Joseph E. Blue

A planar array for the generation of evanescent waves

Wavenumber‐frequency calibration of underwater, planar, receiving arrays requires the ability to generate single‐wavenumber pressure fields over the surface of the array. When the wavenumber‐frequency region of interest is evanescent, transmitting arrays previously constructed have been found to generate fields contaminated with harmonics, acoustic wavenumbers, and nonacoustic wavenumbers from the excitation of antisymmetric Lamb waves. An array that greatly reduces contamination has recently been constructed using a sheet of polyvinylidene fluoride (PVDF) with independent rectangular electrode stripes. The array operates in the frequency range of 500 Hz to 2 kHz and generates evanescent waves with phase speeds between 30 and 150 m/s. Contamination due to the excitation of antisymmetric Lamb waves is eliminated by shifting the phase speed of the Lamb wave out of the region of interest. This is accomplished by bonding the thin sheet of PVDF directly to a thick plate of LEXAN. Contamination from harmonics a...

Low frequency sound source for acoustic sweeps

An underwater acoustic generator comprising a tapered solid positioned in aluid flow to produce a cavitation void. A source of gas supplies the void with gas to produce a resonant gas bubble.

Do manatees utilize infrasonic communication or detection

Some researchers speculate Sirenians might utilize infrasonic communication like their distant elephant cousins; however, audiogram measurements and calibrated manatee vocalizations do not support this contention. A comprehensive series of hearing tests conducted with West Indian manatees yielded the first and most definitive audiogram for any Sirenian. The manatee hearing tests were also the first controlled underwater infrasonic psychometric tests with any marine mammal. Auditory thresholds were measured from 0.4 to 46 kHz, but detection thresholds of possible vibrotactile origin were measured as low as 0.015 kHz. Manatees have short hairs on their bodies that may be sensitive vibrotactile receptors capable of detecting particle displacement in the near field. To detect these signals the manatee rotated on axis, exposing the densest portion of hairs toward the projector. Manatees inhabit shallow water where particle motion detection may be more useful near the water’s surface, where sound pressures are ...

A new concept of a low‐frequency underwater sound source

A new concept of a low‐frequency (<1000‐Hz) underwater sound source has been developed and tested. The oscillation of a rigid body and a means for converting the dipole oscillation of the body to monopole radiation are used in this source. The source can be powered with electric or linear motors or hydrodynamic exciters that convert tow or flow to vibration. To prove the concept, a small version of the electric motor powered source has been built and successfully tested. To extend the bandwidth of the source, the variable resonance frequency and multiresonances of the device are discussed. A gas spring with a frequency‐dependent stiffness is employed for the multifrequency resonant source design. The source promises to be reliable, inexpensive, highly efficient, and powerful.

Underwater noise and zones of masking with respect to hopper dredging and manatees in the St. Johns River in Jacksonville, FL

Underwater noise radiating from dredging can effectively obscure or mask biological and other important sounds. This study recorded underwater acoustic characteristics of hopper dredging in the St. Johns River, Jacksonville, FL, to evaluate noise impacts in the waterway with respect to the endangered West Indian manatee. Of particular interest was the extent and range that dredging noise may mask the sounds of approaching commercial and recreational vessels. Vertical hydrophone arrays and a multi‐channel PC‐based recording system were used to measure dredging noise at various distances. Ambient noise surveys, active propagation of calibrated sources, and controlled boat noise measurements were conducted along the waterway. These data were integrated with behavioral hearing data to estimate zones of masking surrounding dredging. Three discernable noise sources that masked boat noise were (1) cavitation from dredge propellers, (2) draghead vacuuming, and (3) noise from submerged slurry pipelines. Sustained ...

Ship strikes and whales: Shadows, mirrors and paradoxes

Ship collisions with whales and other marine animals occur near the surface. Here Snells law applies and can play a crucial role in the survival and behavioral ecology of marine mammals. In this study we investigated some theoretical constructs of near surface sound propagation that can render whales vulnerable to collisions. Empirical measurements support predictions and demonstrate that propeller noise directly ahead of many large vessels can become indistinguishable from the ambient noise. Large acoustic shadows in front of approaching ships and the confluence of near surface acoustic propagation effects seriously challenge the whales’ ability to detect and locate approaching ships. At the surface, ship noise profiles are loudest at the stern and off the port and starboard sides while remaining remarkably quiet at the bow. Whales with calves swimming near the surface alerted to ship sounds may seek refuge by actively swimming into the quieter shadow zones directly ahead of vessels. Once here, hydrodyna...

Ship strike acoustics: It is all just shadows and mirrors

Whales are vulnerable to collisions when near the surface and in shallow water. Here the physics of near‐surface sound propagation may play a crucial role in their survival. Ships sufficient in size to mortally injury whales generate acoustic spectra dominated by very low frequencies. Since the ocean’s pressure‐release surface severely attenuates frequencies that are generated at distances less than a wavelength from the surface, whales may not detect low‐frequency sounds generated by approaching ships. The Lloyd Mirror Effect predicts sound pressure levels at the surface approximate zero and empirical acoustic measurements support the prediction, while quantifying another important phenomenon associated with large ships, Acoustical Shadowing. The confluence of these acoustic propagation effects pose significant ecological consequences for marine mammals at the surface. While aerial and shipboard monitoring networks help navigate ships away from whales sighted on the surface, these programs are not reliab...

Ultrasonic hearing and vocalizations are used in communication by West Indian manatee mothers and calves.

Ultrasonic hearing thresholds above 46 kHz were first measured with a test‐sophisticated manatee in 1997. Using staircase and method of constants paradigms, pure tones (38–96 kHz) were presented in force‐choice two‐alternative tests. Repeatable detection thresholds were measured at 38, 46, 56, 66, and 76 kHz. These extended ultrasonic hearing results precipitated additional tests using the CHP‐87‐L acoustic tag, which regulatory biologists routinely attach to manatees. The tag produces a 75 kHz pulsed tone at 155 dB. The received SPL at manatee ears approximates 149 dB. Over a tag’s 2 year life expectancy, permanent selective hearing loss may occur. This is a concern because ultrasonic hearing may be important for directional hearing and communication between mothers and calves. Using a four hydrophone array, calls between wild mothers and calves were sampled at 100 kHz. Individual callers were localized, and some individuals could be identified. Peak source levels of 121 dB re 1 Pa at 2 m were directly r...

Field tests of a directional parametric acoustic alarm designed to alert manatees of approaching boats.

The efficacy of the alarm was documented during controlled slow boat approaches under two experimental conditions: (1) approaches without an alarm and (2) same boat approaches with an alarm. Experiments conducted in a NASA security area provided controlled environmental conditions and minimized anthrogenic acoustical variables. An array of GPS instrumented buoys were deployed to acoustically and visually grid sites. HD aerial video synchronized with these buoys documented behavior and acoustic conditions at focal animal positions and throughout sites. Bathymetry and propagation measurements indicate shallow water constraints along with Lloyd’s mirror effect resulting in significant transmission losses at frequencies of 1 kHz. This could account for the lack of response observed during 94% of the boat approaches without the alarm. In contrast, 100%, of the alarm approaches elicited overt avoidance responses. The change in behavior during approach trials was significantly greater during alarm trials F=76.74...

Ship strike acoustics: A paradox with a parametric solution.

Marine mammals are vulnerable to ship collisions when they are near the surface. Here acoustical laws of reflection and propagation can limit their ability to hear and locate the noise from approaching vessels. Defining the physics of near surface acoustical propagation as it relates to ship noise and hearing is central to understanding and mitigating ship strikes. Field data from controlled ship passages through vertical hydrophone arrays demonstrate a confluence of acoustical factors that poses detection challenges including (i) downward refraction, (ii) spreading loss, (iii) Lloyd’s mirror effect, (iv) acoustical shadowing, and (v) masking of approaching ship noise by ambient noise and distant ships. A highly directional, dual‐frequency parametric sonar has been developed to mitigate these challenges and to alert marine mammals of approaching vessels. The system projector is a planar array, comprised of 45 elements, band centered to transmit a high carrier frequency along with a lower side band signal....