Noise Benefits Research Articles

HIGHLIGHTS OF AEROACOUSTICS RESEARCH IN THE U.S.—1998

Highlights of aeroacoustics research in the United States of America during 1998 are reported in a summary compiled from information provided by members of the Aeroacoustics Technical Committee of the American Institute of Aeronautics and Astronautics (AIAA) and other leading research groups in industry, national laboratories, and academia. The past few years have seen significant progress in aeroacoustics. Research has steadily progressed toward enhanced safety, noise benefits, and lower costs. Since industrial progress is generally not published in the archival literature, it is particularly important to highlight these accomplishments. This year we chose to report on five topics of great interest to the aerospace industry including a synopsis of fundamental research at universities and national laboratories. The topics chosen are: (1) advanced subsonic technology (AST), (2) high-speed research (HSR), (3) rotorcraft, (4) weapons bay aeroacoustics control and (5) academic research including computational aeroacoustics (CAA). Although the information presented in this review is not all encompassing, we hope that the topics covered will provide some insights into aeroacoustics activity in the U.S.

Additive noise can enhance temporal coding in a computational model of analogue cochlear implant stimulation

Conventional analogue multichannel cochlear implants are unlikely to convey formant information by the fine time structure of evoked discharges. Theoretically, however, the addition of noise to the channel outputs could enhance the representation of formants by time coding. In this study, the potential benefit of noise in analogue coding schemes was investigated using a computer model of cochlear implant stimulation. The cochlear nerve was modelled by the Frankenhauser–Huxley equations. For all five vowels investigated, the optimal addition of noise to the first channel of the simulated implant (200–671 Hz) caused enhancement of the first formant representation (as seen in amplitude spectra of the simulated discharges). For vowels with a low-frequency second formant, clear enhancement of the second formant resulted from the optimal addition of noise to the third channel (1200–2116 Hz). On the basis of the present computational study, additive noise would be expected to enhance the coding of temporal information by the discharges of a single nerve fiber.

The Benefits of Background Noise

The Benefits of Background Noise

Stochastic resonance and the benefits of noise: from ice ages to crayfish and SQUIDs.

Noise in dynamical systems is usually considered a nuisance. But in certain nonlinear systems, including electronic circuits and biological sensory apparatus, the presence of noise can in fact enhance the detection of weak signals. This phenomenon, called stochastic resonance, may find useful application in physical, technological and biomedical contexts.

Reduction of the noise of a model counterrotation propeller at cruise by reducing the aft propeller diameter

The forward propeller of a model counterrotation propeller was tested with its original aft propeller and with a reduced diameter aft propeller. Noise reductions with the reduced diameter aft propeller were measured at simulated cruise conditions. Reductions of up to 7.5 dB were measured for the aft propeller blade passing tone, and reductions in the harmonics were also measured. Reductions in the interaction tones were observed as the result of the reduced diameter aft blades no longer interacting with the forward propeller tip vortex. Significant reductions in the total noise at each harmonic were observed. The chief noise reduction at each harmonic came from reduced aft propeller-alone noise, with the interaction tones contributing little to the totals at cruise. Total cruise noise reductions, as much as 3 dB at given angles for the blade passing tone, and as high as 10 dB at some of the harmonies were observed. These reductions would improve the fuselage interior noise levels and represent a cruise noise benefit for using reduced diameter aft propeller blades.

Wideband compression and spectral sharpening for hearing‐impaired listeners

Normal‐hearing listeners with hearing losses simulated by masking noise benefit from wideband amplitude compression under conditions (e.g. no interference, most‐comfortable presentation level) for which listeners with sensorineural hearing impairments do not. Such findings suggest that speech intelligibility may be limited by degraded spectral and temporal resolution as well as reduced dynamic range for hearing‐impaired listeners. To combat the combination of reduced dynamic range and reduced frequency selectivity, we have developed a speech‐processing system which provides both widehand compression and a sharpening of short‐term spectral shape. The sharpening is accomplished by an orthogonal decomposition of the critical‐band spectrum (similar to a principal‐component decomposition) followed by expansion of the component weights which specify the peak‐valley structure of the spectrum. We compare speech intelligibility with this system to that with linear amplification and wideband compression alone for hearing‐impaired listeners. Preliminary results for CVC nonsense syllables indicate a modest benefit for the compression/sharpening system for a male speaker, but little benefit for a female speaker. [Work supported by NIH.]

Proceedings of Noise-Con 81: Applied Noise Control Technology

The conference was divided into sessions covering noise control regulations and benefits; noise source identification; barriers and enclosures; mufflers; hearing protection devices; textile and fibre industries; metal fabrication industry; transportation and aircraft noise control; punch-press noise control and miscellaneous topics; woodworking industry; tobacco and packaging industries; community noise; and applications of damping materials. One paper has been abstracted separately.

Variable Stream Control Engine for Supersonic Propulsion

advanced single-stage, high-temperature turbine. The primary burner and the duct burner utilize low-emission, highefficiency combustor concepts. The nozzle is a two-stream, concentric, annular (coannular) design with variable throat areas in both streams and an ejector/reverser exhaust system. The independent temperature and velocity control for both primary and bypass streams provides an inherent reduction in jet noise during takeoff. This noise reduction characteristic is based on an inverse velocity profile, where the bypass stream jet velocity is 50-70% higher than the primary stream velocity. Results from a P&WA model nozzle test program6 sponsored by NASA indicate that noise levels measured for coannular nozzles with this inverted velocity profile are approximately 8 EPNdB (effective perceived noise level in decibels) lower than a single-stream nozzle operating at the same airflow and thrust levels. These results are based on both static tests and wind-tunnel tests simulating takeoff flight conditions. Based on these model tests, the coannular noise benefit represents a breakthrough in jet noise control. Further evaluation (a largescale demonstration of this noise benefit) is in the planning stage. Critical Operating Conditions

Pseudo-transonic equation with a diffusion term

Now we examine the claim, mentioned by Crighton, that the flat part of an elliptical aircraft nozzle jet radiates less noise than a similar circular jet. We can see no instability patterns, or lack of them, on the surfaces of the jet to explain a lower radiation from the wide part of the jet, and indeed Maestrello and McDaid found very little difference in nearfield noise emission from various sides of a rectangular jet. What could be happening, however, is the 90-deg shift in major axis shortly after leaving the aircraft, which then presents a smaller width radiating source to observers on the ground than would a circular jet of the same power. This then could explain the possible noise benefits of elliptical nozzles, although more elaborate theories have been proposed.

Jet Noise Characteristics of Unsuppressed Duct Burning Turbofan Exhaust System

Recent aeroacoustic tests of model coannular nozzles have shown that less noise is generated if the higher velocity jet is exhausted from the outer annular passage rather than from the primary nozzle. These findings are of particular significance to the duct burning turbofan engine being studied for application to an advanced supersonic transport airplane. Unlike conventional turbofan engines that have peak velocities from the primary nozzle, it is possible to design a DBTF engine to have a fan velocity higher than that of the primary flow. In this paper are presented the results of a model test program that covers a range of fan to primary area ratios from 0.75 to 1.2, and a range of fan-to-primar y velocity ratios from 0.4 to 2.8. Correlations are presented that relate radiated sound power to fan velocity, fan-to-primary velocity ratio, and fan-to-primary area ratio. Corresponding exhaust plume velocity traverse data are presented which suggest that the observed noise benefits may be because of the more rapid decay of the annular flow caused by shear stresses on the inner surface which result from the lower velocity primary flow.

Environmental noise nuisance

Severe noise problems involving a small number of people can often be solved following a detailed examination of the factors involved. At very least, provided the necessary resources are available, the noise source and the offended people can usually be separated by distance or by sound insulation. More usually, however, real noise nuisance problems are very complex, involving trade-offs between the benefits of less noise and the costs of lessening it. It is clearly desirable to minimise the harmful effects of noise upon public health but how low must the limits be set and can the costs be met? In general the environmental planner is faced with two problems, firstly to predict the extent and severity of environmental noise impact and secondly to assess the acceptability, or otherwise, of this impact in some absolute sense. The second of these problems is perhaps the more difficult as its solution ultimately rests on the political question of how many affected people can be justified. But the first is of no less importance since the policy-maker must be able to make qualitative evaluations of the effects of different policy options. This paper examines the problem of environmental noise nuisance what it is and how it can be measured and predicted. Detailed methodology is beyond the scope of the paper which concentrates instead upon a discussion of the factors involved and the nature of the relationships between them.

Design Considerations for Power-Plant Pump Noise Reduction

Reduction in vibration, pressure pulsations, and noise in power-plant centrifugal pumps can be achieved by increased impeller to volute tongue distances, improved geometric relationship between the tongue and impeller to minimize the shock of sudden fluid deceleration, control of impeller suction and discharge recirculation effects, and the elimination of hydraulic whirl set up in wearing ring annuli. In addition to the benefits of lower noise and vibration, less destruction of the surfaces of the impeller and volute by cavitation erosion is experienced.

EXPERIMENTAL INVESTIGATION

byK. B. M. Q. Zaman', M. D. Dalai* mad T. J. Bencic tNASA Glenn Research CenterCleveland, OH 44135AbstractConvergent-divergent nozzles, when run atpressure ratios lower than the design value, often un-dergo a flow resonance accompanied by the emission ofacoustic tones. The phenomenon, different in character-istics from conventional 'screech' tones, has been stud-ied experimentally. Unlike screech, the frequency in-creases with increasing supply pressure. There is a'staging' behavior; 'odd harmonic' stages resonate atlower pressures while the fundamental occurs in a rangeof higher pressures corresponding to a fully expandedMath number (Mj) around unity. The frequency 0oN)variation with Mj depends on the half angle-of-diver-gence (0) of the nozzle. At smaller 8, the slope off_,versus Mj curve becomes steeper. The resonance in-volves standing waves and is driven by unsteady shock /boundary layer interaction. The distance between thefoot of the shock and the nozzle exit imposes the length-scale (L ').The fundamental corresponds to a quarter-wave resonance, the next stage at a lower supply pres-sure corresponds to a three-quarter-wave resonance, andso on. The principal trends in the frequency variationare explained simply from the characteristic variation ofthe length-scale L '.Based on the data, correlation equa-tions are provided for the prediction offN. A strikingfeature is that tripping of the boundary layer near thenozzle's throat tends to suppress the resonance. In apractical nozzle a tendency for the occurrence of thephenomenon is thought to be a source of 'internalnoise'; thus, there is a potential for noise benefit simply