Sound Reference Research Articles

Underwater acoustic measurement facilities at NRL/USRD

At the beginning of World War II, the United States recognized the need for establishing systematic methods for calibrating and evaluating sonar transducers and established the Underwater Sound Reference Laboratories. The Underwater Sound Reference Detachment of the Naval Research Laboratory evolved from this beginning and now serves as the focus for underwater acoustic metrology in the United States. It functions as a ‘‘National Bureau of Standards for Underwater Acoustics’’ by providing two types of reference services for a fee. Calibration, test, and evaluation services and standard transducer loan services cover the frequency range of 0.2 Hz to 2 MHz. This paper covers the measurement facilities for carrying out the metrology function and the measurement methods for setting the primary standards and the loan standards. Evaluation of sonar transducers and some of the ongoing research in calibration measurement methodology is also discussed. Finally, some comments are made about calibration system requirements for implementing new measurement techniques.

Transmission coefficient measurement and improved sublayer material property determination for thick underwater acoustic panels: A generalization and improvement of the ONION method

Modifications of the ONION panel-measurement method [J. C. Piquette, J. Acoust. Soc. Am. 85, 1029–1040 (1989)] that allow for simultaneous analysis of transmitted- and reflected-wave data are described. The revised algorithm determines more reliable values for the sound speed and loss of the material of each panel sublayer than does the algorithm that is based exclusively upon analysis of the reflected wave. Included in the revised method is a Taylor series expansion of the sound-speed function of each layer about the steady-state driving angular frequency. This Taylor series is similar to that used for the loss function in the original ONION method, and is introduced here to more accurately accommodate the frequency variation of the phase speed than does the frequency-independent model used previously. Descriptions of successful applications of the revised ONION method to experimental data are provided. The version of the ONION method described in this report has recently been adopted as the standard panel measurement method at the Underwater Sound Reference Detachment of the Naval Research Laboratory (NRL-USRD) in Orlando, FL for tests conducted in the 1- to 20-kHz frequency interval.

Comparison of dielectric and Young's modulus in four types of elastomers

Underwater Sound Reference Detachment, Naval Res. Lab., P.O. Box, 568337, Orlando, FL 32856-8337. The complex Young's modulus is compared with the complex dielectric modulus in four different types of elastomer in the rubbery regime above the glass transition temperature. The elastomers measured included 5109G unfilled neoprene, PR1526 polyurethane, 334-270 natural rubber, and B252 butyl rubber. Results of the comparison support the view that there is a correspondence between the two effects as predicted from thermodynamic considerations. Possible models for the coupling are discussed.

Nearfield calibration array for hydrophones and arrays

A plane‐wave volume was synthesized with a line array of projectors in a 7‐ to 50‐kHz frequency range enabling hydrophones and hydrophone arrays to be calibrated in the nearfield. The plane‐wave volume was created using the Van Buren technique by revolving a suspended line of PZT‐4 type transducers in a semicircular fashion. Results were compared with the known sensitivities and directivity patterns of the hydrophones used. Amplitude shading coefficients for the line array were determined by a computer program furnished by A. L. Van Buren [Underwater Sound Reference Detachment, Naval Research Laboratory].

Calibration of underwater acoustic transducers at NRL/USRD

At the beginning of World War II, the United States recognized the need for establishing systematic methods for calibrating and evaluating sonar transducers and established the Underwater Sound Reference Laboratory. The Underwater Sound Reference Detachment of the Naval Research Laboratory evolved from this beginning and now serves as the focus for underwater acoustic metrology in the United States. This paper covers the measurement facilities and methods for carrying out the metrology function. Evaluation of sonar transducers and some of the ongoing research in calibration measurement methodology are also discussed. Finally, some comments are made about calibration system requirements for implementing new measurement techniques.

The ONION method: A reflection coefficient measurement technique for thick underwater acoustic panels

An earlier report [J. C. Piquette, ‘‘An extrapolation procedure for transient reflection measurements made on thick acoustical panels composed of lossy, dispersive materials,’’ J. Acoust. Soc. Am. 81, 1246–1258 (1987)] described a new technique for evaluating underwater panel measurements. The method is based on least‐squares fitting to a multiple‐layer model. The present article describes further research on the technique, including a description of a successful application to measurements made under nonatmospheric hydrostatic pressure. Also described are enhancements that increase the speed of the algorithm as implemented on a digital computer, and that render the technique practical for use in reducing the massive amounts of data that can be obtained in panel calibration measurements. The method is currently being transitioned to an on‐line panel‐measurement capability in the Anechoic Tank Facility (ATF) of the Underwater Sound Reference Detachment of the Naval Research Laboratory (NRL‐USRD), located in Orlando, Florida.

Laser Doppler anemometry detection of hydroacoustic particle velocity

The method of laser Doppler anemometry is applied to the measurement of particle velocity in a hydroacoustic field. The field is created in an Underwater Sound Reference Detachment-type G19 calibrator. The anemometer is operated in the forward-scattering mode. Seeding particles of varying material and size are dispersed in the acoustic medium (water). The laser Doppler anemometry results are compared with the values for the velocity obtained from the pressure gradient measured by a hydrophone.

The frequency dependence of subjective reaction to vertical and horizontal whole-body vibration at low magnitudes

The assessment of building vibration with respect to human response requires knowledge of the influence of vibration magnitude, frequency, direction, and duration. This article reports an experiment conducted to investigate the effect of three of these factors: frequency, magnitude, and direction. The method of magnitude estimation was employed with a sound reference stimulus (1/3 octave band centered at 1 kHz) so as to determine reaction to vertical (z-axis) and horizontal (y-axis) sinusoidal whole-body vibration. Twenty subjects were exposed to six acceleration magnitudes in the range 0.04–0.4 ms−2 rms at nine frequencies between 4 and 63 Hz. The relation between magnitude estimate ψ and acceleration magnitude φ was determined at each frequency in the psychophysical power form ψ=kφn. The value of the exponent n varied from 1.04 to 1.47 for vertical motion and from 0.68 to 1.99 for horizontal motion. For vertical motion, the effect of frequency was not significant; for horizontal motion, the exponent increased with increasing frequency. Vibration frequency weightings were determined to describe subjective response to whole-body vibration at low magnitudes that might occur in buildings.

The sea-surface microlayer of puget sound: Part I. Toxic effects on fish eggs and larvae

The sea surface is an important habitat for the developmental stages (eggs and larvae) of many fish and invertebrates; it is also a concentration point for anthropogenic contaminants entering the sea. Studies were conducted to determine the extent to which the sea surface of Puget Sound was toxic to the early life history stages of fish. Three urban bays with suspected contamination, a rural reference bay, and a Central Sound site were compared. Surface-dwelling eggs and organisms (zooneuston) were collected with a surface-skimming neuston net and their densities enumerated. Sand sole ( Psettichthys melanostictus) embryos were exposed in the field and laboratory to the sea-surface microlayer. To develop a useful year-round approach to monitoring sea-surface toxicity, larval development of anchovies, kelp bass, and sea urchins was also evaluated as an indication of sea-surface microlayer toxicity. During the spawning season (February and March), urban boys in Puget Sound had lower concentrations of sand sole eggs and neustonic organisms on the sea surface than did the rural bayor Central Sound reference sites. Compared to the reference sites, laboratory exposure to surface microlayer samples collected from urban bay sites generally resulted in more chromsomal aberrations in developing sole embryos, reduced hatching success of sole larvae, and reduced growth in trout cell cultures. In situ hatching success of sole eggs was reduced by half or more in urban bays compared to reference sites. Toxicity was associated with visible surface slicks and, in urban bays, increased with increasing surface pressure (dynes cm −1). Results to be reported separately (Part II) indicate that toxicity is strongly correlated with the presence of high concentrations of polycyclic aromatic hydrocarbons and metals in the sea-surface microlayer. The toxicity of SMIC samples was similar when evaluated by sole, anchovy, kelp bass, or sea urchin tests. A sea-surface monitoring program could use sea urchin embryos to evaluate site-specific sea-surface toxicity throughout the year.

Toxic Chemicals in Marine Sediment and Biota From Mukilteo, Washington: Relationships With Hepatic Neoplasms and Other Hepatic Lesions in English Sole (<italic>Parophrys vetulus</italic>)<xref ref-type="fn" rid="FN2">2</xref><xref ref-type="fn" rid="FN3">3</xref>

High prevalences of idiopathic liver lesions, including 7.5% hepatic neoplasms (e.g., hepatocellular and cholangiocellular carcinomas) and 16.7% foci of cellular alteration (putative preneoplastic hepatic lesions), were found in English sole (Parophrys vetulus) from waters near Mukilteo, a small community on Puget Sound in Washington State. Sediments from the sampling sites contained particularly high concentrations of aromatic hydrocarbons. Substantially lower concentrations of these compounds were found in sediments from another Puget Sound (reference) site in which fish showed no evidence of hepatic neoplasms or foci of cellular alteration. Stomach contents from the fish at Mukilteo contained substantially higher concentrations of the chemicals than did stomach contents of fish from the relatively uncontaminated site. High concentrations of metabolites of aromatic compounds were measured in the bile of fish from Mukilteo. These findings support previously observed relationships between sediment chemicals (e.g., aromatic hydrocarbons) and high prevalences of liver lesions in English sole from Puget Sound. In addition, a dietary route of uptake by English sole of environmental chemicals, including known carcinogens, was documented.

An automated admittance measuring system for electroacoustic transducers

A fully automated method of obtaining admittance loops of piezoelectric transducers has been designed to be utilized in conjunction with the H‐P 4192A impedance analyzer. The algorithm determines the frequencies at maximum and minimum admittances; the series and parallel resonance frequencies; the resonance and antiresonance frequencies as well as the mechanical quality factor of the element under test. The collected data are plotted in the form of admittance loops with evenly spaced points. This program has been implemented successfully at the Underwater Sound Reference Detachment as a means to rapidly obtain the admittance information of various loaded and unloaded underwater transducers and transducer elements. [Work sponsored by NAVSEA.]

5–50 kHz cylindrical nearfield calibration array

In acoustic measurement tanks it is often impossible to directly determine the farfield radiation properties of underwater transducers because of limited space. The Trott nearfield calibration array (NFCA) concept solves the problem by allowing the farfield properties to be obtained by measurements made close to the transducer. We describe here the development of a 5–50 kHz synthetic cylindrical NFCA that is suitable for medium size measurement tanks such as the pressure tank at the Underwater Sound Reference Detachment of the Naval Research Laboratory. The NFCA is synthesized using a single line of 48 piezoelectric ceramic hydrophone elements spaced 2.54 cm apart and amplitude shaded in a special way. The transducer to be calibrated is driven electrically and rotated near the line about an axis parallel to the line. The response of the line is measured at equally spaced angular positions, thereby creating a virtual cylindrical array surrounding the transducer. The azimuthal farfield pattern of the transducer is obtained by computer processing the responses together with a set of complex shading coefficients. We evaluated the line experimentally by calibrating a large piston transducer. The resulting computed farfield patterus are in excellent agreement with corresponding patterns obtained from direct farfield measurements made in a lake. [Work supported by NAVSEA 63R.]

Development of a hydraulically powered underwater acoustic projector for use in the range of 1 to 10 Hz

Development of a hydraulically powered acoustic projector with a minimum source level of 160 dB re:1 μPa at 1 m over the 1 to 10 Hz range will be presented. This projector will be used at the Underwater Sound Reference Detachment for low‐frequency underwater acoustic testing. Design parameters and their relation to the unique capabilities of hydraulic power for this low‐frequency application will be discussed. Construction details, and acoustic as well as mechanical performance will also be presented. [Work supported by Naval Sea Systems Command, Code 63R.]

A standard pressure gradient hydrophone

The Naval Research Laboratory, Underwater Sound Reference Detachment, has developed a standard pressure gradient hydrophone that has a very stable response with hydrostatic pressure to 70 MPa and temperature from 3° to 30°C. The useful frequency range extends from 10 Hz to 8 kHz. The small size of the hydrophone (0.064 m in diameter) allows its use as a reference standard in our low‐frequency pressure vessels.

Computer methods for high-power pulsed immittance measurements

Characterizing the input immittance of underwater sound projectors under normal operating conditions requires high-power free-field testing. Pulse techniques offer a convenient approach to effecting free-field conditions in a limited space. Existing measurement systems estimate immittance at the carrier frequency using the steady-state region of projector voltage and current pulses. In extending the measurement capability at the Underwater Sound Reference Division, Naval Research Laboratory, these projector waveforms are sampled, discrete Fourier transforms taken, and immittance estimated using on-line computer control. The Fourier transform provides, in addition to the steady-state point-by-point results, a broad-band immittance plane pictured for rapid exploration of device characteristics. The impact of sampling rate, pulse shape, and device bandwidth are examined. An additional advantage of using the complete pulse spectrum rather than the steady view is that shorter pulses that yield equivalent results can be used to extend the lower frequency limit for a given space, such as a tank. An alternate definition of impedance is examined using the ideas of real and reactive power.

Measuring the directivity index of underwater sound projectors

The directivity index D i of an underwater sound source usually is approximated because of the difficulty involved in performing the required power measurements and integrating over all angles. A simulation study conducted at the Underwater Sound Reference Division resulted in a model of a D i measuring system that minimizes the complexity of measuring the power profile about a sphere with accuracy to within ±0.1 dB. A system was assembled in accordance with the model and experimental measurements have been made in a shallow lake. The instrumentation and measurement techniques are presented in this paper. A complete three-dimensional power profile can be obtained in approximately 10 min running time with conventional hardware. Measured values of D i are in close agreement with theoretical calculations based on a perfect piston raditors, except for patterns with significant sidelobes.

Engineering Acoustics Workshop on the Accuracy of Calibrating Underwater Sound Transducers

Measurement accuracy in the calibration, test, or evaluation of underwater sound transducers is a somewhat neglected subject. It is the purpose of this workshop to shed some light on the subject and the reasons for its neglect. A panel of four, representing three naval laboratories and an observer of European laboratories will address several questions. What do the laboratories claim for accuracy? What is meant by “accuracy”? What is the claim based on? Can more accurate measurements be made? What would it cost? Is there a need or demand for more accuracy? After a ten-minute informal presentation by each panel member, the floor will be open for comment and contributions by others who perform measurements, use the data, or are otherwise interested in the subject. Representatives of several industrial and university, as well as naval, organizations have been invited to comment. The panel members are: Joseph Blue, Underwater Sound Reference Division, Naval Research Laboratory, Orlando, Florida; Charles Green, Naval Undersea Center, San Diego, California; Joseph Navin, Naval Underwater Systems Center, New London, Connecticut; W. James Trott, Acoustics Division, Naval Research Laboratory, Washington, D. C., and Chairman, Working Group on Hydrophones, International Electrotechnical Commission Technical Committee on Ultrasonics.

Calibration Accuracy for Small Standard Hydrophones

When a standardizing laboratory issues a certificate for a standard, it attributes to that standard a numerical value together with a statement about the uncertainty of this value. In the measurement of the sensitivity of an unknown hydrophone performed once, most activities will claim an accuracy of ±1 dB. Usually no confidence limits are specified, and the accuracy is an educated guess based on experience. This paper discusses the careful “free-field” reciprocity calibration of two small standard piezoelectric hydrophones in a stable environment and the use of these hydrophones in evaluating the calibration performance of six of the established calibration facilities at the Underwater Sound Reference Division of the Naval Research Laboratory. The results of these measurements show that, for the most part, accuracy of the facilities is well within the ±1 dB commonly quoted and is probably within ±0.3 dB with 95% certainty at some of the facilities when calibrating small standard piezoelectric hydrophones.

Low-Noise Broad-Band Hydrophone

A pressure-sensitive noise-measuring hydrophone for the frequency range 10 Hz to 60 kHz that is stable at hydrostatic pressures to 6.9 MPa and temperature from 5° to 35°C has been developed at the Underwater Sound Reference Division, Naval Research Laboratory. The pressure-sensing element is of a novel design that offers high sensitivity, relatively small size, and smooth response. The electrical output impedance of the sensitive element is sufficiently low that low-noise voltage preamplifiers may be used to give an end-of-cable self-noise that is lower than that equivalent to sea state zero.

Speech Intelligibility of Diving Masks and Mouthcups

In order to speak underwater, SCUBA divers ordinarily must replace their mouthpieces with some sort of muzzle, mask, or helmet. The addition of such cavitites to the vocal tract usually results in distortion to the speech signal. Accordingly, it was the purpose of this research to investigate variation in diver/subject speech intelligibility resulting from the use of such devices. Included in this evaluation were all commercially available units plus several prototypes; they were (a) mouthpieces: double-hose regulator bit; (b) mouthcups: Bioengionics, Raytheon, U. S. Divers; (c) full face masks: Scott, MDL, Desco; and (d) helmets: Advanced, Kirby-Morgan. All units were evaluated under similar conditions, i.e., with double-hose regulators and (in most instances) with Aquasonics microphones placed in the breathing cavity. Two studies were carried out. The first in a hyperbaric chamber at the Naval Ship Research and Development Laboratory, Panama City, Florida and the second at NRL Underwater Sound Reference Division, Bugg Spring facility, Leesburg, Florida, and at Rainbow Springs, Dunnellon, Florida. In the first study, diver's speech intelligibility was assessed by standard procedure for five units (bit, Bioengionics, Raytheon, Scott, and MDL) at simulated depths of 0, 12, 50, and 100 ft; a small “wet pot” was utilized for the tests. The second study was carried out for all units in 12 ft of water at Bugg and Rainbow Springs. Divers were held in a fixed position and the speech signal was transmitted by “hard line” directly to a tape recorder at the surface. Data show considerable variability among the units evaluated; highest speech intelligibility was found for one of the mouthcups and for the helmets.