Anechoic Tank Research Articles

Applications of beamforming and acoustic holography in an anechoic tank with surface reflections

Recently, our institute, KRISO/KORDI, constructed a small anechoic tank which has an anechoic lining at the four walls and bottom. Because surface reflections still occur, special care must be taken for acoustic measurements and array signal processing, for example, source localization and acoustic holography. In this paper the effects of the surface reflections to the array signal processing methods are investigated. First, estimated errors by both methods are analyzed for various measurement parameters. Beamforming experiments with vertical line arrays are performed. Holographic reconstruction of simple and complex acoustic sources is also performed to compare the true and estimated results.

An in vitro study of a microbubble contrast agent using a clinical ultrasound imaging system

Optimal insonation settings for contrast imaging are yet to be specified, mainly due to the lack of good understanding of the behaviour of the microbubbles. A satisfactory model that explains the behaviour of individual contrast agent scatterers has not yet been reported in the literature. An in vitro system based on a commercial scanner (ATL HDI3000) has been developed to investigate the backscatter of such agents. Suspensions of Definity® were introduced in an anechoic tank. The frequency of transmitted ultrasound varied from 1 to 5 MHz, pulse period from 2 to 10 periods and peak negative acoustic pressure from 0.08 to 1.7 MPa. The backscatter at the fundamental and second harmonic frequency windows from the agent was normalized in terms of the corresponding components of backscatter from a blood mimicking fluid suspension. The agent provided a dominant resonance effect at 1.6 MHz transmit frequency. Second harmonic normalized backscatter averaged around 9 dB higher than the fundamental. The normalized fundamental backscatter intensity was linear with peak negative pressure. The second harmonic at resonance peaked at 0.5 MPa suggestive of bubble disruption above such pressure. The system proved capable of illustrating the ultrasonic behaviour of Definity® in vitro, and the investigation suggested particular insonation conditions for optimal image enhancement using Definity®.

A new forecasted method of echo stealth effect for underwater target with laying anechoic tiles

Its target strength can be reduced undoubtedly for an underwater target that is possessed of laying anechoic tiles. For evaluating its echo stealth effect, traditional ideas are a numerical evaluation and test of the full-scale target. This paper suggests a new forecasted method named ‘‘The Law of Equivalent Proportions:’’ For laying sound-absorbing material target of sound-absorption coefficient αN at frequency fN with a 1/N target mold, its target strength is reduced to ΔTSN at frequency fN in the anechoic tank test. If a full-scale target is assumed that lay anechoic tiles with sound-absorption coefficient α0 at frequency f0, its target strength is reduced to ΔTS0 at frequency f0, and fN=Nf0 as well as αN=α0, it is ΔTS0=ΔTSN. It is evident from the elliptic cylinder test in the anechoic tank that this conclusion is correct to put the target in a lateral place.

An in vitro system for the study of ultrasound contrast agents using a commercial imaging system

An in vitro system for the investigation of the behaviour of contrast microbubbles in an ultrasound field, that provides a full diagnostic range of settings, is yet to be presented in the literature. The evaluation of a good compromise of such a system is presented in this paper. It is based on (a) an HDI3000 ATL scanner (Bothell, WA, USA) externally controlled by a PC and (b) on the use of well-defined reference materials. The suspensions of the reference ultrasonic scattering material are placed in an anechoic tank. The pulse length ranges from 2 to 10 cycles, the acoustic pressure from 0.08 to 1.8 MPa, the transmit frequency from 1 to 4.3 MHz, and the receive frequency from 1 to 8 MHz. The collection of 256 samples of RF data, at an offset distance from the transducer face, was performed at 20 MHz digitization rate, which corresponds to approximately 1 cm depth in water. Two particle suspensions are also presented for use as reference scatterers for contrast studies: (a) a suspension of Orgasol® (ELF Atochem, Paris, France) particles (∼5 μm mean diameter) and (b) a suspension of Eccosphere® (New Metals & Chemicals Ltd, Essex, UK) particles (∼50 μm mean diameter). A preliminary experiment with the contrast agent Definity (DuPont Pharmaceutical Co, Waltham, MA) showed that the above two materials are suitable for use as a reference for contrast backscatter.

Acoustic measurement facilities at the Applied Research Laboratory

Acoustic measurement facilities, ranging from a water tunnel that is also listed as an ASME Historic Landmark to a new ultrasonic measurement system, are illustrated and described with a brief explanation as to size, measurement equipment available, and frequency ranges that are typically employed. The facilities include a high-pressure tank for hydrophone tests ‘‘at depth,’’ the ultrasonics tank, an anechoic tank for hydrophone calibration, the water tunnel, an air acoustic anechoic room and a large, multipurpose underwater measurement facility.

Evaluation of relative sensitivity of hydrophone with varying hydrostatic pressure employing high pressure tube assembly

The high pressure tube assembly employing active impedance termination method has been found most suited to calibrate deep submergence underwater transducers of low frequencies which cannot be calibrated using conventional anechoic tanks because of fabrication engineering problems as well as multiplying factors of cost. A high pressure tube assembly (12.5 cm internal diameter and 200 cm length) has been employed for the measurements of relative sensitivity of hydrophones with varying hydrostatic pressure. The suitable reference projectors were fabricated which were placed at the end of tube assembly as termination transducers. Four hydrophones as probe hydrophone and Kjaer hydrophone (B&K-8103) for the relative sensitivity measurement have been suitably rigged in the transducer carriage. Free field condition is created with active impedance termination method which is achieved by precise adjustment of amplitudes as well as phases of termination transducers. A method of achieving the free field condition has been evolved which minimizes hit and trial procedure. The variations of the relative sensitivity of a hydrophone (B&K-8103) with hydrostatic pressure up to pressure of 1000 psi have been measured and the values were found close to the values specified (up to 400 meters of ocean depth) by Bruel and Kjaer.

Effects of salt on bubble acoustic radiation in water

Recent measurements of laboratory-generated noise by breaking waves exhibit an increase in sound-pressure levels for salt water compared to fresh water over a broader range of frequencies [A. R. Kolaini, J. Acoust. Soc. Am. 103, 300–308 (1998)]. The surprise increase in sound-pressure levels inspired the study of the effects of salt on the sound radiation by single bubbles released from various size needles. A needle assembly and a false tank (2×2×4 ft), with acoustically transparent walls placed in the middle of an anechoic tank (12×12×8 ft), were used to study the acoustical characteristics of bubbles released from needles. The false tank was filled with water that contained various percentages of sodium chloride (NaCl). In this paper, the results of experiments to examine the variations in radiated sound pressure and the change in the damping coefficient of bubbles as the salinity level increased are reported. The effects of a surfactant agent such as Triton™ 100-X and other mineral acids such as HCl on the bubble acoustic radiation are discussed. The observations show that the change in the local surface tension may not alter the acoustic radiation, whereas the local influence of the ions on water structure, possibly related to the electric repulsion, may play a dominant role in altering the sound pressure and significantly reducing the quality factor of the bubble sound.

Parametric time-frequency estimation of prosthetic heart valve sounds

Prosthetic heart valves have been responsible for extending the life spans and improving the quality of life of many people with serious heart conditions. Even though the heart valves are extremely reliable, eventually they are susceptible to the long-term fatigue and structural failure effects expected for mechanical devices operating over long periods of time. In this paper the application of parametric time-frequency estimation techniques to acoustic measurement of prosthetic heart valve under test in an anechoic tank is described. The analysis reveals interesting signatures when comparing fractured to normal valve sounds. The emphasis is on the application of recursive parametric techniques to ‘‘track’’ changes in the propagating sounds as it is captured by an omnidirectional hydrophone.

Nonlinear, low-frequency sound generation in a bubble layer: Theory and laboratory experiment

The difference-frequency sound generation as a result of interaction of two high-frequency harmonic waves in a bubble layer in water is investigated both theoretically and experimentally. Because the sound speed in the layer is less than that outside, the layer has resonance properties. As was shown before, this can considerably increase the efficiency of the nonlinear frequency transformation. However, unlike the cases considered before, the layer resonance is practically achievable only at the low (difference) frequency, whereas the high-frequency signal (pump) resonates at individual bubbles and then it strongly dissipates. Here the results of an experiment with a bubble layer with a thickness of about 10 cm in an anechoic tank are presented. One of the incident (primary) wave frequencies was 60 kHz, whereas the other could be varied, thus providing the low-frequency signal in the range of 0.8–14.8 kHz. Due to the first-mode layer resonance, this secondary signal had a pronounced maximum at a frequency of 2.4 kHz. The high attenuation of pump waves was due to resonant bubbles. A theory which agrees with the experimental results reasonably well, is developed for this type of interaction.

Localized wave pulses in the Keyport experiment

Localized wave (LW) pulses were produced using a standard Navy array in the anechoic tank at NUWC Keyport. The LW pulses used were the MPS pulse first derived by Ziolkowski, and a new type of pulse based on a superposition of Gaussian beam modes. This new type is motivated by a desire to make a comparison of the MPS pulse with another broadband pulse built from solutions to the wave equation. The superposed Gaussian pulse can be described by parameters which are analogous to those describing the MPS pulse. The directivity patterns and the axial energy decay between the pulses are compared. The behavior of the pulses are found to be similar so that the superposed Gaussian could be another candidate in the class of low diffractive pulses known as localized waves.

Observation of low‐frequency wave generation in a bubble layer.

The difference‐frequency sound generation as a result of the interaction of two high‐frequency harmonic waves in a bubble layer in water are investigated experimentally and theoretically. As it was shown before, the use of the layer resonance can increase the efficiency of the nonlinear transformation of the signal. Here the data of the experiment with a bubble layer of the thickness of about 10 cm in the anechoic tank are presented. One of the incident (primary) wave frequencies was 60 kHz while the another varied from 59 to 50 kHz, thus providing the low‐frequency signal in the range of 1 to 10 kHz. Due to the first‐mode layer resonance, this secondary signal had a pronounced maximum at the frequency of 2.2 kHz (while the primary wave was in resonance with the bubbles of the radius about 45 μm, and no resonance in the layer was observed for it). The bubble volume ratio from these experiments was estimated to be about 1.4×10−4. A theory was also developed for this type of interaction which described well the experimental results. [Work was supported by the Russian Foundation of Fundamental Research Grant No. N0‐93‐05‐8074 and by the Science Opportunity Program of the Office of Naval Research.]

Scattering of sound from laboratory breaking waves

Computer-controlled gravity waves were generated to break in an anechoic tank. A sequence of 15 incident bursts was directed at the bubble cloud entrained by each breaking wave. A burst has seven cycles of a sine wave of frequency ranging from 10 to 15 kHz. The interval between adjacent bursts is set at 0.1 s in order to avoid overlapping the bursts. By analyzing the data of the 15 sound scattering events, the fluctuation was significantly reduced. The bubble clouds were observed to be roughly semicylindrical. The bubble size distribution in an entrained cloud was obtained from video images of a bubble cloud. This experimental information of the shape, size, and bubble size distribution of a bubble cloud was employed as the input into the theory adapted from Sarkar and Prosperetti [J. Acoust. Soc. Am. 93, 3128–3138 (1993)] to estimate the theoretical scattering strength. A comparison between the experimentally measured and the theoretically estimated scattering strengths will be shown. [Work supported by ONR.]

Detection of mechanical changes to prosthetic heart valves by spectral analysis of valve closing sounds.

This study investigates the possibility of using spectral analysis of the closing sounds of a prosthetic heart valve to indicate minor changes to its mechanical state. Sounds are recorded in vitro in a large effectively anechoic tank of water, and the elastically mounted valve is actuated pneumatically within the water. This provides a noise-free environment for recordings. Analysis of the valve sounds is by spectral averaging and digital filtering, with each individual valve used to create its own filter. Results show that it is possible using this technique to detect reliably changes in the mechanical state of a valve, such as the addition of a small mass to the structure of the valve.

Wayne T. Reader: The Vector Research Company years, 1988–1994

After 30 years of distinguished government service Dr. Wayne T. Reader retired in 1988 and joined Vector Research Co., Inc. as principal research scientist. He continued his Navy‐sponsored anechoic materials research program. He recruited a talented research and development team and established a state‐of‐the‐art dynamic material evaluation laboratory. He developed unique experimental procedures for evaluating static and dynamic properties of viscoelastic materials, an extensive material properties database, and analytical computer programs to predict acoustic performance of layered media including anechoic lining systems. He continued to seek economical solutions to acoustic quieting and won a competitive Navy procurement to design and install an innovative lining system to a large anechoic test tank. While continuing his work with passive voided polymers, he began exploring the advantages of composites containing discrete and distributed piezoelectric particles in a polyurethane medium. In addition to his technical work, Wayne Reader established a working environment which met not only his interests but also the company’s and the Navy’s as well. He attracted and mentored several young scientists who continue to work in this field.

Measurements of the scattering of sound from laboratory breaking waves

The results of a laboratory experiment to characterize the backscattered sound field from the bubble cloud generated by spilling breaking waves are described. Gravity waves were generated by a computer controlled plunging-type wavemaker along the length of a 12.7-m-long channel where they were made to break in a 3.6 m×3.6 m×2.4 anechoic tank. An underwater F42A transducer with a special parabolic reflector was used to generate incident bursts of sound ranging from 15 to 40 kHz. This procedure was conducted in the presence of breaking and nonbreaking gravity waves in order to isolate the acoustic scattering strength of the bubble clouds from surface roughness. Detailed measurements of the average void fraction of the bubble cloud at the instant the sound was incident were made. These observations show that there is a significant increase in backscattering strength from the bubble cloud when compared to surface roughness. The scattering strength of the bubble cloud, with an average void fraction of 0.33%, is also shown to increase as the incident frequency approaches the resonant frequency of the individual bubbles in the cloud. A simple theory will be given to predict some of these observations.

Acoustic characterization of laboratory breaking waves in fresh and salt water

Results of a laboratory experiment to characterize the underwater acoustic noise radiated from breaking waves in fresh and salt water are described. The underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz were reported earlier [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am. (in press); 92, 2349(A) (1992)]. These waves were generated by a computer-controlled plunging-type wavemaker and propagated along a 12.7-m-long channel where they were made to break at the midsurface of a 3.6-×3.6-×2.4-m anechoic tank. Specific attention is given to a comparison of the results obtained in salt and fresh water. The sources of acoustic radiation from bubble clouds, the average sound-pressure level, bubble clouds shape, bubble concentration, and size distribution in these medium are discussed. These results may provide considerable insight into the likely source mechanisms for ocean ambient noise.

Observations of underwater sound from laboratory breaking waves and the implications concerning ambient noise in the ocean

The results of an experiment to characterize the underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz are described. Waves are generated by a computer-controlled plunging-type wave maker and propagate along a 12.7-m-long channel where they are made to break at the mid-surface of a 3-×3-×2.5-m anechoic water tank. The individual bubbles and bubble clouds entrained by the breaking wave provide a mechanism for sound production. Using high-speed cinephotography, correlations were established between the hydrodynamic evolution of the cloud and the radiated acoustic emissions. The bubble size distributions inside the cloud were measured with the aid of a high-speed video camera and a fiber optic cable. These measurements indicate that single bubbles with radii as large as 7–8 mm may be entrained in this fresh-water system by moderate spilling breakers. Detailed measurements of the bubble size distribution of the bubble cloud enabled us also to obtain a measurement of the average void fraction in the cloud. These observations reveal that the sources of sound in laboratory spilling breakers is due mostly to single bubble oscillations that can have frequencies as low as 400 Hz; in the case of plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions. The acoustic radiation from bubble clouds is the result of collective oscillations of the bubbles, stimulated by large scale vortices created by the plunging breaker. The sound spectra, averaged over 100 breakers, reveal the following observations. First, the peak frequencies of the average sound spectra shifts from few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker). Second, the sound pressure levels increase in all frequency bands with increasing breaker severity. Lastly, the high-frequency portions of the sound spectra have slopes of about 5–6 dB/oct, which are the slopes observed for the noise spectra of the ocean. These results provide considerable insight into the likely source mechanisms for ocean ambient noise.

Detection of Intense Ultrasound by the Cod Gadus Morhua

ABSTRACT The ability of cod to detect intense 38kHz pulses was studied in an anechoic tank, using a classical cardiac conditioning method. All 15 cod studied reacted to 3ms pulses. The average threshold was 194.4dB re 1μPa (S.D. 5.4dB, S.E.M. 0.3dB). The data do not reveal the sensory modality involved. A number of additional experiments indicate that the ultrasonic energy per se is the effective stimulus. The threshold is too high for most sources of marine ultrasound to be detected, but is low enough to allow the cod to detect echolocating odontocetes at a range of 10–30m. The results also indicate that cod can detect echo-sounders of the fishing fleet at similar ranges.

A simple water‐filled compliant‐tube calibrator

Several geometries for low‐frequency underwater calibrations are in common use, but often require either anechoic walls, rigid walls, or tanks (or lakes) of large dimensions with respect to a wavelength. Such facilities are frequently not practical for use in teaching laboratories either because of inaccessibility or because the facility must be shared with active researchers. The compliant, water‐filled cylindrical tube described in this paper permits low‐frequency calibrations in a laboratory environment, with modest cost and size. The sound speed inside one such device is approximately 300 m/s, which allows 100‐Hz measurements in tubes 1 m long. The limitations of the compliant tube calibrator and examples of measurements made using the calibrator will be discussed.

Sound propagation through a bubble screen

Results of a laboratory experiment to characterize the sound propagation through a bubble screen are described. A bubble screen of various void fractions was positioned in the middle of an anechoic water tank. The cloud was driven using a broadband source and the acoustic pressure was measured on both sides of the layer. The results have revealed that there is significant attenuation near the resonance frequency of the bubbles contained within the screen. When the source frequency is near that of one of the ‘‘collective‐oscillation’’ frequencies of the screen, sound pressure amplification may occur. In this paper the new findings corresponding to the collective mode amplification, high‐frequency attenuation, as a function of the screen’s physical properties are discussed. [Work supported by ONR.]