Helicopter Noise Research Articles

Adaptive Blade Twist – calculations and experimental results

Applying adaptronics to helicopters has a high potential to significantly suppress noise, reduce vibration, and increase the overall aerodynamic efficiency. This paper presents recent investigations on a very promising specific concept described as Adaptive Blade Twist (ABT). This concept allows us to directly control the twist of the helicopter blades by smart adaptive elements. This influences positively the main rotor area which is the primary source for helicopter noise and vibration. Since the interaction of non-stationary helicopter aerodynamics and elastomechanical structural characteristics of the helicopter blades causes flight envelope limitations, vibration and noise, a good comprehension of the aerodynamics is essential for the development of structural solutions to effectively influence the local airflow conditions and finally develop the structural concept. With respect to these considerations, the ABT concept will be presented. This concept is based on the actively controlled tension-torsion-coupling of the structure. For this, an actuator is integrated within a helicopter blade that is made of an anisotropic material based on fiber composites. Driving the actuator results in a local twist of the blade tip in such a way that the blade can be considered as a torsional actuator. Influencing the blade twist distribution finally results in a higher aerodynamic efficiency. The paper starts at giving a review on conventional concepts and potential adaptive solutions for shape control. After, some calculations of the adaptive twist control concept are presented. These are based on a representative model in which the active part of the rotor blade is simplified with a thin-walled rectangular beam, that is structurally equivalent to a model rotor blade of the Bo105 with a scaling factor 2.54. The calculations are performed using an expanded Wlassow theory. The results are valid for static and dynamic conditions. For the dynamic condition excessive deformations near the blade resonance frequency shall be utilised. Therefore, the actuated blade section has to be properly designed for this precondition. This has been demonstrated and verified in experiments, which will not be discussed in this paper. For experimental investigations on the ABT concept the skin of the outer part of the model rotor blade was manufactured of fibre composite material using the above mentioned tension-torsion-coupling effect with an additional uncoupling layer between skin and spar. The experimental results have shown that near to the resonance frequency dynamic forces of 550±550 N are required for a deformation of ± 3 degrees at the blade tip.

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.

CNEL at two sites in Del Mar, due to a hundred helicopter flybys

In the Settlement Agreement in early 1999 of a lawsuit about helicopter noise, in which the City of Del Mar was one of the plaintiffs and the Marine Corps was one of the Federal defendants, there was a requirement that the community noise equivalent level (CNEL) be included as a standard for measuring noise significance. This is a preliminary report about CNEL measured continually at two sites in south Del Mar, California. In both cases the residents had filed official complaints about the noise of helicopters. These residences are distance from heavily traveled streets and highways. The automatic noise monitor consists of a Computer Engineering, Ltd., 493-2 integrating sound-level meter feeding a CEL-238 secondary processor, all in a weather-proof case. The outfit prints, and puts in memory, the one-hour average sound level during a preceding hour and at the end of a day the 24-hour average sound level (24 HL), the day–night average sound level (DNL), and the CNEL. At 22839 Via Grimaldi, Friday to Wednesday 5–10 May 99, the CNELs were 55.8, 55.5, 54.6, 55.9, 58.7, and 58.7 dB. At 421 Ocean Drive, Tuesday to Saturday 11–15 May 99, the CNELs were 58.4, 56.5, 59.6, 58.6, and 60.5 dB.

Military training noise impacts on the endangered Red-Cockaded Woodpecker: Preliminary results

A new three-year study has been initiated to determine the degree of impact of military training noise on the endangered Red-Cockaded Woodpecker (RCW). The study is being conducted on an Army installation. The study design and preliminary results from the first nesting season are presented. Noise sources of interest include artillery and tank live fire blast noise, small arms live fire noise, small arms blanks noise and artillery simulator noise that occurs during maneuver training, and helicopter noise. Response criteria include proximate responses such as flushing from a nest and fitness measures such as nesting success. The objective of the study is to determine, for each type of noise, a dose-response relation and/or response thresholds. The study includes a determination of an audiogram for woodpeckers, which will provide the basis for selecting stimulus characterization metrics that include appropriate frequency weighting and will facilitate interpretation of response data. [Work supported by SERDP.]

Aesthetic, Affective, and Cognitive Effects of Noise on Natural Landscape Assessment

Research has shown that helicopter noise from tourist flights is very common in some national parks and wilderness areas . At Grand Canyon National Park , air craft noise has been found to be as high as 76 dB (A) with as many as 43 noise events in a 20-minute period . The present study examined the influence of 40 dB (A) or 80 dB (A) helicopter noise on assessment of a popular Grand Canyon vista in a laboratory simulation. Participants (44 female and 36 male undergraduates) viewed 68 slides of scenic vistas and assessed them on naturalness, preference, and scenic beauty and evaluated dimensions of freedom, annoyance, solitude, and tranquility. Compared to a control condition of background natural sounds (e.g.,birds, brooks), noise conditions negatively impacted all dependent measures. Although the effects were most pronounced at the 80-dB level, even the 40-dB helicopter noise negatively impacted all dependent variables. Results suggest that helicopter noise interferes with the quality of the visitor experience and even affects the perceived aesthetic quality of landscapes.

The theoretical and practical aspects of producing source data for helicopter noise modeling

Accurate source data are the key to future developments in noise modeling. With the increasing need to model helicopter noise, suitable source data will be required. For helicopters, the directivity of the noise source is an essential component of the model. This paper reviews a system for determining helicopter directivity together with a discussion of future developments. An overview of an international helicopter noise measurement trial conducted in June 1998 to determine directivity will be presented along with some of the data.

Recent developments in helicopter rotor noise prediction at NASA

Significant progress has been made recently toward the efficient and robust prediction of helicopter high-speed impulsive noise. The far-field approximation to the Ffowcs Williams–Hawkings (FW–H) quadrupole has been augmented with a rigorous mathematical development. Two new quadrupole formulations—a retarded-time formulation known as Q1A and an emission-surface formulation known as Q2—have been developed and implemented for subsonic and supersonic source motion, respectively. A key numerical advantage of the far-field approximation is that volume integration of the source region surrounding the rotor blades is only performed once in a preprocessing stage. The resulting quadrupole noise computation is reduced to surface integration over the rotor disk in the vicinity of the blades. The solution-adaptive marching-cubes integration algorithm used for the emission-surface formulation will be described. Another recent advance in helicopter noise prediction is the use of the FW–H equation on a permeable integration surface surrounding the source region. This application of the FW–H equation has been shown to be both analytically and numerically superior to the Kirchhoff approach. Furthermore, numerical evidence is presented to demonstrate that the Kirchhoff method is inappropriate when a wake convects through the surface.

Effects of helicopter and aircraft interior noise and vibration on passengers’ comfort sensation and subjective well-being

Apparently passengers’ sensation of comfort and subjective well-being are key concepts in the research on user acceptance of helicopter and aircraft systems. The design and development of advanced air transportation technologies require a fundamental, theory-based understanding of these concepts. From the passengers point of view, helicopter and aircraft interior noise and vibration are primary physical factors underlying the perception of comfort and well-being. In various judgmental contexts the influence of exposure to combined interior noise and vibration in helicopters and aircrafts will be analyzed by means of combined psychoacoustical, socioacoustical, and psychological research methods: Field experiments in a flight simulator and during real flight situations as well as laboratory experiments using a sound and vibration reproduction system will be carried out in order to examine the interactive effects of noise and vibration. The aim is to develop a passenger comfort or well-being index for helicopter and aircraft interior environments. The index will be based on the integration of physical predictors for the flight situation and psychological predictors for the passengers’ sensation of comfort and well-being.

Spotted Owl reactions to helicopter and chain saw noise

Noise impacts on Mexican Spotted Owls (MSO) were documented during the 1995 non-nesting and the 1996 nesting seasons. Members of the MSO population in the Lincoln National Forest, NM, were exposed to typical helicopter and chain saw noise stimuli. The noise stimuli were presented at a variety of controlled levels and distances. Proximate behavioral responses, nesting success, and nesting productivity were observed and documented. The data were sufficient to establish response threshold distances and noise levels for both types of noise. No spotted owl flushes were recorded when the noise stimulus was >105 m distant, regardless of stimulus type. As stimulus distance decreased, spotted owl flush frequency was shown to increase. In terms of A-weighted noise level, the response threshold for helicopter noise was found to be 92 dB SEL and 46 dB LEQ for chain saws. [Work supported by U.S. Air Force.]

Effects of Helicopter Noise on Mexican Spotted Owls

Military helicopter training over the Lincoln National Forest (LNF) in southcentral New Mexico has been severely limited to protect nesting Mexican spotted owls (Strix occidentalis lucida). To evaluate nesting and nonnesting spotted owl responses to helicopter noise, we measured flush frequency, flush distance, alert behavior, response duration, prey delivery rates, female trips from the nest, and nest attentiveness during manipulated and nonmanipulated periods, 1995-96. Chain saws were included in our manipulations to increase experimental options and to facilitate comparative results. We analyzed stimulus events by measuring noise levels as unweighted one-third-octave band levels, applying frequency weighting to the resultant spectra, and calculating the sound exposure level for total sound energy (SEL) and the 0.5-sec equivalent maximum energy level (LEQ max 0.5-sec) for helicopters, and the 10-sec equivalent average energy level (LEQ avg. 10-se) for chain saws. An owl-weighting (dBO) curve was estimated to emphasize the middle frequency range where strigiform owls have the highest hearing sensitivity. Manipulated and nonmanipulated nest sites did not differ in repro- ductive success (P = 0.59) or the number of young fledged (P = 0.12). As stimulus distance decreased, spotted owl flush frequency increased, regardless of stimulus type or season. We recorded no spotted owl flushes when noise stimuli were >105 m away. Spotted owls returned to predisturbance behavior within 10-15 min after a stimulus event. All adult flushes during the nesting season occurred after juveniles had left the nest. Spotted owl flush rates in response to helicopters did not differ between nonnesting (13.3%) and nesting seasons (13.6%; P = 0.34). Spotted owls did not flush when the SEL noise level for helicopters was <102 dBO (92 dBA) and the LEQ level for chain saws was -59 dBO (46 dBA). Chain saws were more disturbing to spotted owls than helicopter flights at comparable distances. Our data indicate a 105-m buffer zone for helicopter overflights on the LNF would minimize spotted owl flush response and any potential effects on nesting activity.

Broadband Helicopter Noise Generated by Blade Wake Interactions

Broadband Helicopter Noise Generated by Blade Wake Interactions

Auscultation of heart and lung sounds in high-noise environments using adaptive filters

The clinical utility of a stethoscope in an aircraft is limited by high ambient noise. An adaptive noise-reduction stethoscope was implemented and some of its capabilities evaluated. Noise-corrupted heart and lung sounds, and a noise estimate, were collected using a two-microphone coupler in a simulated aircraft environment with 80–100 dB(SPL) ambient noise. Two adaptive filtering algorithms were compared: least mean square (LMS) and normalized LMS (NLMS). For frequencies between 20 and 800 Hz, the NLMS approach yielded roughly a 25% improvement in the extracted signal-to-noise ratio (SNR), while also providing a reduced adaptation time. Coupler chamber geometry was then examined by comparing two designs, one with a cylindrical microphone chamber and the other conical. There was no observable difference between the extracted signals due to this geometry change. Noise source independence was also investigated through comparison of algorithmic performance for C-130 aircraft and helicopter noise. Helicopter noise yielded an 18% lower SNR than C-130 noise due primarily to its relatively larger low-frequency spectral content. Thus a stethoscope using adaptive filters holds promise to extract heart and lung sounds from high ambient noise environments, with the NLMS providing some performance enhancements under certain conditions.

HMM-based strategies for enhancement of speech signals embedded in nonstationary noise

An improved hidden Markov model-based (HMM-based) speech enhancement system designed using the minimum mean square error principle is implemented and compared with a conventional spectral subtraction system. The improvements to the system are: (1) incorporation of mixture components in the HMM for noise in order to handle noise nonstationarity in a more flexible manner, (2) two efficient methods in the speech enhancement system design that make the system real-time implementable, and (3) an adaptation method to the noise type in order to accommodate a wide variety of noise expected under the enhancement system's operating environment. The results of the experiments designed to evaluate the performance of the HMM-based speech enhancement systems in comparison with spectral subtraction are reported. Three types of noise-white noise, simulated helicopter noise, and multitalker (cocktail party) noise-were used to corrupt the test speech signals. Both objective (global SNR) and subjective mean opinion score (MOS) evaluations demonstrate consistent superiority of the HMM-based enhancement systems that incorporate the innovations described in this paper over the conventional spectral subtraction method.

New Computational Methods for the Prediction and Analysis of Helicopter Noise

Abstract : This paper describes several new methods to predict and analyze rotorcraft noise. These methods are 1) a combined computational fluid dynamics and Kirchholt scheme for far-field noise predictions, 2) parallel computer implementation of the Kirchhoff integrations 3) audio and visual rendering of the computed acoustic predictions over large far-field regions, and 4) acoustic tracebacks to the Kirchhoff surface to pinpoint the sources of the rotor noise. The paper describes each method and presents sample results for three test cases. The first case consists of in-plane high-speed impulsive noise and the other two cases show idealized parallel and oblique blade-vortex interactions. The computed results show good agreement with available experimental data but convey much more information about the far-field noise propagation. When taken together, these new analysis methods exploit the power of new computer technologies and offer the potential to significantly improve our prediction and understanding of rotorcraft noise.

Noise compensation methods for hidden Markov model speech recognition in adverse environments

Several noise compensation schemes for speech recognition in impulsive and nonimpulsive noise are considered. The noise compensation schemes are spectral subtraction, HMM-based Wiener (1949) filters, noise-adaptive HMMs, and a front-end impulsive noise removal. The use of the cepstral-time matrix as an improved speech feature set is explored, and the noise compensation methods are extended for use with cepstral-time features. Experimental evaluations, on a spoken digit database, in the presence of ear noise, helicopter noise, and impulsive noise, demonstrate that the noise compensation methods achieve substantial improvement in recognition across a wide range of signal-to-noise ratios. The results also show that the cepstral-time matrix is more robust than a vector of identical size, which is composed of a combination of cepstral and differential cepstral features.

Performance of a digital active noise reduction hearing protector in a reverberant chamber

High-intensity, low-frequency noise in modern helicopters and land vehicles degrades the communication reliability and endangers the hearing of the crew. Passive noise attenuation is not effective in terms of the performance/cost ratio in such environments. While active noise reduction (ANR) headsets that use analog feedback control have been commercially available for some time, this paper will focus on digital ANR hearing protector/headsets, which apply a feedforward control approach. A prototype ANR hearing protector has been developed in the laboratory using a floating-point digital signal processor (DSP) and an adaptive feedforward control algorithm. The ANR performance will be discussed based on recent experimental results measured on five human subjects in a reverberation chamber. For the experiments, 12 to 300-Hz broadband white noise and simulated helicopter noise were used. The results showed good control stability and active noise reduction performance.

HELICOPTER NOISE PREDICTIONS USING KIRCHHOFF METHODS

This paper presents two methods for predicting the noise from helicopter rotors in forward flight. Aerodynamic and acoustic solutions in the near field are computed with a finite-difference solver for the Euler equations. Two different Kirchhoff acoustics methods are then used to propagate the acoustic signals to the far field in a computationally-efficient manner. One of the methods uses a Kirchhoff surface that rotates with the rotor blades. The other uses a nonrotating Kirchhoff surface. Results from both methods are compared to experimental data for both high-speed impulsive noise and blade-vortex interaction noise. Agreement between experimental data and computational results is excellent for both cases. The rotating and nonrotating Kirchhoff methods are also compared for accuracy and efficiency. Both offer high accuracy with reasonable computer resource requirements. The Kirchhoff integrations efficiently extend the near-field finite-difference results to predict the far field helicopter noise.

Robust continuous speech recognition using parallel model combination

This paper addresses the problem of automatic speech recognition in the presence of interfering noise. It focuses on the parallel model combination (PMC) scheme, which has been shown to be a powerful technique for achieving noise robustness. Most experiments reported on PMC to date have been on small, 10-50 word vocabulary systems. Experiments on the Resource Management (RM) database, a 1000 word continuous speech recognition task, reveal compensation requirements not highlighted by the smaller vocabulary tasks. In particular, that it is necessary to compensate the dynamic parameters as well as the static parameters to achieve good recognition performance. The database used for these experiments was the RM speaker independent task with either Lynx Helicopter noise or Operation Room noise from the NOISEX-92 database added. The experiments reported here used the HTK RM recognizer developed at CUED modified to include PMC based compensation for the static, delta and delta-delta parameters. After training on clean speech data, the performance of the recognizer was found to be severely degraded when noise was added to the speech signal at between 10 and 18 dB. However, using PMC the performance was restored to a level comparable with that obtained when training directly in the noise corrupted environment.

Effects of helicopter noise on spotted owls: Methodology

The Endangered Species Act requires federal agencies to assess impacts of their activities on threatened and endangered species and to carry out programs for the conservation of listed species. In some cases activities, including military training, are curtailed because of potential impacts. There is currently no known published research on the possible impact of noise on the spotted owl. The present research addresses the question of the noise impacts of low level helicopter flights and ground activities such as chain saws and motorcycles on Mexican spotted owls (Strixoccidentalislucida). This research will characterize the effect of anthropogenic activity on breeding Mexican spotted owls by developing a dose-response threshold model that quantifies animal response relative to sound stimulus levels and approach distances. Consideration is given to the hearing range and sensitivity of the Mexican spotted owl, the sound level received at roost and nest sites, flight response of nonbreeding owls, effects on nest attentiveness of breeding females and on the rate of prey delivery by breeding males, and development of disturbance-specific management guidelines to minimize potential audio and visual impacts from helicopter and ground activities.

Perpendicular blade vortex interaction

The interaction between a streamwise vortex and a spanwise blade was studied in incompressible flow for blade-vortex separations between ±1/8 chord. Three-component velocity and turbulence measurements were made from 4 chord lengths upstream to 15 chord lengths downstream of the blade using miniature four-sensor hot-wire probes. The interaction of the vortex with the blade causes an almost immediate loss in vortex core circulation. Downstream of the blade the core becomes embedded in the blade wake and begins to grow rapidly. Core radius increases and peak tangential velocity decreases but circulation remains roughly constant. True turbulence levels within the core are much larger downstream than upstream of the blade. Outside of the core the interaction produces a substantial region of turbulent flow associated with the blade and vortex generator wakes. Overall, perpendicular blade vortex interaction substantially alters the flow and produces a much larger and more intense region of turbulent flow than that presented by the undisturbed vortex. These results have significant implications for the prediction of both impulsive and broadband helicopter noise.