James A. McConnell

Analysis of a compliantly suspended acoustic velocity sensor

The dynamics of a compliantly suspended acoustic velocity sensor having a spherical geometry are analyzed using theory and experiment. The analysis starts with a review of the motion associated with an unconstrained solid sphere when subjected to an acoustic plane wave in an unbounded inviscid fluid medium. The theory is then modified to account for the inclusion of an inertial sensor and an external suspension system. Accordingly, the open-circuit receiving response of a geophone-based and accelerometer-based device is derived. Density variations associated with the sphere and the surrounding fluid medium are assessed along with the effects fluid viscosity. Wave effects in the sphere and the suspension system are also analyzed.

Development of a velocity gradient underwater acoustic intensity sensor

A neutrally buoyant, underwater acoustic intensity probe is constructed and tested. This sensor measures the acoustic particle velocity at two closely spaced locations, hence it is denoted a “u-u” intensity probe. A new theoretical derivation infers the acoustic pressure from this one-dimensional velocity gradient, permitting the computation of one component of acoustic intensity. A calibration device, which produces a planar standing-wave field, is constructed and tested. In this calibrator, the performance of the u-u intensity probe compares favorably to that of an acoustic intensity probe which measures both pressure and velocity directly.

Highly directional receivers using various combinations of scalar, vector, and dyadic sensors

The generalized theory of directional sensors is presented in the form of the Taylor series expansion of the acoustic pressure about a point in space. If the expansion is truncated to second order, the analysis of scalar, vector, and dyadic sensors can be made and corresponds to the zeroth‐, first‐, and second‐order gradient of the acoustic pressure. This translates into using a sufficient number of omni‐directional hydrophones or a multimode hydrophone in conjunction with the appropriate finite‐differencing operations to achieve the desired beam patterns. Using the linearized Euler equation, the formulation can be recast in terms of the zeroth‐order gradient of the acoustic pressure along with the zeroth‐ and first‐order gradient of the particle acceleration. In this case, the zeroth‐order terms can be measured directly with an omni‐directional hydrophone and a neutrally buoyant accelerometer. The gradient of the particle acceleration can be measured indirectly using finite differences or directly by measuring the angular acceleration akin to a Rayleigh disk. Of particular interest is the use of scalar, vector, and dyadic sensors to localize sources of sound using arc‐tangent‐squared processing and cardioid‐squared processing as opposed to conventional arc‐tangent and cardioid processing. [Work supported by ONR 321SS.]

Sound‐speed determination in a fluid‐filled elastic waveguide

Theory and test methods are presented for determining the longitudinal sound speed inside a fluid‐filled elastic waveguide. The waveguide is a vertically oriented open‐ended column of water equipped with a moving coil driver. The elastic properties of the waveguide reduce the sound speed to the point where a passive anechoic termination can be used effectively. This facilitates a compact device that exhibits plane progressive waves for sensor calibration at low frequencies. Once the sound speed is determined, then the size of the device and usable bandwidth are known. The sound speed is determined through measurement of the longitudinal resonance frequencies associated with the free vibration case of a duct having ideal rigid and pressure‐release boundary conditions. In practice, however, the moving coil driver presents an impedance boundary condition (e.g., not ideally rigid) that inadvertently couples the mechanical system of the driver to the acoustic system of the waveguide. To circumvent this issue, ...

Forming first‐ and second‐order cardioids with multimode hydrophones

Most everyone in sonar signal processing is familiar with forming a first‐order cardioid using scalar and vector sensors, which are defined as the reciprocal analog of a monopole and dipole radiator, respectively. The second‐order cardioid can be formed using scalar, vector, and dyadic sensors, in which the latter sensor is defined as the reciprocal analog of a quadrupole radiator. Multimode hydrophones configured as piezoelectric cylinders and spheres with segmented electrodes have the ability to form the both types of cardioids. This paper compares and contrasts the use of multimode hydrophones to produce each beam pattern and presents the results of several sensors that were developed. Topics relating to the sensitivity, bandwidth, noise floor, beamwidth, beam steering, and directivity index will be covered. [Work supported by C. M. Traweek at the Office of Naval Research Code 321MS and L. Shumway at the Department of Homeland Security.]

Review of directional sensors for low frequency underwater acoustic applications

In this review, we present some of the more common embodiments directional sensors take on for use in low-frequency underwater acoustic applications. Sensors that fit within this paradigm are typically substantially smaller than an acoustic wavelength and can form first- or second-order cardioid beams at frequencies well below 10 kHz. A mathematical treatment of scalar, vector, and dyadic acoustic fields is presented as it pertains to understanding the phenomenology the sensors are required to measure. Cardioid beam-forming and directivity index are explained to provide context of using directional sensors in aperture constrained situations. The electro-acoustic performance of various sensor concepts is presented using lumped parameter models. Packaging concepts for high-fidelity operation, low self-noise, and low electronic noise in the seawater environment are shown. Special problems such as using directional sensors in deep water or near impedance boundaries is also covered.

Development of a uniaxial pressure-acceleration probe for diagnostic measurements performed on a spherical sound projector

Historically speaking, underwater acoustic vector sensors have seen widespread use in direction finding applications. However, given that this class of sensor typically measures both the acoustic pressure and at least one component of the particle velocity at a single point in space, they can be used effectively to measure the acoustic intensity and/or the acoustic impedance. These metrics can be useful in understanding the acoustic field associated with simple and complex sound radiators. The focus of this paper concerns the development of a uniaxial pressure-acceleration (p-a) probe to measure the specific acoustic impedance of a spherical sound projector (i.e., International Transducers Corporation ITC1001 transducer) over the frequency range from 2.5 to 10 kHz. The design, fabrication, and calibration of the probe are covered along with the results of the aforementioned experiment. Results show that reasonable agreement was obtained between the measured data and an analytical prediction, which models ...

Vector sensors for airborne surveillance applications

The use of vector sensors for airborne surveillance applications (e.g., frequencies below 500 Hz) is discussed with emphasis on transducers that measure the acoustic pressure-gradient. Traditional approaches such as ribbon microphones, hot-wire anemometers, and finite-difference techniques will be reviewed. The crux of the presentation concerns a discussion of diffraction type pressure-gradient microphones that go beyond the classic ribbon microphone and utilize large format membrane transduction elements comprised of piezoelectric and electret materials. The results of analytical, numerical, and experimental evaluations will be presented.

Flow‐induced noise on a pressure‐acceleration underwater acoustic intensity probe

The results of an experiment to determine the effect of flow‐induced noise on a pressure‐acceleration intensity probe are presented. The sensor is a bluff body that conforms to the geometry of a right circular cylinder having a diameter of 10.16 cm and an aspect ratio of unity. A flexural mode hydrophone is used to measure the acoustic pressure and a flexural mode accelerometer is used to measure the acoustic particle acceleration. The hydrophone has a nominal sensitivity of about −185 dB re: 1 V/uPa and the accelerometer has an in‐water acoustic sensitivity of nearly 1 V/g. The bandwidth of the device covers the 10‐Hz to 1‐kHz frequency range and the principle axis of sensitivity is coincident with the axis of the cylinder. The sensor was towed in cross‐flow at speeds ranging from 0.1 to 0.5 cm/s and data were collected over the 10‐Hz to 100‐Hz frequency range. Of particular interest is the decomposition of the intensity spectrum into real and reactive parts along with an assessment of any filtering that...

Development of a compact suspension system for a neutrally buoyant underwater acoustic intensity probe

Practical use of neutrally buoyant underwater acoustic intensity probes requires a suspension system to position them in some preferred orientation with respect to an acoustic field of interest. However, since these probes typically contain a moving coil geophone to facilitate particle velocity measurements, the design of such suspension systems is not trivial. On one hand, theory mandates that the dynamics of the geophone will not be adversely affected by the suspension as long as the mass‐spring system created by the probe and the elastic members has a low‐resonance frequency and high quality factor relative to that of the geophone. However, by meeting these two design criteria, wave effects (e.g., flexural resonances) in the elastic members can become a limiting factor in the performance of the device, particularly at high frequencies. This paper presents theoretical and experimental results of an effort to design a compact suspension system that embodies the aforementioned design considerations, but h...