Soft Baffle Research Articles

Directivity and Frequency-Dependent Effective Sensitive Element Size of a Reflectance-Based Fiber-Optic Hydrophone: Predictions From Theoretical Models Compared With Measurements.

The goal of this work was to measure the directivity of a reflectance-based fiber-optic hydrophone at multiple frequencies and to compare it to four theoretical models: rigid baffle (RB), rigid piston (RP), unbaffled (UB), and soft baffle (SB). The fiber had a nominal 105- [Formula: see text] diameter core and a 125- [Formula: see text] overall diameter (core + cladding). Directivity measurements were performed at 2.25, 3.5, 5, 7.5, 10, and 15 MHz from ±90° in two orthogonal planes. Effective hydrophone sensitive element radius was estimated by least-squares fitting the four models to the directivity measurements using the sensitive element radius as an adjustable parameter. Over the range from 2.25 to 15 MHz, the average magnitudes of differences between the effective and nominal sensitive element radii were 59% ± 49% (RB), 10% ± 5% (RP), 46% ± 38% (UB), and 71% ± 19% (SB). Therefore, the directivity of a reflectance-based fiber-optic hydrophone may be best estimated by the RP model.

Directivity and Frequency-Dependent Effective Sensitive Element Size of Needle Hydrophones: Predictions From Four Theoretical Forms Compared With Measurements.

Directivity is a hydrophone specification that describes response as a function of angle of incidence. The goal of this study was to compare, in the context of needle hydrophones, the commonly used rigid baffle model for hydrophone directivity to three alternative models: soft baffle, unbaffled (UB), and rigid piston (RP). Directivity measurements were performed at 1, 2, 3, 4, 6, 8, and 10 MHz from ±7° in two orthogonal planes for two ceramic and two polymer needle hydrophones with nominal geometrical sensitive element diameters of 200, 400, 600, and 1000 . Effective hydrophone sensitive element radius was estimated by least-squares fitting the four models to directivity measurement data using the sensitive element radius (a) as an adjustable parameter. For > 4 (where and = wavelength), the RP model outperformed the other three models. For , the average error in estimated sensitive element radius was 7% [95% confidence interval (CI): 3%-12%] for the RP model while the lowest average error by the other three models was 46% (95% CI: 38%-54%) for the UB model.

Mechanical fields of the piezoceramic radiator of strength design situated near acoustical baffle

Article describes the mechanical fields of the cylindrical piezoceramic transducer of the strength design, with circular polarization located near cylindrical acoustical soft baffle. As an investigated parameter of the mechanical field of transducer chosen the oscillating velocity of the surface of transducer, because, studying of this parameter has huge interest in practical applications, because on the construction stage of the creating of sonar array we have to know the maximum mechanical loads that arise in the transducer during its operation, and in the classical methods for calculating oscillating velocity of the surface of transducer is set to the constant value, so it can’t identify the changes in the characteristics of the oscillating velocity on the surface of transducer, caused by the interaction between the elements of the array. Unlike the classical methods, oscillating velocity on the surface of transducer is not constant value, and numerical calculations of the physical fields provided, beginning from the electrical input of transducer. Equations, used for calculating numerical values of the oscillating velocity, obtained, using the results of solution of the “pass-through” problem of sound radiation of the cylindrical sonar array formed with cylindrical piezoceramic transducers with circular polarization and cylindrical acoustic soft baffle located in the middle of the array. This solution allows us to take into consideration interaction of the fields in the transducer itself and into array entirely. The numerical analysis of the dependencies of oscillating velocity of the surface of the transducer provided both, for frequency dependence and angle dependence. The analysis provided both, for amplitude and phase characteristics of the velocity. For the frequency dependencies we studied the oscillating velocity in the point on the surface of transducer, that lays in the opposite direction to the baffle, and for angular dependencies we chosen some interesting frequencies to provide the investigations. This frequency and angular characteristics were compared between themselves and with characteristics of transducer itself, without baffle, for situations, when we changed the distance between the transducer and the baffle and the dimensions of the baffle were constant and when we changed the dimensions of the baffle without changing the distance between baffle and the transducer. As a result of investigations we have determined physical causes of the changes in the oscillating velocity of the surface of cylindrical piezoceramic transducer as a part of the system “radiator – baffle”.Ref. 6, fig. 4.

Angular Dependence of the Frequency Response of an Extrinsic Fabry–PÉrot Interferometric (EFPI) Fiber Acoustic Sensor for Partial Discharge Detection

Extrinsic Fabry-Peacuterot interferometric (EFPI) fiber acoustic sensors have been successfully employed in partial discharge detection in a power transformer. In this paper, the angular dependence of the sensor's frequency response is investigated. The interaction between the applied pressure and the EFPI fiber sensor immersed in a fluid was modeled by finite element methods. The angular dependence of the sensor's frequency response was obtained by a coupled structure acoustic analysis in ANSYS and compared to both the soft baffle and unbaffled models and experimental results in a 1-m tank. The measured bandwidth of the sensor's angular dependence of the frequency response is in good agreement with the simulation results with a plusmn3deg 3-dB bandwidth difference and a 0.4-dB difference within a plusmn45deg incident range.

A new calculation procedure for spatial impulse responses in ultrasound

A new procedure for the calculation of spatial impulse responses for linear sound fields is introduced. This calculation procedure uses the well known technique of calculating the spatial impulse response from the intersection of a circle emanating from the projected spherical wave with the boundary of the emitting aperture. This general result holds for all aperture boundaries for a flat transducer surface, and is used in the procedure to yield the response for all types of flat transducers. An arbitrary apodization function over the aperture can be incorporated through a simple one-dimensional integration. The case of a soft baffle mounting of the aperture is also included. Specific solutions for transducer boundaries made from lines are given, so that any polygon transducer can be handled. Specific solutions for circles are also given. Finally, a solution for a general boundary is stated, and all these boundary elements can be combined to, e.g., handle annular arrays or semi-circle transducers. Results from an implementation of the approach are given and compared to previously developed solutions for a simple aperture, a complex aperture, and a Gaussian apodized circular transducer.

Application of the finite element method to two-dimensional radiation problems

Acoustic fields radiated by vibrating elastic bodies immersed in an infinite fluid domain are, in general, quite difficult to compute. This paper demonstrates in the two-dimensional (2-D) case that the radiated near field can be easily obtained using the finite element method if dipolar damping elements are attached to the mesh external circular boundary. These elements are specifically designed to absorb completely the first two components of the asymptotic expansion of the radiated field. Then, the paper provides a new extrapolation method to compute far-field pressures from near-field pressures, using the 2-D Helmholtz equation and its solution obeying the Sommerfeld radiation condition. These developments are valid for any radiation problem in 2D. Finally, two test examples are described, the oscillating cylinder of order m and a finite width planar source mounted in a rigid or a soft baffle. This approach is the generalization to 2-D problems of a previously described approach devoted to axisymmetrical and three-dimensional (3-D) problems [R. Bossut et al., J. Acoust. Soc. Am. 86, 1234–1244 (1989)]. It has been implemented in the ATILA code. It is well suited to the modeling of high-frequency transducers for imaging and nondestructive testing.

Application of direct integral‐equation method in acoustic‐wave diffraction

The direct integral‐equation method (DIEM) is applied to study the diffracted acoustic field of a point source at an arbitrary location relative to a ring aperture in a soft baffle. The velocity potential at an arbitrary field point is expressed in terms of a surface‐source distribution with complex densities. A set of eight real Fredholm integral equations of the second kind is used to determine the surface‐source densities. These equations are transformed into discrete forms by applying the Gauss‐Legendre quadrature formula in the radial direction and the best possible numerical integration formula in the angular direction. In comparison to the boundary‐element method, the DIEM is more efficient, accurate, and flexible. The advantages of DIEM become apparent, especially when the parameters in a given problem vary within a large range. The numerical result of source strengths on the baffle surface in the far field agrees very well with the asymptotic solution. The effect of different parameters on the diffracted acoustic field is systematically studied and compared. These parameters include the location of the acoustic source, the wave number, the size of the ring aperture, as well as the thickness of the baffle. The numerical results show that the baffle thickness and the size of the ring aperture have relatively little influence on the diffracted acoustic field within the tested range. However, the wave number has a significant effect on the diffracted pressure field.

Farfield radiation patterns of a circular piston in a soft baffle with noninfinite extent

This paper presents some numerical results of farfield radiation patterns of a circular piston flush mounted in the end of a semi-infinite soft (pressure release) circular cylinder. The method of equivalent edge acoustic sources is applied to the calculation of the diffracted field due to the finite extent of the end. The results obtained by this method for various radii of the cylinder and locations of the piston demonstrate the effect of a soft baffle with finite extent on farfield radiation patterns of the piston. Experimental results, which agree well with numerical ones even in the shadow region, are also presented.

Fog signal baffle design

As part of its automation program, the U. S. Coast Guard has been replacing multifrequency air-driven sound signals with electric air oscillators. The pure-tone character of this signal has been a cause of noise complaints by residents near the installation. For example, an unbaffled ELG 300/02 dual emitter is capable of producing levels of the order of 80 dBA 1/2 mile to the rear of the signal. Thus, to be effective, a baffle design must provide at least 30–40 dB of attenuation in its dark sector. A program was conducted to develop high attenuation baffles for ELG 300-Hz emitters using the principles of diffraction source interference, acoustically soft baffle edges, and end-fired array lenses. The program utilized 1/8-scale models to test the effectiveness of experimental configurations. The most effective baffle designs were then verified by full-scale field tests. [This research was supported by the Coast Guard under Contract DOT-CG-30200-A.]

Nearfield Pressure Distributions for a Rigid and a Flexing Piston in an Infinite Soft Baffle

The nearfield pressures of a rigid piston (ka = 0.906 at f = 7.55 kHz) and a flexing piston (ka = 2.4 at f = 20.0 kHz) in an infinite soft baffle were measured in a tank by means of a toneburst technique. The resulting pressure profiles are presented as isobaric contours, comparable to those for pistons in a rigid baffle derived analytically by H. Stenzel [Handbook for the Calculation of Sound Propagation Phenomena, A. R. Stickney, Trans. (Springer-Verlag, Berlin, 1939)].

Free Circular Disk with Constant Pressure Distribution

A thin rigid vibrating circular disk with constant-pressure distribution on its surface in an infinite soft baffle corresponds mathematically to the solution of the wave equation with homogeneous Dirichlet boundary conditions. Its characteristics may thus be determined by the evaluation of integrals. The velocity distribution on the disk has been found for various sizes and is compared with those for a disk in an infinite rigid baffle [V. Mangulis, Tech. Rept. TRG-142-TN-64-2 (1964)]. The impedance and radiation pattern is given for various disks (ka<2) and compared with the corresponding quantities for the soft- and rigid-baffle constant-velocity disk as well as with the rigid-baffle constant-pressure disk. The radiation characteristics for the same type of baffle are similar and may be estimated for the constant-velocity disk from the constant-pressure disk for other shapes of radiators. [Hudson Labs. Columbia Univ. Informal Doc. No. 54.] [Work supported by the U. S. Office of Naval Research.]