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NASA NTRS1987

Separation control over an airfoil at high angles of attack by sound emanating from the surface

Active control by sound emanating from a narrow gap in the vicinity of the leading edge of a symmetrical airfoil is used to study the influence of sound on the pressure distribution and the wake at high angles of attack. The results from experiments conducted at a Reynolds number based on the chord of 35,000 show that, with injection of sound at twice the shedding frequency of the shear layer, the region of separation becomes drastically reduced. The shear layer is found to be very sensitive to sound excitation in the vicinity of the separation point. The excitation sufficiently alters the global circulation to cause an increase in lift and reduction in drag. Furthermore, experimental results describing stall and post-stall conditions compare well with the limited data available and indicate that stall is delayed by sound injection into the separated region.

Huang, L. S.↗

NASA NTRS1990

Hearing in three dimensions: Sound localization

The ability to localize a source of sound in space is a fundamental component of the three dimensional character of the sound of audio. For over a century scientists have been trying to understand the physical and psychological processes and physiological mechanisms that subserve sound localization. This research has shown that important information about sound source position is provided by interaural differences in time of arrival, interaural differences in intensity and direction-dependent filtering provided by the pinnae. Progress has been slow, primarily because experiments on localization are technically demanding. Control of stimulus parameters and quantification of the subjective experience are quite difficult problems. Recent advances, such as the ability to simulate a three dimensional sound field over headphones, seem to offer potential for rapid progress. Research using the new techniques has already produced new information. It now seems that interaural time differences are a much more salient and dominant localization cue than previously believed.

Wightman, Frederic L.↗

NASA NTRS1990

Long-range sound propagation: A review of some experimental data

Three experimental studies of long range sound propagation carried out or sponsored in the past by NASA are briefly reviewed to provide a partial prospective for some of the analytical studies presented in this symposium. The three studies reviewed cover (1) a unique test of two large rocket engines conducted in such a way as to provide an indication of possible atmospheric scattering loss from a large low-frequency directive sound source, (2) a year-long measurement of low frequency sound propagation which clearly demonstrated the dominant influence of the vertical gradient in the vector sound velocity towards the receiver in defining excess sound attenuation due to refraction, and (3), a series of excess ground attenuation measurements over grass and asphalt surfaces replicated several times under very similar inversion weather conditions.

Sutherland, Louis C.↗

NASA NTRS1990

Influence of airfoil nose radius on sound generated by gust interactions

The sound radiated by interaction of a short wavelength gust (k much greater than 1) with a symmetric thin airfoil is analyzed. The theory is based on a linearization of the Euler equations about the subsonic mean flow past the airfoil. The sound generation mechanism is found to be concentrated in a local region surrounding the parabolic nose of the airfoil; the size of this local region scales on the gust wavelength. Airfoil thickness produces terms of relative order O(sq rt S) in the far field sound, where S is a Strouhal number based on the airfoil nose radius. At low Mach numbers, entropy gusts generate much less sound than vorticity gusts. However, for O(1) Mach numbers and moderate values of S, entropy and vorticity gusts produce similar sound levels.

Tsai, C. T.↗

NASA NTRS1992

A general introduction to aeroacoustics and atmospheric sound

A single unifying principle (based upon the nonlinear 'momentum-flux' effects produced when different components of a motion transport different components of its momentum) is used to give a broad scientific background to several aspects of the interaction between airflows and atmospheric sound. First, it treats the generation of sound by airflows of many different types. These include, for example, jet-like flows involving convected turbulent motions (with the resulting aeroacoustic radiation sensitively dependent on the Mach number of convection) and they include, as an extreme case, the supersonic 'boom' (shock waves generated by a supersonically convected flow pattern). Next, an analysis is given of sound propagation through nonuniformly moving airflows, and the exchange is quantified of energy between flow and sound; while, finally, problems are examined of how sound waves 'on their own' may generate the airflows known as acoustic streaming.

Lighthill, James↗

NASA NTRS1992

Meteor fireball sounds identified

Sounds heard simultaneously with the flight of large meteor fireballs are electrical in origin. Confirmation that Extra/Very Low Frequency (ELF/VLF) electromagnetic radiation is produced by the fireball was obtained by Japanese researchers. Although the generation mechanism is not fully understood, studies of the Meteorite Observation and Recovery Project (MORP) and other fireball data indicate that interaction with the atmosphere is definitely responsible and the cut-off magnitude of -9 found for sustained electrophonic sounds is supported by theory. Brief bursts of ELF/VLF radiation may accompany flares or explosions of smaller fireballs, producing transient sounds near favorably placed observers. Laboratory studies show that mundane physical objects can respond to electrical excitation and produce audible sounds. Reports of electrophonic sounds should no longer be discarded. A catalog of over 300 reports relating to electrophonic phenomena associated with meteor fireballs, aurorae, and lightning was assembled. Many other reports have been cataloged in Russian. These may assist the full solution of the similar long-standing and contentious mystery of audible auroral displays.

Keay, Colin↗

NASA NTRS1992

Influence of airfoil thickness on sound generated by high-frequency gust interactions

The sound radiated by interaction of a short wavelength gust with a symmetric thin airfoil is analyzed. The theory is based on a linearization of the Euler equations about the subsonic mean flow past the airfoil. The sound generation mechanism is found to be concentrated in a local region surrounding the parabolic nose of the airfoil; the size of this local region scales on the gust wavelength. At low Mach numbers, moderate values of airfoil thickness decrease the sound power, while at higher Mach numbers the sound power tends to increase with airfoil thickness. Airfoil thickness produces dramatic changes in the far field directivity. Both the sound power and the directivity are strong functions of the gust orientation.

Tsai, C. T.↗

NASA NTRS1992

A general introduction to aeroacoustics and atmospheric sound

Lighthill, James↗

NASA NTRS1997

Interaction of Sound from Supersonic Jets with Nearby Structures

A model of sound generated in an ideally expanded supersonic (Mach 2) jet is solved numerically. Two configurations are considered: (1) a free jet and (2) an installed jet with a nearby array of flexible aircraft type panels. In the later case the panels vibrate in response to loading by sound from the jet and the full coupling between the panels and the jet is considered, accounting for panel response and radiation. The long time behavior of the jet is considered. Results for near field and far field disturbance, the far field pressure and the vibration of and radiation from the panels are presented. Panel response crucially depends on the location of the panels. Panels located upstream of the Mach cone are subject to a low level, nearly continuous spectral excitation and consequently exhibit a low level, relatively continuous spectral response. In contrast, panels located within the Mach cone are subject to a significant loading due to the intense Mach wave radiation of sound and exhibit a large, relatively peaked spectral response centered around the peak frequency of sound radiation. The panels radiate in a similar fashion to the sound in the jet, in particular exhibiting a relatively peaked spectral response at approximately the Mach angle from the bounding wall.

Fenno, C. C., Jr.↗

NASA NTRS1998

Refraction of Sound Emitted Near Solid Boundaries from a Sheared Jet

A mathematical model is developed to describe the sound emitted from an arbitrary point within a turbulent flow near solid boundaries. A unidirectional, transversely sheared mean flow is assumed, and the cross-section of the cold jet is of arbitrary shape. The analysis begins with Lilley's formulation of aerodynamic noise and, depending upon the specific model of turbulence used, leads via Fourier analysis to an expression for the spectral density of the intensity of the far-field sound emitted from a unit volume of turbulence. The expressions require solution of a reduced Green's function of Lilley's equation as well as certain moving axis velocity correlations of the turbulence. Integration over the entire flow field is required in order to predict the sound emitted by the complete flow. Calculations are presented for sound emitted from a plugflow jet exiting a semi-infinite flat duct. Polar plots of the far-field directivity show the dependence upon frequency and source position within the duct. Certain model problems are suggested to investigate the effect of duct termination, duct geometry, and mean flow shear upon the far-field sound.

Dill, Loren H.↗

NASA NTRS1998

Experimental Simulation of Active Control With On-line System Identification on Sound Transmission Through an Elastic Plate

An adaptive control algorithm with on-line system identification capability has been developed. One of the great advantages of this scheme is that an additional system identification mechanism such as an additional uncorrelated random signal generator as the source of system identification is not required. A time-varying plate-cavity system is used to demonstrate the control performance of this algorithm. The time-varying system consists of a stainless-steel plate which is bolted down on a rigid cavity opening where the cavity depth was changed with respect to time. For a given externally located harmonic sound excitation, the system identification and the control are simultaneously executed to minimize the transmitted sound in the cavity. The control performance of the algorithm is examined for two cases. First, all the water was drained, the external disturbance frequency is swept with 1 Hz/sec. The result shows an excellent frequency tracking capability with cavity internal sound suppression of 40 dB. For the second case, the water level is initially empty and then raised to 3/20 full in 60 seconds while the external sound excitation is fixed with a frequency. Hence, the cavity resonant frequency decreases and passes the external sound excitation frequency. The algorithm shows 40 dB transmitted noise suppression without compromising the system identification tracking capability.

Source record↗

NASA NTRS1997

Robust Feedback Control of Flow Induced Structural Radiation of Sound

A significant component of the interior noise of aircraft and automobiles is a result of turbulent boundary layer excitation of the vehicular structure. In this work, active robust feedback control of the noise due to this non-predictable excitation is investigated. Both an analytical model and experimental investigations are used to determine the characteristics of the flow induced structural sound radiation problem. The problem is shown to be broadband in nature with large system uncertainties associated with the various operating conditions. Furthermore the delay associated with sound propagation is shown to restrict the use of microphone feedback. The state of the art control methodologies, IL synthesis and adaptive feedback control, are evaluated and shown to have limited success for solving this problem. A robust frequency domain controller design methodology is developed for the problem of sound radiated from turbulent flow driven plates. The control design methodology uses frequency domain sequential loop shaping techniques. System uncertainty, sound pressure level reduction performance, and actuator constraints are included in the design process. Using this design method, phase lag was added using non-minimum phase zeros such that the beneficial plant dynamics could be used. This general control approach has application to lightly damped vibration and sound radiation problems where there are high bandwidth control objectives requiring a low controller DC gain and controller order.

Heatwole, Craig M.↗

NASA NTRS1999

GPS Sounding Rocket Developments

Sounding rockets are suborbital launch vehicles capable of carrying scientific payloads several hundred miles in altitude. These missions return a variety of scientific data including; chemical makeup and physical processes taking place In the atmosphere, natural radiation surrounding the Earth, data on the Sun, stars, galaxies and many other phenomena. In addition, sounding rockets provide a reasonably economical means of conducting engineering tests for instruments and devices used on satellites and other spacecraft prior to their use in more expensive activities. The NASA Sounding Rocket Program is managed by personnel from Goddard Space Flight Center Wallops Flight Facility (GSFC/WFF) in Virginia. Typically around thirty of these rockets are launched each year, either from established ranges at Wallops Island, Virginia, Poker Flat Research Range, Alaska; White Sands Missile Range, New Mexico or from Canada, Norway and Sweden. Many times launches are conducted from temporary launch ranges in remote parts of the world requi6ng considerable expense to transport and operate tracking radars. An inverse differential GPS system has been developed for Sounding Rocket. This paper addresses the NASA Wallops Island history of GPS Sounding Rocket experience since 1994 and the development of a high accurate and useful system.

Bull, Barton↗

NASA NTRS1999

Investigations of Remote Plasma Irregularites by Radio Sounding: Applications of the Radio Plasma Imager on IMAGE

The Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission operates like a radar by transmitting and receiving coherent electromagnetic pulses. Long-range echoes of electromagnetic sounder waves are reflected at remote plasma cutoffs. Thus, analyses of RPI observations will yield the plasma parameters and distances to the remote reflection points. These analyses assume that the reflecting plasma surfaces are cold and are sufficiently smooth that they effectively behave as plane mirrors to the incoming sounder waves, i.e., that geometric optics can be used. The RPI will employ pulse compression and spectral integration techniques, perfected in ground-based ionospheric digital sounders, in order to enhance the signal-to-noise ratio in long-range magnetospheric sounding. When plasma irregularities exist in the remote magnetospheric plasmas that are being probed by the sounder waves, echo signatures may become complicated. Ionospheric sounding experience indicates that while topside sounding echo strengths can actually be enhanced by the presence of irregularities, ground-based sounding indicates that coherent detection techniques can still be employed. In this paper we investigate the plasma conditions that will allow coherent signals to be detected by the RPI and the signatures to be expected, such as scattering and plasma resonances, in the presence of multi-scale irregularities, may possibly have on RPI signals. Sounding of irregular plasma structures in the plasmasphere, plasmapause and magnetopause are also discussed.

Fung, Shing F.↗

NASA NTRS2002

Sounds like Team Spirit

I recently accompanied my son Dan to one of his guitar lessons. As I sat in a separate room, I focused on the music he was playing and the beautiful, robust sound that comes from a well-played guitar. Later that night, I woke up around 3 am. I tend to have my best thoughts at this hour. The trouble is I usually roll over and fall back asleep. This time I was still awake an hour later, so I got up and jotted some notes down in my study. I was thinking about the pure, honest sound of a well-played instrument. From there my mind wandered into the realm of high-performance teams and successful projects. (I know this sounds weird, but this is the sort of thing I think about at 3 am. Maybe you have your own weird thoughts around that time.) Consider a team in relation to music. It seems to me that a crack team can achieve a beautiful, perfect unity in the same way that a band of brilliant musicians can when they're in harmony with one another. With more than a little satisfaction I have to admit, I started to think about the great work performed for you by the Knowledge Sharing team, including this magazine you are reading. Over the past two years I personally have received some of my greatest pleasures as the APPL Director from the Knowledge Sharing activities - the Masters Forums, NASA Center visits, ASK Magazine. The Knowledge Sharing team expresses such passion for their work, just like great musicians convey their passion in the music they play. In the case of Knowledge Sharing, there are many factors that have made this so enjoyable (and hopefully worthwhile for NASA). Three ingredients come to mind -- ingredients that have produced a signature sound. First, through the crazy, passionate playing of Alex Laufer, Michelle Collins, Denise Lee, and Todd Post, I always know that something startling and original is going to come out of their activities. This team has consistently done things that are unique and innovative. For me, best of all is that they are always trying to improve on what they've done before. Second, success in any endeavor stems from people who know how to interpret a composition to sound beautiful when played in a different style. For Knowledge Sharing to work, it must be adapted, reinterpreted, shaped and played with at the centers. In this regard, we've been blessed with another crazy, passionate, inspired artist named Claire Smith. Claire has turned Ames Research Center in California into APPL-west. She is so good and committed to what she does that I just refer people to her whenever they have questions about implementing project management development at the field level. Finally, any great effort requires talented people working behind the scenes, the people who formulate a business approach and know how to manage the money so that the music gets heard. I have known many brilliant and creative people with a ton of ideas that never take off due to an inability to work the business. Again, the Knowledge Sharing team has been fortunate to have competent and passionate people, specifically Tony Maturo and his procurement team at Goddard Space Flight Center, to make sure the process is in place to support the effort. This kind of support is every bit as crucial as the activity itself, and the efforts and creativity that go into successful procurement and contracting is a vital ingredient of this successful team.

Hoffman, Edward↗

NASA NTRS2003

Sound Generation by Aircraft Wake Vortices

This report provides an extensive analysis of potential wake vortex noise sources that might be utilized to aid in their tracking. Several possible mechanisms of aircraft vortex sound generation are examined on the basis of discrete vortex dynamic models and characteristic acoustic signatures calculated by application of vortex sound theory. It is shown that the most robust mechanisms result in very low frequency infrasound. An instability of the vortex core structure is discussed and shown to be a possible mechanism for generating higher frequency sound bordering the audible frequency range. However, the frequencies produced are still low and cannot explain the reasonably high-pitched sound that has occasionally been observed experimentally. Since the robust mechanisms appear to generate only very low frequency sound, infrasonic tracking of the vortices may be warranted.

Hardin, Jay C.↗

NASA NTRS2008

Data Assimilation Experiments using Quality Controlled AIRS Version 5 Temperature Soundings

The AIRS Science Team Version 5 retrieval algorithm has been finalized and is now operational at the Goddard DAAC in the processing (and reprocessing) of all AlRS data. Version 5 contains accurate case-by-case error estimates for most derived products, which are also used for quality control. We have conducted forecast impact experiments assimilating AlRS quality controlled temperature profiles using the NASA GEOS-5 data assimilation system, consisting of the NCEP GSI analysis coupled with the NASA FVGCM. Assimilation of quality controlled temperature profiles resulted in significantly improved forecast skill in both the Northern Hemisphere and Southern Hemisphere Extra-Tropics, compared to that obtained from analyses obtained when all data used operationally by NCEP except for AlRS data is assimilated. Experiments using different Quality Control thresholds for assimilation of AlRS temperature retrievals showed that a medium quality control threshold performed better than a tighter threshold, which provided better overall sounding accuracy; or a looser threshold, which provided better spatial coverage of accepted soundings. We are conducting more experiments to further optimize this balance of spatial coverage and sounding accuracy from the data assimilation perspective. In all cases, temperature soundings were assimilated well below cloud level in partially cloudy cases. The positive impact of assimilating AlRS derived atmospheric temperatures all but vanished when only AIRS stratospheric temperatures were assimilated. Forecast skill resulting from assimilation of AlRS radiances uncontaminated by clouds, instead of AlRS temperature soundings, was only slightly better than that resulting from assimilation of only stratospheric AlRS temperatures. This reduction in forecast skill is most likely the result of significant loss of tropospheric information when only AIRS radiances unaffected by clouds are used in the data assimilation process.

Susskind, Joel↗

NASA NTRS2007

Advanced Systems for Monitoring Underwater Sounds

The term "Passive Acoustic Monitoring System" (PAMS) describes a developmental sensing-and-data-acquisition system for recording underwater sounds. The sounds (more precisely, digitized and preprocessed versions from acoustic transducers) are subsequently analyzed by a combination of data processing and interpretation to identify and/or, in some cases, to locate the sources of those sounds. PAMS was originally designed to locate the sources such as fish of species that one knows or seeks to identify. The PAMS unit could also be used to locate other sources, for example, marine life, human divers, and/or vessels. The underlying principles of passive acoustic sensing and analyzing acoustic-signal data in conjunction with temperature and salinity data are not new and not unique to PAMS. Part of the uniqueness of the PAMS design is that it is the first deep-sea instrumentation design to provide a capability for studying soniferous marine animals (especially fish) over the wide depth range described below. The uniqueness of PAMS also lies partly in a synergistic combination of advanced sensing, packaging, and data-processing design features with features adapted from proven marine instrumentation systems. This combination affords a versatility that enables adaptation to a variety of undersea missions using a variety of sensors. The interpretation of acoustic data can include visual inspection of power-spectrum plots for identification of spectral signatures of known biological species or artificial sources. Alternatively or in addition, data analysis could include determination of relative times of arrival of signals at different acoustic sensors arrayed at known locations. From these times of arrival, locations of acoustic sources (and errors in those locations) can be estimated. Estimates of relative locations of sources and sensors can be refined through analysis of the attenuation of sound in the intervening water in combination with water-temperature and salinity data acquired by instrumentation systems other than PAMS. A PAMS is packaged as a battery-powered unit, mated with external sensors, that can operate in the ocean at any depth from 2 m to 1 km. A PAMS includes a pressure housing, a deep-sea battery, a hydrophone (which is one of the mating external sensors), and an external monitor and keyboard box. In addition to acoustic transducers, external sensors can include temperature probes and, potentially, underwater cameras. The pressure housing contains a computer that includes a hard drive, DC-to- DC power converters, a post-amplifier board, a sound card, and a universal serial bus (USB) 4-port hub.

Lane, Michael↗