Airborne Acoustics Research Articles

Vector sensors—over 40 and still hot

Acoustic vector sensors and gradient arrays have been in use in underwater acoustics for more than 4 decades, so one may wonder why they are featured in a session entitled ‘‘Hot Topics.’’ The reason lies in the recent resurgence in their use. This, in turn, is due primarily to major advances in engineering and signal processing that have been applied to these devices. Historically, theoretical gains have been difficult to achieve with these sensors due to their susceptibility to uncorrelated noise. That is to say, the very process of making a localized measurement of the vector acoustic field lowers the signal-to-noise ratio, relative to a simple pressure measurement. However, with today’s advances in design, manufacturing, and digital signal processing, high-quality performance can be achieved in a very small package size. Moreover, the current interest in these devices is not limited to underwater acoustic applications; rather, it extends across a number of technical areas within the acoustics community. This presentation will begin with a brief introduction to the theoretical foundation and history of these devices. Then, some current applications in both underwater and airborne acoustics will be highlighted.

Unit-cell finite element analysis of the response of complex structures to flow excitation

Analytic estimation of the vibration response of rib-reinforced structures excited by turbulent boundary layer (TBL) wall pressure fluctuations is a subject of ongoing interest for numerous applications in airborne and underwater acoustics. Previous investigators have developed a variety of closed form and approximate analytic methods for estimating the response of structures of interest. Fluid-loading and rib-reinforcement effects make problems involving the response of sea-going vessels particularly challenging. Recent advances in finite element based methods and the enhanced capabilities of modern computer hardware now permit the application of the FEA method to such flow-induced vibration problems. This paper describes the application of the unit-cell capability in BBN’s SARA-2D finite element code to evaluate the response of fluid-loaded, rib-reinforced cylinders to TBL excitation. An overview of the unit-cell modeling method is presented, and example results are provided to illustrate the effects of variations in rib and shell dimensions.

Modulated piezoresistive sensor for airborne infrasound

This paper will describe the design, construction, and testing of a piezoresistive sensor for airborne acoustics in the frequency range 0.005–100 Hz. Intended as a candidate for many-element infrasound arrays where element cost is critical, the sensor was designed around an inexpensive silicon-membrane chip with an implanted piezoresistive bridge. The prototype has a measured self-noise below one millipascal per root hertz at 1 Hz with a 1/f power spectrum to 0.005 Hz, the designed low-frequency rolloff. The responsivity is nominally 20 mV/Pa with a linear signal range to 150 Pa. When the bridge is operated with high-frequency drive and synchronous detection, the self-noise is within a factor of 2 of the intrinisic Johnson noise of the resistive bridge. This self-noise level is well below ambient levels in the microbarometric region from 0.2 to 0.7 Hz. Basic sensor-element limitations will be discussed along with the electronics and package design. In addition, the techniques for calibration and self-noise evaluation will be presented. [Work sponsored by the U.S. Army Space and Missile Defense Command.]

The Acoustical Society of America’s historical contributions to noise and its control

The Society’s contributions to noise and its control, a subfield of the broad subject of acoustics, have been significant and constant over the years since the first noise paper was presented at the Society’s first meeting in 1929. The major contributions are considered in an historical context—during the period before World War II when the Society first produced national standards for instrumentation used for noise measurement, during the War and the Postwar Era when there was considerable government support for airborne acoustics research, during the 1950s when Noise Control magazine was published, followed by the 1960s and the later years of the 20th Century. Of the ASA members expressing interest in the 13 different subfields of acoustics, the number concerned with noise is near the top of the list. But ASA is no longer alone. Nearly a dozen other professional organizations in the U.S.A. have groups that are involved with noise and its control. Over the past three-quarters of a century, ASA has played a leadership role in developing the subfield of acoustics focused on noise control, and in producing a broad range of American National Standards on noise and vibration. In the future, ASA must take concrete, positive steps to retain its position of leadership.