Parametric Acoustic Array Research Articles

Nonlinear parameter estimation in water-saturated sandy sediment with difference frequency acoustic wave

Difference frequency acoustic wave from nonlinear interaction of two primary acoustic waves at frequencies of 76 and 114 kHz was utilized with a parametric acoustic array theory to estimate the nonlinearity parameter of water-saturated sandy sediment. Such nonlinearity parameter can be used as background information for the nonlinear acoustic investigation of bottom or sub-bottom profiling in the ocean sandy sediments. Because of its lower attenuation the difference frequency acoustic wave method can be usefully applied to estimate the nonlinearity parameter of ocean sediment in the ocean as well as under laboratory conditions. The nonlinearity parameter β for the water-saturated sandy sediment used as a reference in this study was estimated as β = 80.5 ± 5.1 at the difference frequency of 38 kHz. It was agreed very well with that estimated at the difference frequency of 67 kHz, when two primary frequencies were 137 and 204 kHz. The estimated nonlinearity parameter of water-saturated sandy sediment in this study was also compared and analyzed with those estimated in previously published literatures. It was suggested that the difference frequency wave method used to estimate the nonlinearity parameter of water-saturated sandy sediment can be employed as a good method to estimate the nonlinearity parameters of fluid-like granular media.

A stepped plate transducer as ultrasonic range sensor

A new type of highly directional ultrasonic transducer is designed and tested as an ultrasonic range sensor by using the parametric acoustic arrays. To get intensive primary waves, a modified stepped plate transducer is proposed. Gallego‐juarez et al.(1978) first proposed stepped plate transducer for high power radiation at one frequency. The steps with the height of half‐wavelength of sound in air compensate discrete phase difference on the vibrating plate. However, two collimated beams are required for the parametric acoustic array. The position and height of the steps are modified to compensate the flexural vibration for two frequencies in this transducer. The transducer diameter is 50mm. The optimal primary frequencies are designed as 80kHz and 120kHz to generate difference frequency of 40kHz efficiently. The proposed transducer has the HPBW of 5° that is much higher directivity than general ultrasonic range sensor(Generally 20° on same size). The maximum SPL is 130dB at primary frequencies and 95dB a...

Acoustic‐laser land mine detection: Operationally enabling technologies

Reliable and cost effective methods are critically needed to locate the vast number of land mines left behind from military conflicts. Many mine location systems being developed fall short of being operationally practical since they may detect only metal or a specific mine feature, have limited standoff ranges, or have slow search speeds. An approach that may overcome operational issues is the acoustic‐laser method. Acoustic waves generated by a noncontact/standoff source and transmitted through air can couple to the ground and induce resonances from a large variety of mines. The vibration field above these mines can be measured remotely using laser Doppler vibrometry. In this paper, enabling technologies that can significantly increase search area speed, improve standoff range, and reduce size in an acoustic‐laser system are discussed. We take a novel approach to excite mines by using a high powered parametric acoustic array source developed at MIT Lincoln Laboratory that transmits a highly directive sound beam from a safe distance. The avalanche photo diode (APD) array developed at MIT Lincoln Laboratory will also be discussed as the enabling technology of a proposed 4096 multipixel laser system that can sense a wide vibration field out in front of a moving vehicle.

A HIGHLY-DIRECTIONAL ULTRASONIC RANGE SENSOR USING A STEPPED-PLATE TRANSDUCER

A new type of highly-directional ultrasonic range sensor is designed, fabricated, and tested in this paper. To improve directivity, the parametric acoustic array, a nonlinear effect of media between two intensive waves, is applied to an ultrasonic range sensor. Additionally, a new type of the stepped-plate transducer is applied for generating high amplitude waves consisting of two frequencies. The proposed transducer shows a half power beam width(HPBW) of 5° at 40 kHz that is much higher directivity than the general ultrasonic range spatial sensor (typically 20°). Therefore, this research shows the possibility to improve the resolution of the ultrasonic range sensor.

Evaluation of parametric acoustic array technology in application to the acoustic landmine and improvised explosive device (IED) detection problems

There has been recent interest in using the parametric acoustic array for obtaining highly directional low frequency sources in acoustic landmine and IED detection. Experiments (using a commercial parametric array (0.5 m diam) located 1.5 m directly over the target and driven by a swept sine audio modulated 60 kHz tone) were performed to compare the ‘‘on’’ to ‘‘off’’ target soil surface particle velocity of an inert VS 1.6 (plastic) anti‐tank landmine buried 2.5 cm deep in masonry sand in a concrete box located in the anechoic chamber facility. The laser Doppler vibrometer response (normalized to the microphone) was measured from 100–1,100 Hz using a spectrum analyzer. A SPL of 40 dB re 20 μ<th>rmPa at 150 Hz was insufficient, where the mine was known to have its largest resonance. Resonances between 300–600 Hz also went undetected, but amplitude contrast ratios of 20 and 3 were measured at 850 and 1,050 Hz, respectively, upon signal averaging. Limitations in carrier SPL, auditory damage, saturation effects, distortion, and low efficiency need to be addressed to achieve the required 60–80 dB SPL needed near 100 Hz. Mines or IEDs buried at 15 cm require more SPL. [Work supported by ARL.]

Professor Peter Westervelt and the parametric array

Work on the interaction of large-amplitude waves in fluids, leading up to the discovery of the parametric array, is mentioned, starting with Thuras, Jenkins, and O’Neil [‘‘Extraneous frequencies generated in air carrying intense sound waves,’’ J. Acoust. Soc. Am. 6, 173–180 (1935)]. Westervelt’s theory for the scattering of sound by sound [‘‘Scattering of sound by sound,’’ J. Acoust. Soc. Am. 29, 934–935 (1957)], is reviewed. The discovery of arraylike and directivity properties of colinear waves interacting nonlinearly, as remembered by Peter Westervelt (private communication to Muir, 2005), is discussed. The formulation of the parametric array, illustrated from the original manuscript [Westervelt, P. J., ‘‘Parametric acoustic array,’’ J. Acoust. Soc. Am. 35, 535–537 (1963)], is discussed, expounding on its elegance and utility in opening new avenues for research and development, as well exploring its assumptions and limits of applicability. Subsequent benchmark experiments and theoretical mode relaxing the assumptions and extending the range of applicability, are mentioned. The First and Second International Symposia on Nonlinear Acoustics (1968 and 1969) are remembered, photographically, with mention of several discoveries made by the luminaries at that time. Numerous practical applications, developed over the past 45 years, are then briefly illustrated. Westervelt’s mentorship with graduate students at Brown is discussed. [Work in large part, supported by the U.S. Navy.]

Parametric acoustic array and P. J. Westervelt: A tribute

Discovery of the parametric acoustic array, that is, its principle, by P. J. Westervelt [J. Acoust. Soc. Am. 35, 535–537 (1963)] is justly famous. Reasons are given for this claim, including its unprecedented character and its usefulness, which is witnessed by a number of operational acoustical devices whose applications span subbottom profiling, marine archaeology, mine detection, and fish swimbladder resonance absorption spectroscopy, among others. New applications continue to be found, as to backscattering by swimbladder-bearing fish for sizing and quantification. These applications may be supported and extended by current efforts to develop a standard-target calibration protocol. While the larger community has pursued such applications, while also defining limits to the performance of the parametric acoustic array, the achievement represented by the essential discovery remains undiminished. In this tribute, some comments are also made of a personal sort on Westervelt’s approach to science and teaching.

Nonlinear parameter estimation in water-saturated sandy and muddy sediments

Nonlinear acoustic responses of water-saturated sediments are very important to understand nonlinear phenomena of gassy ocean sediments. Especially, the second harmonic, the sum, and the difference frequency acoustic waves in water-saturated sediments can provide practical criteria to estimate the nonlinear parameters of gassy sediments. In this paper, the difference frequency acoustic waves in water-saturated sandy and muddy sediments were observed in a water tank experiment using a pulse transmission technique. Their pressure levels were 10 dB higher than the background noise level at a maximum driving pressure of source acoustic transducer. The experimental results were compared with theoretical estimations of the parametric acoustic array. The nonlinear parameters of water-saturated sandy and muddy sediments were estimated as a few tens in their comparison. The nonlinear parameters were also compared with those obtained with the second harmonic acoustic wave. [Work supported by MOCIE (Ministry of Commerce, Industry and Energy).]

Forty years of nonlinear ultrasound

Nonlinear ultrasound forms an integrated discipline of nonlinear acoustics founded in 1755. A short outline of the state-of-the-art in nonlinear ultrasound in 1960 forms the introduction to this paper. Some of the most important contributions to the development in the theoretical, analytical and numerical basis of nonlinear ultrasound and in experimental investigations of nonlinear ultrasonic processes published during the period of 1960 through 2000 are discussed and their successes and failures in practical exploitation are illuminated. A more detailed treatment is given of research achievements in nonlinearity of fluids, in focused ultrasonic field, in parametric acoustic arrays and in thermoacoustics. An attempt is made to point out some fields of research in nonlinear ultrasound where future efforts should be concentrated.

A non-contact technique for evaluation of elastic structures at large stand-off distances: applications to classification of fluids in steel vessels

A novel technique for non-contact evaluation of structures in air at large stand-off distances (on the order of several meters) has been developed. It utilizes a recently constructed air-coupled, parametric acoustic array to excite the resonance vibrations of elastic, fluid-filled vessels. The parametric array is advantageous for NDE applications in that it is capable of producing a much narrower beamwidth and broader bandwidth than typical devices that operate under linear acoustic principles. In the present experiments, the array operates at a carrier frequency of 217 kHz, and the sound field several meters from the source is described spectrally by the envelope of the drive voltage. An operating bandwidth of more than 25 kHz at a center frequency of 15 kHz is demonstrated. For the present application, the array is used to excite vibrations of fluid-filled, steel containers at stand-off distances of greater than 3 m. The vibratory response of a container is detected with a laser vibrometer in a monostatic configuration with the acoustic source. By analyzing the change in the response of the lowest order, antisymmetric Lamb wave as the interior fluid loading conditions of the container are changed, the fluid contained within the steel vessel is classified.

Investigation of generation of low-frequency fields produced by parametric acoustic array, located in the waveguides with rough surfaces

Under natural conditions the parametric acoustical arrays (PAA) work as a rule near the bottom or near the sea surface. If there are free surfaces in the area of interacting of the high-frequency waves (HFW) then the multi-lobe directivity pattern of the PAA is formed in the horizontal plane. However, these effects appear only in waveguides which are bordered smooth surfaces, because these boundaries do not violate conditions of coherence for interacting waves. If there is a rough surface in the area of interacting of HFW, it leads to limit the length of PAA. In this case a single-lobe directivity pattern on the low-frequency (LF) is formed [Gurbatov et al., Radiophys. Quantum Electron. 38, 82–86 (1995)]. In this paper the results of laboratory investigations of the LF fields angular distribution in waveguides with the rough surfaces are presented. It is shown that the rough boundaries leads to increasing of angular distributions width in composing with similar characteristics in boundless space, and the quality kind of field distribution in the horizontal plane depends not only on its mode structure, but on characteristics of rough waveguide boundaries. [Work supported by RFBR Grants No. 99-02-18354, No. 98-02-16402, INTAS Grant No. 11134.]

Field of acoustic parametric array generated by a point source in a cylindrical waveguide with rough boundaries

Consider the possibility of using obtained theoretical solving for the diagnostics and checking a structure of the nonlinear acoustic parametric array (APA) field in inwardly closed or half-closed volumes of diagnosing structures via the determination of field structure over distinctive parameters: pressures, displacing, oscillatory velocities, etc., with allowance for a physical nonlinearity of the media, hydrodynamic, boundary and other conditions of ultrasonic field forming. A theoretically designed method of solving an interior problem on the finding of ultrasonic field in a cylindrical waveguide with rough surface at irregularity description functions h(a,z), where the method of consequent approximations is used for obtaining and representing boundary conditions under small magnitudes of irregularities, commensurable with magnitudes of acoustic field wavelengths. Herewith roughness (irregularities) is considered to be, for example, a border-reflecting surface, beside which is a geometric with a small change in the scale of working wavelength, in our case, field of primary pumping waves (PPW). The physical problem model represents itself as consideration of an acoustic field of the monochromatic point source of an inwardly circular cylinder with radius a, the physical characteristics of which are assumed as ideal reflecting, and the irregularity (roughness) of its surface is given in general type by the function of roughness’ description h(a,z).

Formation of characteristics of a parametric acoustic array in the vicinity of a rough surface

We present an experimental study of the fields of difference frequency waves (DFW) generated by a parametric acoustic array in laboratory conditions. The spatial distribution of the DFW field of a parametric acoustic array with oblique incidence of the source radiation onto a rough water-air interface that models a disturbed ocean surface is considered. It is experimentally shown that the presence of a reflecting surface in the region of interaction of the pump wave results in a significant distortion of the angular dependences of the DFW fields in a reflected beam.

Methods for calculation of the fields of parametric acoustic arrays in oceanic waveguides

Methods for calculation of the fields of parametric acoustic arrays operating in an inhomogeneous medium under regimes of low-frequency wave generation similar to the Westervelt and Berktay regimes for free space are discussed. Mode and ray representations of difference-frequency waves are used. Both plane isovelocity waveguides and refractive sound channels are investigated.

深海無人探査機用パラメトリック・サブボトムプロファイラ

A sub-bottom profiler system for remotely operated vehicle in deep sea was developed to observe surface sedimentary layers of several thousand meter-depth. Using a parametric acoustic array, we realized a very narrow beam width of 4 degrees with the frequency of 3.5 kHz. In this report, first, the dependency of the secondary pressure levels of parametric acoustic sources on the depth of water is calculated by the use of the physical data obtained in real sea. As the result, it is shown that the second pressure levels become slightly higher in deep sea of 6, 000 m than the values at sea surface. Next, a prototype of the parametric source for the experiments under ordinary ambient pressure is developed and tested. The experimental data obtained in water tank agree with the calculated values. Then, in the field experiments of the sub-bottom profiling at the wharf in comparison with the conventional source. Finally, we developed a parametric sub-bottom profiler for 10, 000 m class ROY, and acoustic characteristics of these equipments measured in water tank and experimental results in shallow water are shown.

An Acoustic Parametric Array for Measuring the Thickness and Stratigraphy of Contaminated Sediments

Acoustic mapping and monitoring of thin contaminated sediments in inland waters requires an optimized acoustic source. An experiment conducted in Hamilton Harbour, Lake Ontario, demonstrated that high resolution recording of sediment layering is possible when a parametric acoustic source is used. Good correlation was obtained with the stratigraphy of cores from the test site, and parametric records revealed a 4 cm ooze layer at the surface that could not be resolved using a conventional acoustic source. The encouraging performance of the parametric source for measuring thickness of sediments, which are typically not amenable to acoustic measurement, suggests that an acoustic parametric array might be a useful tool for monitoring sedimentation processes or for high-resolution surveys of modern sediment thickness and volume.

A study of improvement in efficiency for parametric acoustic array using a nonlinear vibration of bubble under water

A study of improvement in efficiency for parametric acoustic array using a nonlinear vibration of bubble under water

Study of acoustic transmission loss in sediments at low frequencies using parametric acoustic arrays

Investigations have been carried out on the acoustic transmission loss in clay, sand, and gravel in the frequency range 6–10 kHz. Parametric acoustic arrays have been used for this study and have been generated using amplitude modulation of a primary transducer of frequency about 300 kHz in which the carrier was suppressed. Source level and beam width of the parametric signal have been ∼ 155 dB re: |μPa/m2 and 3.6°, respectively. Experiments have been conducted in the laboratory taking the sediment in a rectangular container of dimensions 0.7×0.49×0.6 m3. The acoustic transmission loss using parametric arrays has been compared with the measured loss with a nonparametric source. The results show a dependence of transmission loss on the particle size and are discussed in the light of previous studies.

Parametric waveforms and spectra of the primary pulses with two kinds of envelopes

The integral of the pulse self-demodulation in acoustic parametric array can be calculated in light of the Fourier convolution theorem. Instead of a rectangular pulse with two discontinuous edges, two kinds of primary envelopes are used, the waveforms and their spectra are investigated. An optimal ratio of the primary duration has been found theoretically. Experimental results support this theory.

Effects of noncollinear interaction on parametric acoustic arrays in dispersive fluids

The effect of noncollinear interaction on radiation of the difference-frequency signal from a parametric array is analyzed for small angles of intersection formed by two primary beams radiated from the same source. Although variations in the angle of intersection affect the phasing of the array, it is shown that for no practical situations can noncollinear interaction be used to offset detrimental effects due to dispersion. Instead, reduction in length of the interaction region as a result of noncollinear interaction adversely affects the farfield radiation. Length reduction is avoided when one primary beam completely overlaps the other, which is the situation encountered with the parametric receiving array.