Hydroacoustic Devices Research Articles

Гидроакустические технологии дистанционного управления поведением рыб и щадящего отвода морских млекопитающих от орудий лова

The results of the development of hydroacoustic devices for the direction and retention of fish during their fishing and scaring of pinnipeds from fishing gear are presented. The devices allow generating underwater sounds that simulating the biological signals of predatory cetaceans. The characteristics of the sounds emitted and the methods of their application to increase the efficiency of fishing is shown. The results of experimental studies of the assessment of fish and pinnipeds reaction to the signals of hydroacoustic emitters - imitators of killer whales sounds are overviewed.

Investigation of the Variability Characteristics of Sound Waves Reflected on the Sea Surface Considering Surface Wavelength

Many kinds of hydroacoustic devices are used in the field of ocean development of resources and observations. The effects of sound reflection from the sea surface are often evaluated in estimating the performance of these devices and in analyzing acoustic data obtained in actual seas. When sea surface waves are sufficiently calm, the assumption of specular reflection of sound waves is valid for the estimation of acoustic propagation characteristics. On the other hand, actual surface waves change randomly over time because of wind and gravity, and consequently reflected sound waves also fluctuate randomly. In this study, we evaluated the variability characteristics of reflected sound waves from the sea surface by acoustic simulation using the finite-difference-time-domain method. Our results clarify the effect of the wave height and wavelength at the sea surface on the variability characteristics of reflected sound waves.

Hydroacoustic Bottom Soil Profiling Device with Synthetic Aperture

Introduction. Acoustic profiling is a conventional method for studying the geological structure of the seabed. To this end, a low-frequency acoustic profiler with an operating frequency range of 1…14 kHz can be used. However, under lower operating frequencies, the difficulty of achieving the required resolution arises. The problem of improving the angular resolution of a hydroacoustic device, particularly in the direction of the carrier movement, remains to be a priority task in the search and detection of objects on the seabed, as well as the study of the bottom soil structure. Angular resolution can be improved through several approaches, including an algorithm for synthesizing the antenna aperture based on the law of the phase change of the reflected signal. This approach is relevant in the design of highresolution hydroacoustic tools.Aim. To demonstrate the possibility of constructing a bottom soil profiling device, as well as the possibility of increasing its angular resolution based on an algorithm for synthesizing the antenna aperture of an acoustic profiler.Materials and methods. The study employed the groundwork data obtained for side-scan sonars with a synthetic aperture in terms of constructing an antenna device and the methods of radio-wave excitation developed for radar systems.Results. The possibility of synthesizing the antenna aperture for an acoustic profiler of the bottom soil was studied. An algorithm for synthesizing the antenna aperture was investigated along with phase distortions of the trajectory signal and their influence on the sonar image. The fundamental principles of processing the trajectory signal were considered.Conclusion. An approach to increasing the resolution of an acoustic profiler when performing search and survey tasks is proposed. The proposed design of the acoustic part of a profiling device can be used in the development of search and survey hydroacoustic devices with a high angular resolution.

Joint Use of Mechatronic Systems to Organize Effective Counteraction to the Coordinated Action of Enemy Torpedoes

The article proposes an approach to the organization of counteraction to the probable enemy on the basis of the joint use of mechatronic devices using the phenomenon of electric explosion to counteract homing systems and solving a group control task in order to protect the target from defeat. The above class of tasks involves the use by the attacked of mechatronic devices that play the role of mobile defenders, in the quality of which can act as a self-propelled source of interference, distracting the attacking object (missile or torpedo) by intercepting it, and a shock RTK aimed at destroying a dangerous object. A characteristic feature of the considered tasks is the incompleteness of a priori information about the distance to the attacking object and the possibility of instant use of active means. It is shown that the presence of even only a second defender is advisable, since it significantly increases the interception time compared to the case of only one defender. Also, an additional defender increases the probability of choosing one of the defenders for the intermediate pursuit instead of the main target. Under the conditions set in the considered models for starting positions, speeds and power reserve, in the case of a real torpedo attack on defenders, when following the constructed schemes for the release of defenders, the torpedo will not have enough fuel to defeat the main target. The methods of interference formation based on electric explosion are considered. It is proposed to use mechatronic hydroacoustic counteraction devices capable of moving in an aquatic environment and creating broadband interference of a given intensity as such sources. A model example of the use of mechatronic devices in a given scenario is considered.

Subwavelength bandgap phononic crystals with mixed resonant modes

A new kind of subwavelength bandgap phononic crystals with water host is proposed. Scatterers of these phononic crystals are Helmholtz resonators decorated by bubbles. The subwavelength ratio can reach 0.005; the relative bandwidth can reach 1.38, which is far larger than those (0.25) of existing subwavelength bandgap phononic crystals with water host. The formation mechanism of these subwavelength band gaps is achieved, i.e., bubbles cause a significant reduction of the elasticity coefficient of springs in the ‘mass-spring’ model. Two broadening mechanisms of subwavelength band gaps are obtained: one is the interaction between scatterers is relatively large, the other is there are several different resonant modes in a band gap. The latter was not mentioned in the previous literature. These conclusions are important to design new types of subwavelength hydroacoustic devices.

Теоретическое и экспериментальное обоснование принципов построения гидролокаторов обзора донной обстановки

The paper investigates the principles of constructing sonars for surveying the bottom situation. The issue of choosing the characteristics of side-scan sonar is considered in order to effectively solve the problems of surveying and searching for objects on the seabed. Possible options for applying methods to reduce the "dead zone" of a hydroacoustic device are considered. The issue of constructing the acoustic part of the sonar in order to increase the resolution is touched upon. A comparative analysis of the performance characteristics of hydroacoustic samples of domestic and foreign production is given.

Результаты военно-технического эксперимента по построению быстроразвертываемой системы освещения подводной обстановки с использованием робототехнических комплексов и инновационных гидроакустических средств

The results of a military-technical experiment on the construction of a rapidly deployable underwater lighting system using robotic systems and innovative hydroacoustic devices are presented. Scientific and technological problems are identified and directions for further work on the creation of a promising system of technical support and the development of methods for the operation of marine robotic systems in the course of their group use, including in the far sea zone, are identified.

Spatial and temporal distribution of anchovy density in Inner Ambon Bay on dry season (December-February)

Inner Ambon Bay is a small pelagic fish fishing ground, especially anchovy. Anchovy plays important role in the skipjack fishery as live bait in Ambon city. Nevertheless, research for its resource management is rarely done. The aims of the research are to obtain information of density, vertical distribution and horizontal distribution of anchovy on Dry Season. Densities data of anchovy were collected using hydro-acoustic device at six parallel transect lines and one cross- parallel transect line. Geo-statistical analysis technique was used to describe horizontal distribution of anchovy meanwhile, to observe vertical distribution pattern, vertical anchovy data was plotted on the graph. Result shows that lower average density of anchovy was found in January while higher density was in February. On vertical distribution, anchovy was distributed from near surface to 35 m depth layer, but higher densities were found at 25 m depth layer in December and February respectively and in January at 30 m of depth layer. On horizontal distribution, lower density of anchovy occupies a large space, in contrast, higher density was in small fish schools occupy a small space and are scattered in given parts of Inner Ambon Bay but at differ locations between months.

Utilization Of Underwater Submersible Light Led To Fish Swimming Activity On Fixed Liftnet Operated In Mangrove

Fixed chart fishing focuses on utilization of light aids for attract fish. Underwater submersible light ((Lacuda) LED is one of the assistive technologies that are environmentally friendly, be the subject of study in determining the characteristics of the movement or fish swimming groupings based on the operating time of the fishing gear and can be detected by hydroacoustic devices. The research method used is descriptive exploratory method by participating in fishing operations in the waters of Laikang Bay from April to September 2020. The results of the observations made at 18:00-05:00 Wita every trip shows that fish swimming activity after midnight tends to be higher than before midnight and at midnight. Based on the hydroacoustic detection of fish swimming activity in the form of solitary, medium and large groups at an average depth of 2-6 meters at 03:00-05:00 Wita. The amount of catch in hauling I is 138,4 Kg while hauling II is 157,3 Kg. The type of catch is small pelagic fish and demersal fish.

Squared figures of merit and electromechanical coupling factors of a novel lead-free 1–3–0 composite for sensor and energy-harvesting applications

Piezoelectric sensitivity and energy-harvesting parameters are studied for a novel lead-free 1–3–0 composite where domain-engineered single-crystal rods are surrounded by a polymer matrix containing two types of air pores. Correlation between some effective parameters, concerned with the high piezoelectric sensitivity of the composite, and the elastic properties of its porous matrix is first stated. Diagrams are first built to show volume-fraction ranges wherein conditions for large squared figures of merit (Q33*)2=d33* g33*≥200⋅10−12 Pa−1 (longitudinal) and (Qh*)2=dh* gh*≥50⋅10−12 Pa−1 (hydrostatic) hold, where d33*, dh*. g33*, and gh* are piezoelectric coefficients of the composite. The studied composite is also of interest due to its large electromechanical coupling factors kt* ≈ 0.8–0.9 (thickness) and kh* ≈ 0.5–0.6 (hydrostatic). Piezoelectric sensitivity, electromechanical coupling factors and other characteristics are compared to those of a lead-containing 1–3–0 composite and a few 1–3 composites. Correlation between the effective parameters of the 1–3–0 composite and elastic properties of its porous matrix is discussed. Replacing the parallelepiped-shaped single-crystal rods with the cylindrical rods in the 1–3–0 composite leads to minor changes in the aforementioned parameters. The performance of the studied lead-free 1–3–0 composite proves its suitability for modern piezoelectric sensors, energy-harvesting and hydroacoustic devices.

Theoretical and numerical investigations on resonant characteristics of two piezoelectric plates coupled with a finite fluid field

The vibration characteristics of two piezoelectric cantilevered plates in air and water are investigated in this study. The two-piezoelectric-plate system can provide different vibration characteristics compared to the previous studies in which only one cantilever plate is used because the anti-phase motion can be evoked in this newly designed system. First, the vibration characteristics of the plates coupled with a bounded compressible inviscid fluid are derived by theoretical analysis. The governing equation of the piezoelectric plate and the superposition method are used to analyze the vibration displacement of the piezoelectric plate, and the acoustic wave equation is used to derive the fluid equation. The frequency eigenfunction of the coupled system is derived by the boundary conditions of hydrostatic equilibrium. After solving the characteristics equation, the resonant frequencies and corresponding mode shapes for assumptions about both compressible and incompressible fluids and the velocity and pressure fields of the fluid can be obtained. Next, finite element analysis (FEA) is used to verify the theoretical results. It is shown from this study that the theoretical predictions of resonant frequencies and the corresponding mode shapes have a good agreement with those of the FEA for water and compressible fluid assumption of air. The unique high pressure value in the outer or middle layer in an air-coupled condition does not occur in water because water’s compressibility is relatively smaller than that of air. The influence of the depth and properties of the fluid on the behavior of the coupled solid–fluid is also investigated. For anti-phase motion, the frequency variation with change in the depth of the middle layer is similar to that of the outer layer for both air- and water-coupled conditions. For in-phase motion, the variation in frequency with the change in the depth of the middle layer is negligible. The particular modes in fluid compounded by modes in vacuum are analyzed on the distortion of mode shapes at higher resonant modes. These results can provide complete information on the vibration characteristics of a solid–fluid coupled system and will contribute to the evolution of hydroacoustic devices in which two piezoelectric plates are used.

A geophysical, geochemical and microbiological study of a newly discovered pockmark with active gas seepage and submarine groundwater discharge (MET1-BH, central Gulf of Gdańsk, southern Baltic Sea)

High-resolution bathymetric data were collected with a multi-beam echosounder in the southern part of the Baltic Sea (region MET1, Gulf of Gdańsk) revealing the presence of a 10 m deep and 50 m in diameter pockmark (MET1-BH) on the sea bottom (78.7 m). To date, no such structures have been observed to reach this size in the Baltic Sea. The salinity of the near-bottom water in the pockmark was about 2 PSU (about 31.22 mmol/l Cl−), which clearly indicated the presence of a submarine groundwater discharge (SGD). Water column, sediments and the seabed structure were investigated in the MET1-BH area using various hydroacoustic devices: multi-beam and splitbeam echosounders and a sub-bottom profiler. Geochemical analyses of sediment pore waters (CH4, Cl−, Br−, F−, SO42−, Ca2+, Mg2+, K+, Na+, ∑H2S, dP, dSi, NH4+, DIC, DOC) and microbiological analysis of sediments (16S rRNA) were performed. The content of CH4 and CO2 in the outflowing gas and its origin (δ13C-CH4 and δ2D-CH4) were determined. Hydroacoustic data showed that gas was emitted intensively from the inside of the structure. The height and intensity of the gas flares varied depending on the hydrostatic pressure. The gas contained 76.1% of CH4, 17.6% of CO2 and 0.39% of He. Methane source was microbial. Geophysical investigation revealed the presence of dislocations in sub-surface sediment layers in the MET1 region, which could have created a passage for groundwater and gas. Geochemical analyses pointed to intensive processes of organic matter decomposition in this area, active methanogenesis in the surface sediment layer, lack of the sulphate-methane transition, and freshwater seepage at a depth of ~88 m (bottom of the pockmark), probably from Upper Cretaceous deposits. The Prokaryota composition, atypical for marine surface sediments, resulted from the combination of freshwater and high organic matter content, and reflected active in situ methanogensis.

Sidescan sonar meets airborne and satellite remote sensing: challenges of a multi-device seafloor classification in extreme shallow water intertidal environments

Tidal ecosystems like the Wadden Sea are particularly valuable for their ecological and economic importance. Here, the natural dynamics of the abiotic and biotic processes is threatened by the human pressure, and great efforts are made on mapping and monitoring programs. Remote sensing techniques (e.g., satellite and airborne sources) are commonly used on land and intertidal areas, whereas hydroacoustic devices are deployed in the subtidal zones. The overlap of hydroacoustics (sidescan sonar) and airborne Lidar data in such sensitive transitional zone (inter- to subtidal) is rather uncommon. In order to test the limitations of both techniques in extremely shallow waters (0.7 m min, water depth) and to find the most efficient methods for the spatial classification of intertidal areas, a portion of the backbarrier tidal flat of Norderney was investigated. Lidar bathymetric data were used for extracting high resolution morphological information. Sidescan sonar mosaics were collected in two following years under contrasting weather conditions. An expert classification based on sidescan sonar backscatter intensity, seafloor texture, morphology, and surface sediment data subdivided the research area into 10 classes. The outcomes were compared with an existing RapidEye-based classification. The tested methods showed both advantages and limitations, which were discussed based on statistical analyses. Satellite and Lidar approaches were most suitable for mapping biogenic features (e.g., shellfish beds) over large areas, whereas sidescan sonar was superior for detail detection and discrimination of morpho-sedimentary regions. As an outlook, it is postulated to perform ground-truthed hydroacoustic mapping on small testing areas, and to use the obtained classification for training satellite-based classification algorithms.

ПАРАМЕТРИЧЕСКИЙ ГИДРОЛОКАТОР ТРАВЕРЗНОГООБЗОРА ДЛЯ ОБНАРУЖЕНИЯ И МОНИТОРИНГА РЫБНЫХ СКОПЛЕНИЙ НА МЕЛКОВОДЬЕ

Currently, the state of Affairs for fishing for biological resources of the world's oceans is changing dramatically. Work related to environmental monitoring of marine areas is carried out at the Federal, regional, territorial and local levels. The main purpose of monitoring the state of water bodies is to provide competent authorities and nature users with information about the quantitative and species composition of living organisms, as well as information support for deci-sion-making procedures in the field of environmental protection and environmental safety. Sys-tematic generalization of information about the world's oceans, biological phenomena in the body of knowledge, is necessary for understanding the distribution, abundance and availability of fish. In order not to undermine the biological stocks of hydrobionts of the world's oceans, it is neces-sary to regularly conduct monitoring and research work on the quantitative accounting of biologi-cal resources, regulate quotas for fish and shellfish. Due to the complexity of accounting for bio-logical resources in shallow areas of The world ocean using existing instruments, issues related to the development and research of a traverse survey sonar for monitoring marine hydrobionts in shallow water are considered, since this topic is relevant, and the issue of preserving the biologi-cal diversity of reservoirs has become acute. The technology of traverse survey using sonar is characterized by a high degree of automation, information content, efficiency of research and low labor costs compared to traditional methods based on the use of trawls, dragging. It should also be noted that due to the use of this method, there is no injury to the inhabitants of reservoirs, since often injured hydrobionts die. In the traverse view sonar, a "non-petal " narrow- directional direc-tional pattern is formed in the parametric mode in the vertical plane, which allows increasing the range of action to 700 meters or more. And in the horizontal plane, to increase echo contact and search performance, a wide directional pattern (30–40 deg) is formed by using several pumping antennas installed at a certain angle relative to each other. The development and further applica-tion of the above-mentioned hydroacoustic device will allow creating a kind of database with in-formation about the territorial distribution of fish accumulations in shallow water areas. However, for the practical implementation and creation of such complex technical devices, a whole range of applied research and development work is required. The grandiosity of all that is connected with the human idea of the ocean, served as the basis for a widespread opinion about the unlimited and inexhaustible resources of the world's oceans. However, this opinion is not valid for biological resources. This is evidenced by signs of depletion of fish stocks and other fisheries in irrationally fished areas of the ocean.

Quantitative evaluation for variability characteristics of reflected sound waves from sea surface

Many kinds of hydroacoustic devices are used in the field of ocean development of resources and observations. In evaluating the effect of sound propagation in water, it is important to evaluate the characteristics of sound reflection on sea surface. Amplitude and phase of reflected sound waves often fluctuates because of the changing sea surface with waves. In this paper, we evaluated the amplitude and phase variability characteristics of reflected sound waves on sea surface by acoustic simulation with the finite difference time domain method and water tank test. It is revealed that the energy ratio γ for amplitude variability is uniquely determined by Rayleigh roughness parameter when the sea surface is sufficiently random or the number of surface waves included in the irradiation range of sound waves is large. In addition, the standard deviation of phase for the phase variability is determined by regardless of that is, sea surface spectrum.

Status of the large-scale commercial trawl fisheries in southern Lake Malawi

A study to assess the status of the large scale commercial fisheries operating in southern Lake Malawi was conducted using generalised linear modelling and Principles of Precautionary Approach. The generalised linear modelling analysis was used to standardise catch per unit of effort data from 1976 to 2009 and to determine the effects of fishing vessel characteristics such as engine size and hydro-acoustic devices on fish catch rates. The Precautionary Approach, through reference points like Bmax (reference point corresponding to period of relatively high fish catch rate), Bpa (reference point at which precautionary management action should be undertaken), Blim (limit reference point beyond which a fishery is considered seriously depleted) and Bcur (reference point corresponding to current fish catch rate), was used to assess the status of fish stocks using standardised fish catch rate which approximated fish biomass or stock size. Results of the assessment indicated that the current fish biomass level in all three categories, bottom pair trawl, stern bottom trawl and stern mid-water trawl fisheries, is well above Blim, but is 9–12% above Bpa, which is very close suggesting that the fishery is fully exploited and precautionary management measures are required. Suggested measures included introduction of fishing closed season for the large scale commercial fisheries which will be dependent on the biology of the most important target fish stocks, relocation or reduction of fishing effort in the current fishing grounds and intensification of patrols to reduce incidences of illegal, unreported and unregulated fishing activities.

1‐3‐Type Composites Based on Ferroelectrics: Electromechanical Coupling, Figures of Merit, and Piezotechnical Energy‐Harvesting Applications

AbstractThe physical and microgeometric factors that are able to improve the piezoelectric performance, anisotropy, and energy‐harvesting characteristics of modern 1‐3‐type composites based on ferroelectrics are discussed. The composite connectivity patterns of particular interest for this study include 1‐3‐0, 1‐0‐3, and 1‐2‐2. The active components of the studied composites are chosen from conventional perovskite‐type ferroelectric ceramics, lead‐free materials, or domain‐engineered single crystals, all of which exhibit particularly intriguing electromechanical properties. Examples of the large anisotropy of piezoelectric coefficients, electromechanical coupling factors, squared figures of merit, and large hydrostatic parameters of the three‐component 1‐3‐type composites are considered in the context of their piezotechnical applications. The applications of these materials include piezoelectric transducers, sensors, energy‐harvesting, and hydroacoustic devices.

Hydroacoustic methods and tools for fish stock assessment and fishery maintenance Part 2. Methods and tools of fishery biohydroacoustics

Studies on influence of hydroacoustic fields on behaviour of commercial species and using of hydroacoustic tools for management of fish and squids behavior to increase the fishing efficiency are overviewed. The methods and means of fisheries biohydroacoustics are considered critically and the reasons of their unsatisfactory using in fishery are analyzed. Sounds with a certain spectrum and level are still applied for influence on fish behaviour without sufficient scientific and technical substantiation, so a complex approach to development of effective hydroacoustic tools for remote control of fish movement is necessary. Results of studies on acoustic reception and acoustic activity for schooling physostomous fishes are presented. Spectral-power and temporal parameters of the sounds and their frequency differentiation by fish size are determined. Sound-generating mechanisms of fish are considered and signal significances of the sounds radiated by fish are recognized. Stereotypes of acoustic behaviour are revealed for toothed whales during their hunting upon fish: these predatory cetaceans have special acoustic manipulators able to generate signals for concentration and holding the fish, adapted for hearing system of the prey. Results of hydrobionic modelling of organs and mechanisms for sound generation of marine animals and their technical realization in hydroacoustic devices are presented. The developed devices allow to generate underwater pulse sound signals simulating biological signals of certain physostomous fish species and predatory cetaceans (dolphins and killer whales). Efficiency of these simulating signals influence on behaviour of fish is proved by behavioral experiments and fishing tests. Applications of these devices for various fisheries are discussed.

2014년 동계 북부 동중국해 저층 트롤 및 음향학적 조사에 의한 저서어족 생물자원의 분포특성 비교

This study examined the spatiotemporal distribution of demersal fish aggregations in the Northern East China Sea by conducting a trawl survey with hydroacoustic devices. A bottom trawl was used for this survey and fish density was determined from the catch data. Acoustic data were collected at frequencies of 38 and 200 kHz from November to December 2014 and converted into the nautical area scattering coefficient (NASC, <TEX>$m^2/n{\cdot}mile^2$</TEX>). In the catch data analysis, the range of catch per unit area by station was <TEX>$26-8,055kg/km^2$</TEX> and for the acoustic data, that of the NASC was <TEX>$0.45-34.80m^2/n{\cdot}mile^2$</TEX>. The values were significantly correlated. The combined results of both surveys found that the density was highest at St. 5 (<TEX>$33^{\circ}$</TEX> 10.3', <TEX>$126^{\circ}$</TEX> 23.3') and lowest at St. 8 (<TEX>$33^{\circ}$</TEX> 20.7', <TEX>$127^{\circ}$</TEX> 36.3'). The application of hydroacoustic methods offers a new approach for estimating the biomass of demersal fish aggregations.

Solid–liquid coupled vibration characteristics of piezoelectric hydroacoustic devices

This study investigated the vibration characteristics of piezoelectric bimorphs with regard to the solid–liquid coupling effect in piezoelectric–acoustical devices. Experimental measurements and finite element numerical calculations were used to determine the resonant frequencies and mode shapes of piezoelectric bimorphs, which could be used to promote the flow of air, water, and glycerin. Two piezoelectric devices were designed to verify the solid–liquid coupled vibrations of the piezoelectric bimorphs: (1) Type A with bimorph located above the surface of the fluid and (2) Type B with bimorph located within the fluid. The first type has a piezoelectric bimorph bonded to PDMS polymer, such that resonant vibrations produce changes in the volume of the chamber, which increases the flow of fluid. Another is a biomimetic design, in which a piezoelectric bimorph embedded in the chamber imitated the tail of a fish in order to promote the flow of liquid. Three experimental techniques were used to determine the solid–liquid coupled vibration characteristics. Electric speckle pattern interferometry (ESPI) was used to measure the resonant frequencies and mode shapes associated with the vibration of piezoelectric bimorphs interacting with fluids. Second, a laser Doppler vibrometer (LDV) was used to obtain the frequency spectrum of vibrating displacement using dynamic signal swept-sine analysis. The third experiment involved analyzing impedance in the piezoelectric bimorphs in order to identify the resonant frequencies and anti-resonant frequencies of the piezoelectric material under the influence of a fluid. Finite element method (FEM) was used for the analysis of vibration characteristics associated with the interaction between the fluids and piezoelectric-solid elements paired with (1) acoustic elements only or (2) acoustic elements and solid elements with viscoelastic properties. The numerical calculations of solid–acoustical coupled vibration are in good agreement with solid–liquid coupled experimentally results. The FEM solid elements with viscoelastic properties can be applied to predict the dynamic behaviors for lower frequency of the solid–acoustical coupled vibration. This study proposes an efficient methodology of the development of piezoelectric hydroacoustic devices by FEM, which has been verified by experimental measurements in this paper.