by ONR.]
Coupled-mode solutions in the complex plane and benchmarking with the parabolic equation method
Authors: Stotts Steven A. , Knobles David P. , Koch Robert A.
Published: October 2013

A complex plane extension of a previously developed two-way coupled mode model is presented. Coupling coefficients based on horizontal layer propagators using complex Airy solutions are evaluated analytically [Stotts, J. Acoust. Soc. Am. 111, 1623–1643 (2002)]. This approach significantly reduces memory storage requirements and improves the computational efficiency over numerical depth integration for the coefficients. The integral equation method introduced by Knobles [J. Acoust. Soc. Am. 96, 1741–1747 (1994)] is used to solve the coupled equations. Benchmarking with the parabolic equation (PE) approach will be presented for several examples including the ASA continental shelf benchmark and up-slope/down-slope wedge propagation. Since Tappert’s introduction, PE has been accepted as the de-facto model for range variable calculations. Nevertheless, having an independent verification by a normal mode model for general environments is advantageous. Differences between real and complex k-plane solutions are emphasized. Comparisons to numerically calculated key quantities are presented. Application to a range-dependent data set recorded recently off the coast of Florida is discussed.
Dependence of reverberant energy on source distance in enclosures
Authors: Larsen Erik , Feng Albert S.
Published: October 2013

It is known that the direct-to-reverberant energy ratio has a strong influence on speech intelligibility, and also on the performance of adaptive beamforming algorithms. It is also a cue for source distance perception. According to traditional acoustic assumptions, reverberation is homogenous and isotropic, yielding a decay of −6 of direct-to-reverberant energy ratio per doubling of source distance. However, the extent to which real-world data deviates from this ideal prediction and the underlying physical parameters that affect the outcome are unknown. A series of experiments and simulations were conducted in which the reverberant energy was determined at various distances from the sound source, in various listening rooms. It was found that reverberant energy decreased systematically with an increase in source distance, while reverberation time increased orderly. Also, source distance at which direct-to-reverberant energy ratio was zero corresponded well with the critical distance as given by standard acoustic theory. The decay of reverberant energy with distance varied for the different rooms tested, roughly between −1 and −3 dB per doubling of source distance. An evaluation of these results in terms of the room’s acoustic parameters will be given.
The effects of stage configurations in a recital hall
Authors: Yang Wonyoung , Hodgdon Kathleen K. , Tichy Jiry
Published: October 2013

The room acoustical parameters, the reverberation time (RT), the early decay time (EDT), the bass ratio (BR), the clarity factor (C80), the initial time delay gap (ITDG), and the interaural cross-correlation coefficient (IACC) were measured for acoustical evaluation both in the audience seats and on the stage, using six different stage configurations in a 450-seat unoccupied hall. The stage configurations consisted of combinations of varying the position of the drapes with and without the reflector at the back of the stage. The listening test was conducted in a laboratory using high fidelity headphones to verify the subjective preference for the seats and the presence of the reflector. Analysis showed that the draped area of the stage was related to the RT, EDT, C80, BR, while the ITDG was strongly dependent on the presence of the reflector. There was not a strong impact observed in the IACC for the varying configurations. In general, the reflectors improved most of the acoustical parameters. The loudness of the musical sound was the primary parameter used to decide the subjective preference, which correlated with the C80 and the ITDG with the reflector.
Measures of uncertainty in geoacoustic inversion
Authors: Chapman N. Ross
Published: October 2013

Inversion methods for estimating geoacoustic model parameters from acoustic field data have been applied with considerable success over the past decade. The most effective methods that have been developed generally fall into two categories, nonlinear methods that are posed as optimization problems, and linearized methods that invert quantities such as horizontal wave numbers that are derived from the acoustic field data. For either case, the complete solution of the geoacoustic inverse problem requires a measure of the error for the estimated parameter, as well as the estimate itself. This paper describes an approach for specifying an error measure in nonlinear inversion processes based on matched field processing. The inversion uses an optimization algorithm that combines global and local search processes to sample the model parameter space. An effective error measure is obtained from information in scatter plots of the cost function versus parameter values for models that were tested in the search process. Examples are presented to demonstrate the application of the method to test cases from the recent Geoacoustic Inversion Benchmark Workshop, and from experimental data from vertical line arrays and seafloor horizontal arrays. [Work supported by ONR.]
Frechet derivatives for shallow water ocean acoustic inverse problems
Authors: Odom Robert I.
Published: October 2013

For any inverse problem, finding a model fitting the data is only half the problem. Most inverse problems of interest in ocean acoustics yield nonunique model solutions, and involve inevitable trade-offs between model and data resolution and variance. Problems of uniqueness and resolution and variance trade-offs can be addressed by examining the Frechet derivatives of the model-data functional with respect to the model variables. Tarantola [Inverse Problem Theory (Elsevier, Amsterdam, 1987), p. 613] published analytical formulas for the basic derivatives, e.g., derivatives of pressure with respect to elastic moduli and density. Other derivatives of interest, such as the derivative of transmission loss with respect to attenuation, can be easily constructed using the chain rule. For a range independent medium the analytical formulas involve only the Green’s function and the vertical derivative of the Green’s function for the medium. A crucial advantage of the analytical formulas for the Frechet derivatives over numerical differencing is that they can be computed with a single pass of any program which supplies the Green’s function. Various derivatives of interest in shallow water ocean acoustics are presented and illustrated by an application to the sensitivity of measured pressure to shallow water sediment properties. [Work supported by ONR.]