James E. Barger

The air gun impulsive underwater transducer

Air guns are effective transducers of impulsive low‐frequency underwater sound. They are commonly arrayed to be the sound source in marine geophysical seismic exploration systems. Individual guns from these arrays radiate up to 180 kJ per shot in about a 5‐Hz frequency band centered at about 10 Hz. The bandwidth and fundamental frequency can both be increased by increasing the detonation depth. It is necessary to modify the seismic air guns, however, if their radiated energy is to be as large at frequencies above about 50 Hz as it is at lower frequencies. Three different air guns are described, each one having good acoustical efficiency over the range of fundamental frequencies extending from 10 to 120 Hz. A summary for each air gun describes its power requirements, and its fundamental frequency and energy source level as functions of depth. It is observed that air guns that make cylindrical bubbles operate at higher fundamental frequencies than do air guns that make spherical bubbles having the same bubb...

Thresholds of Acoustic Cavitation in Water

Cavitation thresholds in water have been measured as a function of frequency, dissolved gas, ambient pressure, and suspended‐particle size. The apparatus used comprises a 2‐1 volume of water enclosed by a spherical glass shell driven at its radially symmetric resonance frequencies by 8 multiresonant piezoelectric transducers. Large acoustic pressures can be produced, ranging from 10 bar at 27 kc/sec to 200 bar at 1.16 Mc/sec. The threshold data can be divided into three regions: In region A—defined by f 600 mm Hg—small air bubbles grow by rectified diffusion and stabilize at the pressure nodes. In region B—defined by f 3 bar, and Ps 200 kc/sec, Pa>5 bar, for any value of Ps—transient cavities are formed, but their presence can be detected acoustically only. The threshold tends to a slope of 12 dB/octave for frequencies a...

Electrostrictive polyurethane projector

Projectors for the acoustical radiator for tactical search (ARTS) program must be very thin, to minimize drag, while at the same time produce large surface displacements in the 70‐ to 300‐Hz band. To accommodate these requirements, the projectors are flat pads of thickness 0.1 m laminated from 1400 layers of polyurethane film that has deposited, on one side, a 300‐A aluminum electrode. The individual projector pads are to be arrayed in a self‐propelled vehicle that is 100 m deep, 3 m long, and 0.2 m wide, and that is capable of stable operation to 15 kn. Polyurethane was chosen for the transduction material mostly because of its small modulus (15 MPa) and large displacement coefficient (10 A/V). The low‐Q resonance for an individual projector pad, determined by the stiffness of the pad and radiation mass, occurs at about 200 Hz; but this frequency is expected to be lowered to 150 Hz for the array of projectors, owing to a greater radiation mass. Since polyurethane film has not before been used as a transd...

Solid‐Core Probe Hydrophone

Analysis and experiment have shown that the sensitivity of small probe hydrophones using a ceramic tube as the active element can be substantially improved by properly proportioning the cylinder and mounting it on a solid metallic core. The solid core raises the resonance frequency of the hydrophone by more than an octave, thereby extending also the frequency range of uniform response.

Signaling along elastic plates with wideband acoustic pulses

High data‐rate signaling with acoustic pulses along an elastic plate is made difficult by the conversion of a transmitted pulse into many component pulses that arrive separately during a time span that can exceed the travel time of the first component pulse to arrive. In addition, the amplitude of the received signals fluctuates widely with changes in pulse center frequency. But to obtain high data‐rate signaling, many pulses with different center frequencies must be transmitted at closely spaced time intervals. It is important in these circumstances to understand the principal features of the component pulses that originate from a single transmitted pulse. This paper derives analytical expressions for the arrival times, the peak envelope amplitudes, and the center frequencies of the arriving component pulses that are transmitted by wideband line forces acting normal to the plate surface. Each component pulse is the manifestation of a different mode of propagation. By carefully picking the bandwidth and center frequency of the launching pulse, the number of its component pulses (or, alternatively, excited modes) that have significant amplitude can be minimized, if the pulse center frequency is low enough. If the frequency is high enough, so that more than about six propagating modes can exist, then the number of arrivals cannot be reduced significantly. At higher frequencies, several of the modes merge to form the Rayleigh wave, which is centered within both earlier and later modes. A series of photographs show how transmitted Gaussian pulses look when the pulse center frequency is picked to illustrate three regimes; where the Rayleigh wave is not formed; where it is partially formed; and where it is fully formed.

Thoughts on the future of acoustics

This is a brief report of the final Plenary Session of the anniversary meeting of the Acoustical Society of America in Cambridge, Massachusetts in June 1979. It summarizes briefly conjectures about the future of acoustics with special reference to interdisciplinary technology, hearing problems, noise, and physical acoustics.

Ira Dyer and the BBN Applied Research Division

Ira Dyer was one of the first hires by Leo Beranek for the fledgling company Bolt, Beranek and Newman (BBN). Ira in turn hired many of the now recognized pioneers in ocean, structural; and ocean acoustics and formed the BBN Applied Research Division. Members investigated all aspects of sound and vibration in ships, submarines, aircraft, and spacecraft. An example includes during the mid-1950s, Ira helped design the US Navy X-1 Submarine, a small four-man quiet diesel-electric submarine for ADM Rickover. BBN designed an innovative engine mounting system that quieted the vehicle and led the way for ultra-quiet submarines in the future Navy. This submarine is now on display at the Navy Submarine Museum at Groton, CT. Other contributions by members include SEA (statistical energy analysis) and criteria for modeling flow-excitation forcing functions to structural radiation. Ira created a golden era for acoustics at BBN.

Helmholtz-Rayleigh Interdisciplinary Silver Medal in Underwater Acoustics and Engineering Acoustics

The SilverMedal is presented to individuals, without age limitation, for contributions to the advancement of science, engineering, or human welfare through the application of acoustic principles, or through research accomplishment in acoustics.

Sound and structural destruction

Our honoree is well known for his, and David Feit’s, book on the interaction of sound with structures. In this work, equations representing both sound and structure are linearized. Results apply to most cases, even when sound pressures exceed safe hearing levels. But there are occasions when it is wanted to destroy the functionality of a structure by a directed shock wave in its surrounding fluid. This type of interaction exceeds yield stress in the structure, causing permanent deformation. This paper describes both an explosive and a nonexplosive underwater shock generation device and shock propagation to a 0.8 m diam metal sphere. The damage is computed by finite element methods, and compared to actual damage. Insights are found from the linear analysis.

Source level and Urick

Source level (SL) is the first term in Bob Urick’s sonar equation, and it represents the root‐mean‐square pressure of sound radiated at the target, in decibels. The fundamental measure of source level is the signal‐energy level (SEL) represented by the time integral of sound pressure squared, to which the detection index is proportional, in decibels. Despite using a form of sonar equation more natural for long‐pulse transducers than for impulsive ones, he only described explosives in detail in his chapter on generation of underwater sound. When ‘‘Principles of Underwater Sound for Engineers’’ was published, there was a tendency to use impulsive sound sources to study sound interactions with the sea and to use narrow‐band long‐pulse transducers in sonar systems. The diminution of this tendency has been a very important development since Bob’s book was written, and it has occurred because of improvements in both kinds of transduction. Transducer bandwidth has been increased, both by materials with larger el...