Perceived Noise Level Research Articles

Noise monitoring system and MatLab application for aircraft noise level calculation

This paper presents the use of noise monitoring system, terminal NMT 3637B and software 7802, Brüel & Kjær in process of jet aircraft noise certification according to standard ICAO Annex 16 and the algoritam for calculating EPNL noise level (Effective Perceived Noise Level) that we created in MatLab environment. This software sublimated all necessary input parameters, acoustic and non acoustic in purpose of getting final result and intermediate data. Those data are important for reporting to the certificating authority.

On the potential limitations of conventional sound metrics in quantifying perception of nonlinearly propagated noise

The use of conventional metrics to quantify the perception of nonlinearly propagated noise has been studied. Gaussian noise waveforms have been numerically propagated both linearly and nonlinearly, and from the resulting waveforms, several metrics are calculated. These metrics are overall, A-, C-, and D-weighted sound pressure levels, perceived noise level, Stevens Mark VII perceived loudness, Zwicker loudness, and sharpness. Informal listening demonstrations indicate that perceived differences in annoyance between linearly and nonlinearly propagated waveforms are substantial. Because the metrics studied seem inadequate in representing the perceived differences, rigorous subjective testing is encouraged to properly quantify and understand these differences.

Fan Noise Reduction with Increased Bypass Nozzle Area

A model turbofan was tested in the NASA Glenn 9- by 15-ft low speed wind tunnel to explore far-field acoustic effects of a variable area bypass nozzle using three fixed nozzle flows. The baseline nozzle was sized to produce maximum stage performance at cruise condition. However, wind tunnel testing is conducted near sea level condition. Therefore, to simulate and obtain performance at other operating conditions, two additional nozzles were tested-one with +5% increase in weight flow (+5.4% area increase), sized to simulate the performance at the stage design point (takeoff) condition, and the other with a +7.5% increase in weight flow (+10.9% area increase) sized for maximum weight flow at sea level condition. Measured acoustic benefits with increased nozzle area showed effective perceived noise level reductions of 2 or more dB (for a 1500 ft fan flyover with a 3.35 scale factor) while the stage thrust actually increased by 2 to 3%. Noise reductions, principally in the level of broadband noise, were observed everywhere in the far field. Laser Doppler velocimetry measurements downstream of the rotor showed that the total turbulent velocity decreased with increasing nozzle flow, which may explain the reduced rotor broadband noise levels.

균등 및 비균등 배열 헬리콥터 반토오크 시스템 성능 및 소음특성

This paper described the performance test and sound characteristics of helicopter `Tail-Fan` anti-torque system. In this research, Korea Aerospace Research Institute(KARI) developed `Tail-Fan` anti-torque system for a helicopter and carried out performance and sound capturing tests of even and uneven tail fans. The performance test is carried out and the noise signals which are generated during the test are saved with microphones at the same time. The performance test`s results meet the design requirements. Tone-corrected perceived noise level is reduced by replacing the even product with the uneven one.

Response Surface Method Optimization of Uniform and Axially Segmented Duct Acoustics Liners

An extensive duct acoustics propagation study is presented that has been conducted to assess the design of a liner for an aeroengine inlet duct. The aim is to predict how different liner configuarations, at various flight conditions affect the attenuation of sound in an inlet. Two different noise source models are used: single mode and multimode. These represent the two principal fan noise sources: tonal and broadband noise. The two noise source models are then combined to predict the overall attenuation. An optimization procedure based on a response surface model is presented, to investigate a uniform and an axially segmented acoustic liner. The objective function used in the optimization is based on an approximate calculation of the perceived noise level. The aim is to utilize and axially segmented liner to increase, compared to a unifrom liner, the overall sound attenuation that is predicted. The main feature that emerges is that it is possible to increase the attenuation with an axially segmented liner only when a limited number of propagating modes are present.

Elegant Concept for Reduction of Jet Noise from Turbofan Engines

Jet noise is a major component of total aircraft noise at takeoff, even for modern aircraft powered by high-bypass-ratio (BPR) turbofan engines with BPR of around five. The reduction of jet noise at fixed BPR has proven to be a formidable challenge, and no practical design that produces substantial aeroacoustic benefit has evolved despite significant effort. A simple concept of a beveled nozzle is adapted for realistic engine geometries. The strategy for noise reduction in the peak sector of noise radiation is based on our understanding of the characteristics of the noise sources in a dual-stream exhaust system. Specifically, the noise generation mechanisms of the inner shear layer are modified by beveling the primary nozzle to achieve noise reduction. Detailed aeroacoustic measurements are carried out statically and in the presence of a flight stream to assess the noise benefit of the modified exhaust system, relative to a conventional arrangement, at realistic engine cycle conditions. Two beveled nozzles of bevel angles 24 (bevel24) and 45 deg have been evaluated. The bevel24 nozzle has a low performance penalty (∼0.2%) at cruise power and produces substantial reduction in noise levels in the aft quadrant. The reduction in overall sound pressure levels is ∼5 to ∼7 dB near the spectral peak in the angular sector >110 deg. There is a slight increase (∼1 dB) in the overall sound pressure levels at the lower polar angles. The magnitude of noise reduction is strongly dependent on the velocity of the inner stream. The reduction in levels occurs over a wide frequency range; there is no increase at the higher frequencies, which is a typical trend for other noise reduction concepts. The effects of forward flight on the measured spectra are different at different azimuthal angles; further, the flight effects in the azimuthal plane are a function of the bevel angle. A total noise reduction of ∼4 dB effective perceived noise level is demonstrated at takeoff (sideline and overhead) at realistic flight Mach numbers with the bevel24 nozzle.

An estimation of annoyance due to various public modes of transport in Delhi

Measurements of noise levels associated with different types of vehicles plying the roads in Delhi were made. From the data, noise level indices L(10) , L(90) and Leq were determined. In addition, spectra of noise for different vehicles at 1- octave band frequencies were also obtained. The time-averaged noise spectra reveal that the noise intensities are significantly higher in the frequency range of 0.5 kHz to 2 kHz for all types of vehicles. Perceived noise levels (PNdB) and the total noisiness measured on NOY scale indicate that rural transport vehicles (RTVs) are most annoying, followed by buses, auto-rickshaws and taxis.

Stated choice valuations of traffic related noise

This paper reports a novel application of the stated choice method to the valuation of road traffic noise. The innovative context used is that of choice between apartments with different levels of traffic noise, view, sunlight and cost with which respondents would be familiar. Stated choice models were developed on both perceived and objective measures of traffic noise, with the former statistically superior, and an extensive econometric analysis has been conducted to assess the nature and extent of householders’ heterogeneity of preferences for noise. This found that random taste variation is appreciable but also identified considerable systematic variation in valuations according to income level, household composition and exposure to noise. Self-selectivity is apparent, whereby those with higher marginal values of noise tend to live in quieter apartments. Sign and reference effects were apparent in the relationship between ratings and objective noise measures, presumably reflecting the non-linear nature of the latter. However, there was no strong support for sign, size or reference effects in the valuations of perceived noise levels.

Far-Field Acoustic Investigation into Chevron Nozzle Mechanisms and Trends

Chevron nozzles currently offer one of the most feasible methods of reducing jet exhaust noise in medium to high-bypass turbofan engines. Tests were conducted in the University of Cincinnati Nozzle Acoustic Test Facility, simulating a separate flow exhaust system to provide insight into some of the basic mechanisms and trends of this emerging technology. For this study, a baseline inner nozzle and three chevron nozzles were tested over a wide range of operating conditions, including dual and single flow. Chevrons with varying numbers of lobes and levels of penetration were selected for this study to provide insight into the impact of these geometric parameters on the noise level. Spectral and directivity results from heated, coaxial flow tests showed that the chevron nozzles are most effective at lower frequencies and at aft directivity angles. Reductions in overall sound pressure level (SPL) ranging from 3 to 6 dB were documented. Calculations of perceived noise level directivity also showed 4-6 dB reduction at aft angles. The data also illustrated clear and consistent trends with respect to the chevron geometric parameters. Specifically, the chevron penetration was determined to be a primary factor in controlling the tradeoff between low-frequency reduction and high-frequency SPL increases. Although slight differences were observed with varying chevron lobe numbers at a fixed penetration, it appears that the effect is less significant than the penetration. Finally, the data indicated clear dependence of the chevron benefit on the velocity difference between the inner and outer streams.

New Method for Jet Noise Reduction in Turbofan Engines

A new method for reducing large-scale mixing noise from dual-stream jets is presented. The principle is reduction of the convective Mach number of turbulent eddies that produce intense downward sound radiation. In a jet representing the coaxial exhaust of a turbofan engine, this is achieved by tilting downward, by a few degrees, the bypass (secondary) plume relative to the core (primary) plume. The misalignment of the two flows creates a thick low-speed secondary core on the underside of the high-speed primary flow. The secondary core reduces the convective Mach number of primary eddies, thus hindering their ability to generate sound that travels to the downward acoustic far field. Tilting of the bypass stream is possible by means of fixed or variable vanes installed near the exit of the bypass duct. Subscale aeroacoustic experiments simulated the exhaust flow of a turbofan engine with bypass ratio 6.0. Deflection of the bypass stream resulted in suppression of the peak overall sound pressure level by 4.5 dB and the effective perceived noise level by 2.8 dB. For the nozzle configuration used, the thrust loss is estimated at around 0.5% with the vanes activated and 0.15% with the vanes deactivated.

Distributed exhaust for jet noise reduction

Acoustic detection of low flying, jet-powered military air vehicles and takeoff noise levels from commercial jets are often driven by jet mixing noise radiated from the exhaust. Cueing provided by the noise signature results in increased opportunity for a ground-based visual observer against such a target. Significant reductions in detectability and takeoff noise levels can be achieved through reductions in jet noise. Research has been conducted over the past several years to develop innovative, quiet, distributed exhaust nozzle (DEN) concepts, which attempt to achieve revolutionary reductions in jet mixing noise while minimizing propulsion penalties. The DEN’s benefit relies on discharging the exhaust flow through many miniature nozzles rather than one or two large nozzles. Noise suppression from the DEN concept results from a favorable shift in frequency content compared to conventional jets. Significant increases in atmospheric attenuation and decreases in the ear’s sensitivity at these higher frequencies result in much reduced detection ranges and perceived noise levels. This technology is applicable to both subsonic and supersonic aircraft, and particularly to low flying fixed-wing unmanned air vehicles, special operations forces transports, and future commercial transports with especially stringent noise reduction requirements. [Work supported by NASA LaRC.]

Engine Cycle and Exhaust Configuration for Quiet Supersonic Propulsion

The thermodynamics and acoustics of a fixed-cycle, bypass ratio 3 supersonic engine with an innovative noise suppression scheme is explored. The silencing method entails installation of variable turning vanes in the bypass exhaust of a separate-flow turbofan engine. During noise-sensitive segments of flight, the vanes impart a slight downward tilt to the bypass plume relative to the core plume, thus thickening the bypass stream on the underside of the jet. This results in a reduction of the convective Mach number of instability waves that produce intense downward sound radiation. Subscale experiments show that, relative to the mixed-flow exhaust, the coaxial separate-flow exhaust with vanes reduces the peak overall sound pressure level by 8 dB and the effective perceived noise level by 7 dB. The noise-equivalent specific thrust on takeoff is reduced from 490 to 390 m/s. Compared to a current-generation low-bypass turbofan engine, the bypass ratio 3 engine is estimated to be 13 dB quieter with the mixed-flow exhaust and 20 dB quieter with the aforementioned suppression scheme. The vane configuration of this study is estimated to cause a thrust loss of 1% at takeoff and 0.25% at supersonic cruise.

On noise reduction by flap side edge fences

This paper discusses the effects of fences on side edge cross flows. Experimental data are reviewed to show that fences can modify the cross flow in the side edge region of aircraft flaps in such a way that noise reduction can be achieved. Near field pressure data reveal that fences reduce the characteristic frequency of the flow fluctuations. This leads us to postulate that the fences make the far field noise spectra shift downward in frequency. Because airframe noise spectra typically have a negative slope in the most important mid to high frequency domain, this downward shift in frequency leads to an equivalent reduction of noise levels, when measured by conventional metric such as the effective perceived noise level in aircraft certification. To explain the decrease of the characteristic frequency of the flow fluctuations due to the fences, an analytical model is developed, based on vortex dynamics. The predictions from the model are shown to be in very good agreement with measured data, both for the dominant frequency in the baseline case without the fences and for the trends of the decreasing frequency with increasing fence height.

Shoehorning in the jet age

The introduction into commercial service of jet air transports preceded the codification of U.S. federal aviation noise regulatory policy by nearly two decades. The immediate problem that had to be solved to permit jet operations in 1958 at New York airports was to determine that the noisiness of jet airplane would be no greater than that of the largest propeller-driven aircraft then in operation. This paper describes the efforts made for the Port of New York Authority to meet and enforce that mandate. BBN made noise measurements of many propeller aircraft takeoffs in the community off the main runway at Idlewild at distances of 2.5 miles and greater from the start of take-off roll. Each measurement was associated with airplane type, gross weight, and altitude. The Boeing 707 was similarly measured at Boeing’s airport. Relative annoyance judgments were made in the laboratory to establish equivalent noisiness of jet and propeller spectra, and ‘‘Perceived Noise Levels’’ were determined by a process similar to loudness calculations. Boeing was forced to equip the 707 with multi-tube mufflers and, and jet take-off procedures had to be modified to maintain equivalent perceived noise levels. The substantive findings and politics of these efforts are discussed.

Acoustics and Thrust of Quiet Separate-Flow High-Bypass-Ratio Nozzles

TheNASAGlennResearchCenterrecentlycompletedanexperimentalstudytoreducethejetnoisefrommodern turbofan engines. The study concentrated on exhaust nozzle designs for high-bypass-ratio engines. These designs modie ed the core and fan nozzles individually and simultaneously. In comparison with chevrons, tabs appeared to be an inefe cient method for reducing jet noise. Data trends show that interaction between fan e ow and the core cowl could strongly impact noise and cruise performance irrespective of the mixing device installed. The study demonstrates that modie cations of the core nozzle are generally more advantageous than of the fan nozzle. Even greater advantage in noise reduction and associated cruise thrust penalties is demonstrated through simultaneous modie cation of both nozzles. Thebestnozzledesign had a 0.06% cruisethrustlossand, corrected fortakeoff thrust loss, a 2.7-EPNdB reduction for the effective perceived noise level metric. This design simultaneously employed chevrons on the core and fan nozzles. Last, e ve nozzle cone gurations with cruise thrust loss of less than 0.5% and noise reductions of over 2.5 EPNdB are identie ed as candidates for full-scale engine and e ight demonstrations.

Not another airport noise metric

The assessment of airport noise in communities has been fraught with confusion by the proliferation of metrics. This paper will discuss the origins and uses of unusual acoustic metrics such as Time Above, Community Noise Index (CNI), Noise per Seat Index (NPSI), D-weighting, and exposure (EXP); and compare them with more ordinary ones such as Effective Perceived Noise Level (EPNL), Day–Night Equivalent Sound Exposure Level (DNL), A-weighting, maximum level (Lmax), and signal-to-noise ratio (S/N). The metrics will then be scrutinized from the technical, airport, and community viewpoints, and the strengths and weaknesses of each will be discussed. Nonacoustic factors, such as listener expectations, will also be discussed. It will be seen why so many different types of decibel have led to confusion.

The Effect of the Urban Street Canyon on the Noise from Low Flying Aircraft

The effect of urban streets on the noise from a low-flying aircraft has been investigated. The study first involved the development of a prediction method based on geometrical acoustics with the assumption of specular reflections. The effect of a variety of street configurations on the transmission gain was investigated and analysed in terms of changes in street geometry. The relative Effective Perceived Noise Level was also predicted. The predictions show that for specular reflections the relative Effective Perceived Noise Level increases in proportion to the ratio of building height to flying altitude, however, changing the street width has only a minor effect. The effect of mixed specular and diffuse reflections was investigated using a commercial acoustical modelling package and it was shown that although the effect on the transmission gain was small, the scattering of sound energy out of the street channel resulted in a decrease in the value of the relative Effective Perceived Noise Level with the ratio of building height to flying altitude resulting in a value of the relative Effective Perceived Noise Level which was negative for most street configurations examined. It can be concluded that although simple specular models can predict the transmission gain with reasonable accuracy, more complex models involving the effects of diffuse reflections are required for noise units involving integration over time.

항공기 소음 환경 기준 설정을 위한 연구

Rating scales for environmental noise are varied in their calculation procedure. Among them WECPNL (weighted equivalent continuous perceived noise level) is the rating scale for aircraft noise currently being used in domestic and applied only for aircraft noise. However <TEX>$L_{dn}$</TEX> calculated from <TEX>$L_{eq}$</TEX> <TEX>$L_{eq}$</TEX> is used as a rating scale for not only aircraft noise but also environmental noise. Besides, it is easy to calculate and internationally preferred. It is, therefore, not adequate for the evaluation of residents' exposure. Moreover it is very difficult to measure the aircraft noise by WECPNL due to the complicated calculating procedures if automatic measuring system is not used. Accordingly. this study aims to propose alternative evaluation procedure for the aircraft noise. To achieve this purpose, the data measured by automatic measuring system were gathered and calculated with three evaluation procedures : WECPNL <TEX>$L_{eq}$</TEX> and <TEX>$L_{dn}$</TEX>, and the results calculated from different methods were compared and analyzed.

Hot topics in noise

In response to increasing worldwide public concern about noise, new methods and means for assessing and controlling noise have been developed. Outdoors, advancements have been made in understanding the role that soundscapes play in the quality of life in our communities, which range from urban to rural environments. Recent efforts have been devoted to quantifying and preserving the resource of natural quiet in US National Parks. The impact of theme parks and amphitheaters on their host communities is an active area of research. International development of environmental noise policy is producing significant action. Indoors, new investigations on room noise descriptors are advancing that area of noise practice. The development of standards for classroom acoustics, including practical limits on noise, has the potential to improve the educational experience for all children. Product design research for sound quality will reduce perceived noise levels, and also make the resulting sound more pleasant. Important recent findings on occupational noise effects include new criteria for noise exposure, and the real-world effectiveness of hearing protectors. For many people, acoustics is chiefly represented by noise. These advances in the practical application of acoustics promise to contribute to a more civilized and hospitable society.

ヘリコプタのブレード/渦干渉騒音の解析

Two types of prediction tools for helicopter noise have been developed under the cooperative research between Advanced Technology Institute of Commuter Helicopter Ltd. (ATIC) and National Aerospace Laboratory (NAL) . One of them is a combined method of CAMRAD II, interpolation code for blade motion and wake geometry, aerodynamic code of 3D unsteady Euler solver, and aeroacoustic code based on Ffowcs Williams and Hawkings (FW-H) formulation. The other consists of CAMRAD II, 3D unsteady Euler solver using moving overlapped grid method, and FW-H code. The acoustic waveform of Blade-Vortex Interaction (BVI) noise predicted by the former tool is in good agreement with the experimental data of 1/7-scale model AH-1 Operational Loads Survey (OLS) rotor. This method is applied to investigate the effect of blade-tip shape on the intensity of BVI noise. As a result, it is shown that anhedral and swept-forward tip shapes effectively reduce the BVI noise of OLS rotor in a descent flight condition. The predicted Effective Perceived Noise Level (EPNL) of a helicopter is also compared with the experimental data obtained by ATIC and reasonable correlation is obtained. The latter tool successfully predicts the distinct spikes in the BVI wave-form of ATIC model rotor tested in German-Dutch Wind Tunnel (DNW) . In the comparison of measured and calculated carpet noise contours, reasonable agreement is obtained. The present tools are expected to be useful for the design of low-noise helicopters in the future.