Eunghwy Noh

Validation of Two-dimensional Performance of Low-frequency Active Echo Reduction Technique

In an earlier paper we showed that the active control technique using tile projectors was effective in reducing low-frequency echoes from an underwater object. But the validation of the technique was limited to only one dimension in space. To fully evaluate the echo reduction performance, it is necessary to compare the acoustic field before and after the control in at least two dimensions. In this paper, expanding upon our earlier study, we evaluate the performance of the aforementioned technique via two-dimensional scans of the acoustic field. The measurements show that one could achieve at least 6 dB reduction in echo strength throughout the entire scan range.

Active control of low-frequency underwater echoes using an array of tile projectors

With the introduction of low-frequency active sonar in anti-submarine warfare, there is a growing need for a novel underwater invisibility device that could replace the existing passive anechoic tiles. In this talk, we describe an experiment on active reduction of underwater echoes at low frequencies using an array of tile-shaped projectors. Each tile projector was designed with the aid of finite-element computations and tested in an acoustic tank for transmit voltage sensitivity and directivity. An array of tile projectors, covering a scale model submarine in a large acoustic tank, were driven by control signals that were intended to produce the impedance match between water and the object. Depending on the frequency of the incident wave, echo reduction as large as 6 dB was achieved. [This work has been supported by the Low Observable Technology Research Center program of Defense Acquisition Program Administration and Agency for Defense Development.]

Optimizing an underwater transmit array of projectors with large variations

Uniformity of constituent projector elements is a prerequisite for building a high-performance underwater transmit array. Unless the element-to-element variations in transmitting voltage response (TVR) and directivity fall within specified limits, the resulting acoustic performance of the array will suffer significantly in terms of its sound output and directivity. Here, we consider a case in which projector elements exhibit a large scatter in TVR, say, over 6 dB, and present a design guide to still get the most of the array. The focus of the study is to explore how the relative placement of projector elements affects their mutual interaction and ultimately leads to the overall characteristics of the array. Finite element computations are carried out to derive an optimized design and the design guide. [This work has been supported by the Low Observable Technology Research Center program of Defense Acquisition Program Administration and Agency for Defense Development.]

Comparison of computational schemes in the design of a large-scale projector array

Modeling and simulation for the design of a projector array involves computations in the transducer and acoustic domains. In the transducer domain, each transducer is often described by the two-port, equivalent-circuit model, in which the radiation impedance containing the influence of the neighboring transducers as well as the medium is not known a priori. To obtain the radiation impedance and the subsequent transducer response a set of coupled transducer equations must be solved simultaneously in conjunction with the (computationally expensive) wave-field calculation in the acoustic domain. In this talk, we compare two computational schemes for speed, which differ in the way the mutual interaction of transducers is treated: the first scheme explicitly accounts for the mutual interaction in terms of the mutual impedance, whereas in the second scheme the interaction is reflected implicitly in the iterative solution of the transducer equations. Here, the speed is largely determined by the number of wave-fi...

Investigation of nonlinearity parameter B/A of saturated, unconsolidated marine sediments via a statistical approach

In this talk, we present a theory and the supporting experiment for the nonlinearity parameter B/A of saturated, unconsolidated marine sediments. The model is composed of the quadratic nonlinearity of the equivalent suspension of grains and the interstitial fluid and the nonlinearity resulting from the random grain contacts. To capture the random nature of grain contacts, the number of Hertzian contacts on each grain and the distribution of contact forces between grains are treated statistically. Predictions of B/A are compared with measurements performed with the finite amplitude insert-substitution (FAIS) method. Dependence of B/A on external compression and the incident sound pressure level is also discussed. [This work was conducted in the Unmanned Technology Research Center (UTRC) sponsored by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD) in the Republic of Korea.]

Conceptual Design of Cylindrical Hydrophone Arrays for Stabilization of Receiving Characteristics under Ocean Ambient Noise

An underwater sound surveillance system detects and tracks enemy ships in real-time using hydrophone arrays, in which seabed-mounted sensor arrays play a pivotal role. In this paper the conceptual design of seabed-mounted, cylindrical hydrophone arrays for use in shallow coastal waters is performed via finite element calculations. To stabilize the receiving characteristics under the ocean ambient noise, a technique for whitening the ambient noise spectrum using a metal baffle is proposed. Optimization of the array configuration is performed to achieve the directivity in the vertical and azimuthal directions. And the effects of the sonar dome shape and material on the structural vibration and sound scattering properties are studied. It is demonstrated that a robust hydrophone array, having a sensitivity deviation less than 4 dB over the frequency range of interest, can be obtained through the whitening of the ambient noise, the optimization of the array configuration, and the design of acoustically transparent sonar domes.

Derivation of the nonlinearity parameter B/A of saturated, unconsolidated marine sediments via a statistical approach

Marine sediments are known to exhibit the nonlinearity parameter B/A that is much greater than that of homogeneous fluids and solids. The enormous value of B/A is particularly sensitive to the state of grain contacts. Although there exists a large body of literature that deals with the B/A of marine sediments, few studies have addressed how the random nature of grain contacts affects B/A. Here, we develop a model for the B/A of saturated, unconsolidated marine sediments, which accounts for random grain contacts. The number of contacts per grain and the distribution of contact stress between grains are treated statistically. The quadratic nonlinearity of the equivalent suspension of grains and the interstitial fluid is then combined with the nonlinearity arising from the random Hertzian contacts to produce the effective B/A of marine sediments.

Comparison between the Biot-Stoll model, the grain-shearing model, and the effective density fluid model with respect to sediments in the Yellow Sea

There are some distinctive geophysical properties of sediments in the Yellow Sea, where mudflats are developed extensively. To select a physical model for acoustic wave propagation in the sediments is a critical issue in the analysis and the conceptual design of buried object scanning sonars. We consider three different sediment models to describe physical and geoacoustic properties of the sediments in the Yellow Sea: the Biot-Stoll model, the grain-shearing model, and the effective density fluid model (EDFM). These sediment models are evaluated in terms of their predictions of sediment properties compared with the measurement data. [This work was conducted in the Unmanned Technology Research Center (UTRC) sponsored by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD) in the Republic of Korea.]

Transducer models for simulating detection processes for underwater mining

Numerical simulations on propagating and scattering processes of sound waves in water and sediment may be useful for designing a detection system for underwater mining. Here a transducer model is developed for the numerical simulation to implement radiating and receiving processes of transducers into numerical calculations. Since the Rayleigh integral approach is adopted for acoustic radiation, the accurate velocity profiles over the radiating surfaces of a transducer array should be estimated considering the mechano-acoustic interactions including the dynamics of unit drivers and the acoustic radiation loadings on the radiation surfaces. We adopted the approach that the surface velocity is calculated using the transducer model with the acoustic loading while the loading effects are estimated via calculating the radiation impedance of transducer array using Rayleigh integrals. The estimated velocity profile of the transducer surface is used for calculating the accurate sound fields generated by the transd...

A modeling and simulation suite for design of buried object scanning sonars

In this talk we highlight a work in progress, concerning the development of a comprehensive modeling and simulation (M&S) suite for design of buried object scanning sonars. The M&S suite is expected to cover almost all aspects of physical and engineering acoustics involved in the design process, ranging from transducers, sound propagation, sediment acoustics, backscattering by buried objects, to sonar image processing. The overview of the M&S suite is given along with a preliminary demonstration in the context of a cylindrical object buried in sandy sediment. [This work was conducted in the Unmanned Technology Research Center (UTRC) sponsored by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD) in the Republic of Korea.] In this talk we highlight a work in progress, concerning the development of a comprehensive modeling and simulation (M&S) suite for design of buried object scanning sonars. The M&S suite is expected to cover almost all aspects of physical and engineering acoustics involved in the design process, ranging from transducers, sound propagation, sediment acoustics, backscattering by buried objects, to sonar image processing. The overview of the M&S suite is given along with a preliminary demonstration in the context of a cylindrical object buried in sandy sediment. [This work was conducted in the Unmanned Technology Research Center (UTRC) sponsored by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD) in the Republic of Korea.]