NASA Scientific and Technical Aerospace Reports

characteristics of the technology demonstrator version of the Instrumented Prodder and set-up of the tests under controlled
conditions are described in Chapter 3. The results of the tests are given in Chapter 4. Finally, in Chapter 5 an assessment of
the test results is reported, as well as information on the utility of the Instrumented Prodder obtained fiom the discussions with
the usergroup and the demining community. Chapter 5 concludes with a number of options for the continuation of the product
development path of the Instrumented Prodder. The main conclusions are summarised in Chapter 6.
Author
Product Development; Project Management; Mine Detectors; Sensors; Feedback Control
20040111235 NASA Langley Research Center, Hampton, VA, USA
Active Control of Fan-Generated Tone Noise
Gerhold, Carl H.; [1995]; 7 pp.; In English
Report No.(s): CEAS/AIAA Paper 95-029; No Copyright; Avail: CASI; A02, Hardcopy
This paper reports on an experiment to control the noise radiated from the inlet of a ducted fan using a time domain active
adaptive system. The control ,sound source consists of loudspeakers arranged in a ring around the fan duct. The error sensor
location is in the fan duct. The purpose of this experiment is to demonstrate that the in-duct error sensor reduces the mode
spillover in the far field, thereby increasing the efficiency of the control system. The control system is found to reduce the blade
passage frequency tone significantly in the acoustic far field when the mode orders of the noise source and of the control source
are the same, when the dominant wave in the duct is a plane wave. The presence of higher order modes in the duct reduces
the noise reduction efficiency, particularly near the mode cut-on where the standing wave component is strong, but the control
system converges stably. The control system is stable and converges when the first circumferential mode is generated in the
duct. The control system is found to reduce the fan noise in the far field on an arc around the fan inlet by as much as 20 dB
with none of the sound amplification associated with mode spillover.
Author
Acoustics; Active Control; Aerodynamic Noise; Ducted Fans; Noise Reduction; Noise Generators
20040111236 NASA Langley Research Center, Hampton, VA, USA
The Kirchhoff Formula for a Supersonically Moving Surface
Farassat, F.; Myers, M. K.; [1995]; 6 pp.; In English; First Joint CEAS/AIAA Aeroacoustics Conference, 12-15 Jun. 1995,
Munich, Germany
Report No.(s): AIAA Paper 95-062; No Copyright; Avail: CASI; A02, Hardcopy
The Kirchhoff formula for radiation from stationary surfaces first appeared in 1882, and it has since found many
applications in wave propagation theory. In 1930, Morgans extended the formula to apply to surfaces moving at speeds below
the wave propagation speed; we refer to Morgans formula as the subsonic formulation. A modern derivation of Morgans result
was published by Farassat and Myers in 1988, and it has now been used extensively in acoustics, particularly for high speed
helicopter rotor noise prediction. Under some common conditions in this application, however, the appropriate Kirchhoff
surface must be chosen such that portions of it travel at supersonic speed. The available Kirchhoff formula for moving surfaces
is not suitable for this situation. In the current paper we derive the Kirchhoff formula applicable to a supersonically moving
surface using some results from generalized function theory. The new formula requires knowledge of the same surface data
as in the subsonic case. Complications that arise from apparent singularities in the new formulation are discussed briefly in
the paper.
Author
Wave Propagation; Supersonic Speed; Aircraft Noise; Aerodynamic Noise; Aeroacoustics
20040111317 NASA Langley Research Center, Hampton, VA, USA
Prediction and Measurement of the Vibration and Acoustic Radiation of Panels Subjected to Acoustic Loading
Turner, Travis L.; Rizzi, Stephen A.; [1995]; 4 pp.; In English; No Copyright; Avail: CASI; A01, Hardcopy
Interior noise and sonic fatigue are important issues in the development and design of advanced subsonic and supersonic
aircraft. Conventional aircraft typically employ passive treatments, such as constrained layer damping and acoustic absorption
materials, to reduce the structural response and resulting acoustic levels in the aircraft interior. These techniques require
significant addition of mass and only attenuate relatively high frequency noise transmitted through the fuselage. Although
structural acoustic coupling is in general very important in the study of aircraft fuselage interior noise, analysis of noise
transmission through a panel supported in an infinite rigid baffle (separating two semi-infinite acoustic domains) can be useful
in evaluating the effects of active/adaptive materials, complex loading, etc. Recent work has been aimed at developing
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