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Nov 1981

Volume 70, Issue S1, pp. S1-S109

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back to top Session I. Engineering Acoustics I: Low‐Frequency Underwater Sound Sources
Invited Papers
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Radiation of low‐frequency sound (A)

A. L. Van Buren

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S18-S18 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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The fundamentals and problems of low‐frequency sound radiation are reviewed. Emphasis is on the constraints that physics imposes on the development of high‐power, low‐frequency, underwater sound sources. Included are discussions of such constraints as a highly reactive radiation impedance, the requirement of large displacements of the radiating surfaces, strong radiation coupling between elements in an array, and the possibility of cavitation.
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Constraints on engineering (A)

John V. Bouyoucos

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S18-S19 (1981); (2 pages)

Online Publication Date: 12 Aug 2005

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The practical requirements for low‐frequency underwater sound sources are reviewed, with the volume velocity and force levels needed as a function of power level and frequency placed in perspective. The principal features of a representative selection of low‐frequency transducers in use today are set forth, and the opportunities for improvements in performance are assessed with regard to the limitations of materials, seals, pressure compensation, and other aspects that affect size, weight, reliability, cost, etc. Finally, selected comments are made on suggested research and development or other studies that might lead to improvements in the state of the art of low‐frequency transduction.
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Constraint on source output imposed by nonlinear transmission effects in the medium (A)

David T. Blackstock

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S19-S19 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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An important limit to the transmission of sound is set by nonlinear propagation effects. Because of these effects the peaks of a high‐intensity sound wave tend to travel faster than the troughs. A sinusoidal wave tends to distort into a sawtooth wave, which because of its rich high‐frequency content, suffers rapid absorption by the medium. The distortion therefore leads to extra attenuation of the wave. The extra attenuation can become so severe that saturation occurs. That is, the sound pressure level at the receiver reaches a limit that cannot be exceeded regardless of the acoustic power generated by the source. Extra attenuation also affects beam patterns because the most intense part of the beam is attenuated faster than other parts. The severity of nonlinear propagation distortion depends on frequency and source size as well as on amplitude. A relatively simple graph has been developed that indicates the importance of nonlinearity for a given source. [Work supported by ONR.]
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Recent developments in large‐scale, moving‐coil underwater sound sources (A)

Bernard S. Willard

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S19-S19 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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The intrinsic advantages of moving‐coil transducers as low‐frequency, broadband acoustic projectors are discussed, as well as the relative merits of single, large‐scale units versus arrays of smaller units. Recently developed low‐frequency projectors which can be towed behind a ship at significant speeds and depths are shown. The principal areas of study were heat dissipation, coil and coilform configurations, structural design, and magnetic materials. Of particular interest is the use of samarium cobalt magnetic material which offered many more advantages to this field than was originally expected. Several novel approaches to transducer configuration are also shown.
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The Helmholtz resonator for use as a low‐frequency underwater acoustic source (A)

A. M. Young

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S19-S19 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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The principle of the Helmholtz resonator as applied to low‐frequency underwater acoustic sources is discussed. The advantages, disadvantages, and limitations of this type of transducer are addressed in general and then applied to a design example. The example will be followed from the equivalent circuit analysis and fabrication of a working scale model through to the experimental evaluation of the full‐scale transducer.
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Characteristics and capabilities of the flextensional projector (A)

Edward F. Rynne

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S19-S19 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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A perennial interest in low‐frequency projectors has led to the examination of the air‐backed class IV flextensional projector technology. This technology is capable of providing high power at low frequency in a compact, lightweight package. Despite such recognized capabilities it has seen only limited application due to the complexities of design and manufacture and to the scarcity of data on performance and reliability. In this paper, the results of tests at Seneca Lake on four identical, state‐of‐the‐art flextensional projectors will be presented. The four projectors were tested both individually and in a line array configuration. The test results will be compared with the predictions of a mathematical model developed at the Naval Ocean Systems Center. The high level of agreement between model and experiment allows the model to be used in assessing design tradeoffs for various applications. Specific tradeoffs in terms of power bandwidth, size, weight, and thermal characteristics will be discussed. [Work sponsored by NAVSEA 63R12.]
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Development of hydraulically powered, low‐frequency, underwater sound projectors (A)

G. D. Hugus III

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S19-S20 (1981); (2 pages)

Online Publication Date: 12 Aug 2005

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Hydraulic power is attractive for use in underwater acoustic projectors in the low audio and infrasonic frequency range. This is because of the capability of hydraulic power systems to produce the necessary large linear displacements and forces. The advantages, disadvantages, and design considerations of hydraulically powered underwater projectors of different configurations will be discussed in this paper. Also discussed will be the development of a hydraulically powered projector to be used for transducer calibration. This projector has the design requirement to produce 160 dB re 1 μPa source level at 1 m in the frequency range of 1 to 10 Hz.
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Mechanical linkage projectors (A)

Howard A. Wilcox

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S20-S20 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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Fundamental constraints pertaining to resonant types of low‐frequency underwater projectors explain why a nonresonant approach has permitted the design of a relatively small, efficient, and inexpensive projector whose output frequency, piston motion amplitude, and operating depth are all independently controllable. The resulting unit is inherently a “standard” source not requiring calibration by auxiliary instruments. Overall energetic efficiencies of 1% to 2% have been achieved in operating at 15 Hz and shallow depths (500 ft). Design principles have been worked out for achieving instantaneous switching of output amplitude, frequency, and phase as well as for the simultaneous output of two or three anharmonically related signals. The first unit displaces about 216 lb, uses a 1/12th HP electric motor to generate a 15‐Hz signal at a source level of 171 dB re 1 μPa at 1 m, and would cost about $7000 each to manufacture in lots of ten.
Contributed Papers
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A high‐power, low‐frequency hydroacoustic sound source (A)

Max Utterback and Steve Nichols

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S20-S20 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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A low‐frequency sound source of high acoustic output (214 dB re 1 μPa at 1 m) has been developed by Hydroacoustics Inc. The projector has been configured to operate over two distinct frequency bands, with a bandwidth of approximately 40% of each band‐center frequency. The conversion between the two bands requires relatively simple hardware changes, principally in the radiating structure. The primary input power to the source is in the form of three‐phase electrical power to an electric motor driven hydraulic power supply which is an integral part of the projector structure. Conversion of the input power to acoustic energy is accomplished through a rugged hydroacoustic power amplifier which responds to a low level signal voltage and reproduces this waveform as acoustic pressure/flow acting in a centrally located fluid cavity. Two opposed, edge‐supported radiating flexural disks are center driven by drive pistons which communicate with this fluid cavity. The entire structure is very rugged and capable of operation to great depths. Analytical modeling of this projector has been developed by the Naval Ocean Systems Center in conjunction with Hydroacoustics. This model includes the radiation loading, the flexural modes of the radiating disks, the hydraulic power supply, and the two stage hydraulic amplifier—including those nonlinearities associated with large power transduction. In water calibration test results in both bands will be presented, as well as a comparison between these results and the modeling effort. [Work supported by the Naval Electronics System Command.]
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A modified AO projector (A)

B. A. Armstrong and G. W. McMahon

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S20-S20 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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The AO projector is a WWII British Admiralty minesweeping source that operated at 230 Hz with a source level of about 198 dB re/μPa at 1 m. Being virtually indestructible, many still exist in operating condition. To improve their utility as research sources, three AO's have been modified to resonate at 110 Hz. A SCUBA pressure compensation system has been installed to extend their operating depth capability. The source level at resonance is displacement limited to 195 dB and a current‐limited, broadband source level of at least 182 dB is available above resonance up to the diaphragm breakup at 700 Hz. The modification method will be described and performance predictions given for a modified AO with an 80‐Hz resonance.
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Dipole characteristics of a magnetostrictive rare earth iron “square‐ring” transducer (A)

S. M. Cohick and J. L. Butler

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S20-S20 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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A magnetostrictive transducer has been constructed of rare earth Terfenol D (Tb0.3Dy0.7Fe2) rods in a “square‐ring” configuration, which is essentially cylindrical. The transducer, 6 in. in diameter and 2 in. high, is resonant at 2.0 kHz in the omnidirectional mode. Some underwater detection systems also require directional information. A means of operating the “square‐ring” in a dipole mode is developed. The physical mechanism of the dipole transducer behavior is described. The modification to the transducer and the driving circuit are presented. The acoustic calibration of the transducer in the dipole mode shows acceptable performance from 1.5 to 5 kHz. The dipole resonance is 2.4 kHz. [Work supported by NSWC in cooperation with NADC.]
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