Stephen E. Forsythe

Low-frequency echo-reduction and insertion-loss measurements from small passive-material samples under ocean environmental temperatures and hydrostatic pressures.

System L is a horizontal tube designed for acoustical testing of underwater materials and devices, and is part of the Low Frequency Facility of the Naval Undersea Warfare Center in Newport, Rhode Island. The tube contains a fill fluid that is composed of a propylene glycol/water mixture. This system is capable of achieving test temperatures in the range of -3 to 40 deg Centigrade, and hydrostatic test pressures in the range 40 to 68,950 kPa. A unidirectional traveling wave can be established within the tube over frequencies of 100 to 1750 Hz. Described here is a technique for measuring the (normal-incidence) echo reduction and insertion loss of small passive-material samples that approximately fill the tube diameter of 38 cm. (Presented also is a waveguide model that corrects the measurements when the sample fills the tube diameter incompletely.) The validity of the system L measurements was established by comparison with measurements acquired in a large acoustic pressure-test vessel using a relatively large panel of a candidate material, a subsample of which was subsequently evaluated in system L. The first step in effecting the comparison was to least-squares fit the data acquired from the large panel to a causal material model. The material model was used to extrapolate the panel measurements into the frequency range of system L. The extrapolations show good agreement with the direct measurements acquired in system L.

A scanning laser Doppler vibrometer acoustic array

Experiments confirm that a laser Doppler vibrometer can be used to detect acoustic particle velocity on a fluid-loaded acoustically compliant, optically reflective surface. In these experiments, which were completed at the Acoustic Test Facility of the Naval Undersea Warfare Center, Scotchgard™ reflective tape was affixed to the interior surface of a standard acoustic window. The polyurethane array window had a thickness of 0.9525cm (0.375in.) and a material density of 1000kg∕m3. The surface velocity measured, using a commercial scanning laser vibrometer system (SLVS), was beamformed conventionally and flawlessly detected and localized acoustic signals. However, the laser Doppler vibrometer used in the experiments had relatively poor acoustic sensitivity, presumably due to high electronic noise in the photodetector, speckle noise, standoff distance, and drifting laser focus. An improved laser Doppler vibrometer, the simplified Michelson interferometer laser vibrometer sensor (SMIV), is described in brief....

Dolphin-inspired combined maneuvering and pinging for short-distance echolocation

The biorobotic emulation of swimming and flying animals carrying out short-distance echolocation while maneuvering is considered. A simple and lightweight sonar for use on a small, maneuverable underwater vehicle for short-distance echolocation is explored. This sonar has four sensors and uses broadband, high-frequency signals to echolocate. The frequency-time characteristics of these signals are compared to those of bats and dolphins. The biosonar is paired with a biologically inspired, maneuverable, underwater vehicle, the combined use of sensors and maneuverability being analogous to animal behavior. Homing experiments have been carried out in an acoustic test facility where identification and localization of multiple targets is based on fusion of acoustic returns from multiple pings.

Ocean Variability Effects on High‐Frequency Acoustic Propagation in KauaiEx

During the Kauai experiment in summer of 2003 a bottom‐mounted vertical line array containing 8 hydrophones spaced 0.6 meter apart was deployed in a 100‐m shallow water region near the Pacific Missile Range Facility. The acoustic source was placed about 2 km away on a flat sea bottom at 95 meter water depth. The element spacing was sufficiently small to allow measurements of the temporal variability of time‐angle intensity fluctuations of the acoustic energy. Measurements were made simultaneously of the broadband acoustic pulse transmissions (8–50 kHz) and environmental parameters. The latter measurements included current, temperature and salinity profiles, directional surface wave spectra, as well as wind speed and direction above the sea surface. Arrival time‐angle fluctuations were found to be correlated with the environmental variability due to ocean dynamics in this region. It is shown that variations of the sea surface dynamics exhibit different temporal effects than those occurring within the water...

Acquiring acoustic data using a scanning laser vibrometer.

Scanning laser vibrometers have been in use for many years in acoustic measurements. Unique methods of characterizing both active and passive devices or structures with vibrational modes have been developed by NUWC personnel; these are combinations of signal generation techniques and physics‐based data analysis tools. The signal generation techniques reduce the time for potentially lengthy scans at multiple frequencies through the use of a “comb” waveform, [J. Acoust. Soc. Am. 116(5), (2004)], if the device under test is known to be linear. Scans can be done in either air or water. An analysis technique using the Helmholtz integral (using NUWC’s MATLAB‐based CHIEF code; Schenck and Benthein, NOSC 1970, 1988) allows analysis from an in‐air scan of a transducer face (before encapsulation) to predict the transducer’s far‐field beam patterns under some conditions. This is especially valuable as a time‐saving tool to ensure that the transducer is on track to achieve the final design intent. Examples of the tec...

Use of complex source points to simplify numerical Gaussian beam synthesis

It is often desirable to generate the acoustic field due to a so‐called Gaussian beam. One way to do this is to use the free‐space Greens function for the acoustic field and to sum small area sources over a circular plate with the appropriate shading for the desired Gaussian beam. For very high frequencies and narrow beams, the computation time to give an accurate sum can be large when calculating the sum for many points in the acoustic field. An alternate approach comes from the use of a single point source with complex coordinates R=[Xr+iXi,Yr+iYi,Zr+iZi]. When this complex source point is used in the free‐space Greens function, the formal expressions for pressure and particle velocity can be used if careful attention is paid to the interpretation of the complex distance, r, that arises in the exp(ikr)/r term. The singularity is no longer a single point in the case of a complex source, but a circular disk. The far field of a complex source point is a good approximation to a Gaussian beam. Several comput...

Relating ocean dynamics and sea state to time‐angle variability of HF waveforms

One of the objectives of the Kauai experiment was a better understanding of the ocean dynamics effects on the propagation of high‐frequency acoustic signals. Due to a unique oceanographic feature of the shallow water region near the Pacific Missile Range Facility in Kauai, a bottom mounted vertical line array containing eight elements was deployed with sufficiently small element spacing to measure the acoustic energy near the bottom. Simultaneous environmental parameters including current, temperature and salinity profiles, directional surface wave spectra, as well as the wind speed and direction above the sea surface were measured. High correlation between the environmental variability and the received acoustic signals is observed. To interpret the results broadband PE and Gaussian beam ray tracing models were utilized. Arrival time‐angle statistics are correlated with the environmental variability due to ocean dynamics in this region. It is shown that variations of the sea surface dynamics exhibit diffe...

Use of complex source points to simplify Gaussian beam synthesis

It is often desirable to generate the acoustic field due to a so‐called Gaussian beam. One way to do this is to use the free‐space Greens function for the acoustic field and to sum small area sources over a circular plate with the appropriate shading for the desired Gaussian beam. For very high frequencies and narrow beams, the computation time to give an accurate sum can be large when calculating the sum for many points in the acoustic field. An alternate approach comes from the use of a single point source with complex coordinates R=[Xr+iXi,Yr+iYi,Zr+iZi]. When this complex source point is used in the free‐space Greens function, the formal expressions for pressure and particle velocity can be used if careful attention is paid to the interpretation of the complex distance, r, that arises in the exp(ikr)/r. The singularity is no longer a single point in the case of a complex source, but a circular disk. The far field of a complex source point is a good approximation to a Gaussian beam. Several computational uses of the technique will be demonstrated. [Work supported by ONR.]

Current tomography in coastal regions

Reciprocal acoustic transmissions have proved successful for tomography in deep water. However, relatively little effort has been focused on shallow‐water environments where sound propagation and scattering takes place often over very short ranges in both the ocean (due to volume and surface) and the seabed. In these regions the effect of depth variations on sound speed is negligible and the salinity and temperature are dominant causes of volume fluctuations. Nevertheless, the methodology can still be utilized to determine the current. In a well‐calibrated high‐frequency (0.6–18 kHz) acoustic experiment, the range averaged horizontal current in two different directions is determined based on the differential travel times recorded from three reciprocal source–receiver stations. These tripod stations were arranged in a triangular configuration. Data were obtained in nearly isovelocity condition with slight variations in the salinity profile during several tide cycles. The feasibility of obtaining range aver...

Intensity variations of high‐frequency broadband acoustic scatter from sea surface in coastal environment

Data from a broadband propagation experiment for mid‐to‐high‐frequency (0.6–18 kHz) in the very shallow water (15 m) of Delaware Bay are studied over a wide range of environmental conditions. Physical parameters such as temperature and salinity as well as hydrodynamic parameters such as surface waves, tide, and current were measured simultaneously with acoustic transmissions. Three hydrophones were utilized to record the received signals in the source/receiver range of 389 m. A beamforming technique is used to allow examination of the arrival angle as a function of arrival time. Statistical correction is applied to normalize the arrival signals and to correct for sidelobes. Four ray paths having only one interaction and scattered by the sea surface into micro‐multipaths are considered for analysis. The focus of this study is on the evaluation of these signals as a function of sea surface roughness during several tide cycles. To analyze the data, PE calculations are conducted and parameter studies for diff...