Heui‐Seol Roh

Acoustic wave propagation in bovine cancellous bone: Application of the Modified Biot–Attenborough model

Acoustic wave propagation in bovine cancellous bone is experimentally and theoretically investigated in the frequency range of 0.5-1 MHz. The phase velocity, attenuation coefficient, and broadband ultrasonic attenuation (BUA) of bovine cancellous bone are measured as functions of frequency and porosity. For theoretical estimation, the Modified Biot-Attenborough (MBA) model is employed with three new phenomenological parameters: the boundary condition, phase velocity, and impedance parameters. The MBA model is based on the idealization of cancellous bone as a nonrigid porous medium with circular cylindrical pores oriented normal to the surface. It is experimentally observed that the phase velocity is approximately nondispersive and the attenuation coefficient linearly increases with frequency. The MBA model predicts a slightly negative dispersion of phase velocity linearly with frequency and the nonlinear relationships of attenuation and BUA with porosity. The experimental results are in good agreement with the theoretical results estimated with the MBA model. It is expected that the MBA model can be usefully employed in the field of clinical bone assessment for the diagnosis of osteoporosis.

Correlations between acoustic properties and bone density in bovine cancellous bone from 0.5 to 2 MHz

Correlations between acoustic properties and bone density were investigated in the 12 defatted bovine cancellous bone specimens in vitro. Speed of sound (SOS) and broadband ultrasonic attenuation (BUA) were measured in three different frequency bandwidths from 0.5 to 2 MHz using three matched pairs of transducers with the center frequencies of 1, 2.25, and 3.5 MHz. The relative orientation between ultrasonic beam and bone specimen was the mediolateral (ML) direction of the bovine tibia. SOS shows significant linear positive correlation with apparent density for all three pairs of transducers. However, BUA shows relatively weak correlation with apparent density. SOS and BUA are only weakly correlated with each other. The linear combination of SOS and BUA in a multiple regression model leads to a significant improvement in predicting apparent density. The correlations among SOS, BUA, and bone density can be effectively and clearly represented in the three-dimensional space by the multiple regression model. These results suggest that the frequency range up to 1.5 MHz and the multiple regression model in the three-dimensional space can be useful in the osteoporosis diagnosis.

Passive acoustic threat detection in estuarine environments

The Maritime Security Laboratory (MSL) at Stevens Institute of Technology supports research in a range of areas relevant to harbor security, including passive acoustic detection of underwater threats. The difficulties in using passive detection in an urban estuarine environment include intensive and highly irregular ambient noise and the complexity of sound propagation in shallow water. MSL conducted a set of tests in the Hudson River near Manhattan in order to measure the main parameters defining the detection distance of a threat: source level of a scuba diver, transmission loss of acoustic signals, and ambient noise. The source level of the diver was measured by comparing the divers sound with a reference signal from a calibrated emitter placed on his path. Transmission loss was measured by comparing noise levels of passing ships at various points along their routes, where their distance from the hydrophone was calculated with the help of cameras and custom software. The ambient noise in the Hudson River was recorded under varying environmental conditions and amounts of water traffic. The passive sonar equation was then applied to estimate the range of detection. Estimations were done for a subset of the recorded noise levels, and we demonstrated how variations in the noise level, attenuation, and the divers source level influence the effective range of detection. Finally, we provided analytic estimates of how an array improves upon the detection distance calculated by a single hydrophone.

Parallel capillary-tube-based extension of thermoacoustic theory for random porous media

Thermoacoustic theory is extended to stacks made of random bulk media. Characteristics of the porous stack such as the tortuosity and dynamic shape factors are introduced into the thermoacoustic wave equation in the low reduced frequency approximation. Basic thermoacoustic equations for a bulk porous medium are formulated analogously to the equations for a single pore. Use of different dynamic shape factors for the viscous and thermal effects is adopted and scaling using the dynamic shape factors and tortuosity is demonstrated. Comparisons of the calculated and experimentally derived thermoacoustic properties of reticulated vitreous carbon and aluminum foam show good agreement. A consistent mathematical model of sound propagation in a random porous medium with an imposed temperature is developed. This treatment leads to an expression for the coefficient of the temperature gradient in terms of scaled cylindrical thermoviscous functions.

Acoustic diagnosis for porous medium with circular cylindrical pores.

Acoustic transmission coefficient and phase velocity of a Lucite slab with circular cylindrical pores with a nonrigid pore frame were experimentally and theoretically investigated. For theoretical investigation a new phenomenological model, the modified Biot-Attenborough (MBA) model, was proposed. The MBA model takes into account both the first kind and the second kind of waves introduced by Biot. It also separately considers viscous and thermal effects with three new phenomenological parameters: boundary, phase velocity, and impedance parameters. The theoretical estimation with three phenomenological parameters shows reasonably good agreement with the experimental data. The physical characteristics of porous medium such as porosity and pore size can be inversely analyzed in terms of the acoustic data such as the transmission coefficient and phase velocity as the functions of porosity and frequency. This makes acoustic diagnosis possible for noninvasively investigating physical characteristics of porous media such as bones and ocean sediments.

Acoustic measurements of bubbles in the wake of ship model in tank

The interest to bubble generation by moving ships is connected with large area of bubble wake reaching several kilometers that can be used for ship detection. We developed acoustic system for the measurements of bubble density and conducted measurements in 100 m long towing tank. The developed system measured attenuation of ultrasound in wide frequency band from 50 to 800 kHz between two acoustic sensors placed on the distance 20cm. The attenuation of sound produced by bubbles was observed during several minutes after model of ship passed the point of measurement. The attenuation was recalculated to the bubble size distribution for bubbles from 4 to 65 microns using theory of resonance bubble attenuation. The measured bubble size distribution can be interpolated by power dependence n(R) R (‐3.5) that is typical for bubbles at sea subsurface layer. The dependencies of bubble concentration of model ship speed and type of propeller were investigated. The generation of bubbles was observed when the model sp...

Determination of acoustic attenuation in the Hudson River Estuary by means of ship noise observations

Analysis of sound propagation in a complex urban estuary has application to underwater threat detection systems, underwater communication, and acoustic tomography. One of the most important acoustic parameters, sound attenuation, was analyzed in the Hudson River near Manhattan using measurements of acoustic noise generated by passing ships and recorded by a fixed hydrophone. Analysis of the ship noise level for varying distances allowed estimation of the sound attenuation in the frequency band of 10-80 kHz. The effective attenuation coefficient representing the attenuation loss above cylindrical spreading loss had only slight frequency dependence and can be estimated by the frequency independent value of 0.058 dBm.

Acoustic noise produced by ship traffic in the Hudson River estuary

This paper presents results of measurements of acoustic noise in Hudson River Estuary near Manhattan in the frequency band 10–100 kHz. The Estuary has very complex sound propagation conditions due to the extremely shallow and high time‐ and space variability of the water characteristics. The acoustic noise was recorded by a set of hydrophones and the acoustic measurements were accompanied by ship traffic video recording using the video‐based Surface Traffic Surveillance system. This video system allowed us to map various boats and ships and to find distances between them and the hydrophone system. The measurements provided acoustic noise data for different kinds of ships in Hudson River, their dependencies on frequencies, and distances. The measurements of noise for various distances were applied for estimation of sound attenuation in a wide frequency band. We calculated the sound attenuation coefficient showing the attenuation loss of an acoustic signal in addition to cylindrically spreading loss. The re...

Estimation of passive acoustic threat detection distances in estuarine environments

The Maritime Secure Laboratory (MSL) at Stevens Institute of Technology supports research in a range of areas relevant to harbor security, including passive acoustic detection of underwater threats. The difficulties in using passive detection in an urban estuarine environment include intensive and highly irregular ambient noise and the complexity of sound propagation in shallow water. MSL measured the main parameters defining the detection distance of a threat: source level of a scuba diver, transmission loss of acoustic signals, and ambient noise. The source level of the diver was measured by comparing the divers sound with a reference signal from a calibrated emitter placed on his path. Transmission loss was measured using the transmission of a sweep signal (1–100 kHz) from the calibrated emitter. The passive sonar equation was then applied to estimate the range of detection. Estimations were done for various recorded noise levels, demonstrating how fluctuations in noise level and the mobility of the diver influence the effective range of detection. Finally, analytic estimates of how a hydrophone array improves upon the detection distance calculated by a single hydrophone are shown. [This work was supported by ONR project No. N00014‐05‐1‐0632: Navy Force Protection Technology Assessment Project.]

Capillary‐tube‐based extension of thermoacoustic theory for a random medium

Thermoacoustic theory for a single capillary tube is extended to random bulk medium on the basis of capillary tubes. The characteristics of the porous stack inside the resonator such as the tortuosity, dynamic shape factor, and porosity are introduced for the extension of wave equation by following Attenborough’s approach. Separation of the dynamic shape factor for the viscous and thermal effect is adopted and scaling using the dynamic shape factor and tortuosity factor is demonstrated. The theoretical and experimental comparison of thermoviscous functions in reticulated vitreous carbon (RVC) and aluminum foam shows reasonable agreement. The extension is useful for investigations of the properties of a stack with arbitrary shapes of non‐parallel pores.