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Flow Acoustics

2008;():1-8. doi:10.1115/NCAD2008-73008.

Flow-induced vibration tests were performed on a beam with a square cross section. The test article was induced into a plunge mode self-excited vibration by forcing air across the beam while it was suspended by leaf springs. The natural frequency and the damping were varied during these tests to alter the variables that contribute the onset of flow-induced vibration. Additionally, cantilevered beams were tested in the same wind tunnel and results proved to be consistent with results from the leaf spring supported beam. The slope of the vertical force coefficient was determined from this testing; this coefficient was determined to be approximately unity, which was roughly 1/3 of the value found in other published data.

Commentary by Dr. Valentin Fuster
2008;():9-15. doi:10.1115/NCAD2008-73012.

Crowd noise levels in large stadiums are often discussed in the press and by the casual stadium patron; however, there has been little scientific evaluation of these levels and their corresponding effects. An effort was undertaken at Penn State University to measure noise on the field at Beaver Stadium (State College, PA: capacity 107,282) during a PSU football game. The two-fold purpose of these measurements was to evaluate the noise levels with respect to how they affect the game and make real-time, calibrated recordings for future use in the football team’s indoor practice facility. Measurements were taken on the field level using sound levels meters and DAT recorders throughout the PSU vs. Ohio State University football game on 27 October 2007. As a point of comparison, similar measurements were conducted by a team of researchers from Virginia Tech during the VT vs. University of Miami game on 17 November 2007 at Lane Stadium (Blacksburg, VA: capacity 66,233). The crowd noise levels are compared between the two stadiums taking into account the circumstances of the two games, time of day, and design of the stadiums. The levels on the field are evaluated in terms of speech intelligibility, i.e. the ability of the players to communicate with each other. In addition, a hearing loss safety assessment for both fans and players is presented.

Topics: Noise (Sound) , Fans
Commentary by Dr. Valentin Fuster
2008;():17-29. doi:10.1115/NCAD2008-73016.

Lock-in occurs between many different types of flow instabilities and structural-acoustic resonators. Factors that describe the coupling between the fluid and structure have been defined for low flow Mach numbers. This paper discusses how different flow instabilities influence lock-in experimentally and analytically. A key concept to the lock-in process is the relative source generation versus dissipation. The type of fluid instability source dominates the generation component of the process, so a comparison between a cavity shear layer instability with a relatively stronger source, for example wake vortex shedding from a bluff body, will be described as a coupling factor. In the fluid-elastic cavity lock-in case, the shear layer instability produced by flow over a cavity couples to the elastic structure containing the cavity. In this study, this type of lock-in was not achieved experimentally. A stronger source, vortex shedding from a bluff body however, is shown experimentally to locks into the same resonator. This study shows that fluid-elastic cavity lock-in is unlikely to occur given the critical level of damping that exists for a submerged structure and the relatively weak source strength that a cavity produces. Also in this paper, a unified theory is presented based on describing functions, a nonlinear control theory used to predict limit cycles of oscillation, where a self-sustaining oscillation or lock-in is possible. The describing function models capture the primary characteristics of the instability mechanisms, are consistent with Strouhal frequency concepts, capture damping, and are consistent with mass-damping concepts from wake oscillator theory. This study shows a strong consistency between the analytical models and experimental results.

Commentary by Dr. Valentin Fuster
2008;():31-42. doi:10.1115/NCAD2008-73019.

This paper examines the interaction of nonuniform flows with propeller blades in a submerged elastic duct. The acoustic radiation from the duct is calculated and correlated to the flow nonuniformities and the propeller and duct characteristics. The case of a stiffened duct with ribs is also considered and the dispersion relation of the duct modes is compared with that of a regular duct. The dispersion relation of the stiffened duct has a periodic structure similar to that of connected oscillators with large number of independent modes. Because of our interest in the acoustic radiation of such a system, we focus our attention on the flexure modes. The model is first tested with simple internal forces such as monopoles and dipoles. The results for unstiffened ducts show strong directivity as the dipole radial location moves closer to the duct wall. For stiffened ducts, the magnitude of the acoustic response as well as the directivity vary strongly and show large peaks near the stiffened duct free modes. For a propeller, an Euler code provides the pressure distribution along the blades. This represents the dipole strength distribution. Its radiated sound is calculated by summing up the contribution of the distributed dipoles. In this process, compact source effects are also taken into account.

Commentary by Dr. Valentin Fuster
2008;():43-51. doi:10.1115/NCAD2008-73020.

Beamforming in reverberant environments is important to locate and quantify noise sources in turbofan engine nacelles, automobile interiors, factories, and architectural settings. In order to validate and explore the limits of this approach, a beamforming experiment was conducted in a reverberation chamber using a 32 channel planar phased array and a B&K sound power source. In the reference configuration, the source was located 2 m from the 1.1 m diameter array, and neither was close to the chamber walls. A less-demanding case was constructed by adding some foam absorber to the room to reduce reverberation. A difficult case resulted when the source was placed 5 m from the array, about 1 m from a corner of the chamber. Conventional frequency-domain beamforming with diagonal deletion was applied. The sound source was accurately located at the 2 m distance, with and without the added absorber. In the 5 m case, the sound source could be located at only a few frequencies and only when the processing bandwidth was increased from 48.8 Hz to 781 Hz. Processing individual eigenvectors of the CSM separated the direct and reflected source in 5 m case. The error in the deduced broadband sound power was 0.72 dB in the baseline case, 1.47 dB with the added absorber, and 5.41 dB with the speaker in the corner. Application of CLEAN-SC did not improve the accuracy of the corner results. Use of a Green’s function that attempts to account for reflections was ineffective. It is concluded that beamforming in highly reverberant environment with the free space Green’s function is practical, provided the array is designed and positioned correctly for the environment and source location.

Commentary by Dr. Valentin Fuster
2008;():53-57. doi:10.1115/NCAD2008-73021.

An acoustic localization method is applied in a reverberant environment to locate the sources of discrete sounds having unknown timing and waveform. In particular, the localization method is applied to study low event rate cavitation in a vortical flow in a water-tunnel test-section with characteristic cross section dimension of 0.3 m. The primary frequency and bandwidth of the acoustic pulses from the small isolated cavitation bubbles are 10 kHz and 200 kHz respectively, and the measured pulse duration is ∼15–20 micro-seconds. The localization method involves using an array of receiving hydrophones to record the cavitation sound pulses. These hydrophone recordings, which include direct-path signal, reflected path signal, and noise, are time windowed and cross-correlated to obtain direct-path arrival-time differences. These arrival time differences are used in conjunction with a simple ray-based acoustic model to estimate the source location in three dimensions via a robust Monte-Carlo routine. The ratio of the primary-frequency wavelength to the water-tunnel cross-section dimension is ∼1/2. Consequently the time-windowing is tight; only 1 to 1.5 center-frequency cycles at the beginning of a signal pulse are readily useful for localization purposes. The remainder of the signal is contaminated by reflections and is not used in the present effort. To check and validate the results of the acoustic method, two-camera high-speed video data was taken synchronously with the acoustic data for 53 cavitation events. The acoustic localization scheme provided an unambiguous location estimate for all 53 cavitation bubbles. The average distance between the optical and acoustic measurement of the bubble location was 18.4 mm, or ∼1/8 of the wavelength of the primary signal frequency.

Topics: Acoustics , Sound
Commentary by Dr. Valentin Fuster
2008;():59-67. doi:10.1115/NCAD2008-73029.

Sound generation in low Mach number turbomachines is typically dominated by unsteady fluid forces on rigid surfaces. As a result, the radiated sound is closely related to the unsteady flow field. The present study focused on the self noise that is generated by a ducted rotor separate from the effect of noise due to inflow turbulence. The flow rate through the rotor was independently varied in order change the mean lift on the blades. Measurements of the flow field around a ducted rotor were found to provide insight to the various mechanisms of sound that are present at different mean loading conditions. At lower flow rates the blades were partially stalled, resulting in significantly increased noise levels. The measurements included rotor wake measurements using hot-wire anemometry and far field sound. A simple model to predict the radiated self noise based on the hot-wire measurements is presented.

Commentary by Dr. Valentin Fuster
2008;():69-77. doi:10.1115/NCAD2008-73031.

The actual speed of sound in the exhaust medium of an engine plays an extensive role in the noise attenuation characteristics of the engine’s muffler system. For 2-stroke engine applications, the speed of sound in the exhaust gas also greatly affects how the expansion chamber is tuned to maintain maximum power output. The combustion process in an engine creates exhaust gases that differ from the composition of atmospheric air. This difference in chemical composition and humidity content yield a different density and ratio of specific heats. These ultimately yield different sound speeds in the exhaust gases compared to atmospheric air. This paper performs a full chemical analysis of the combustion process in an internal combustion gasoline engine to yield the chemical composition of the of the exhaust gases. An algorithm is written to calculate the speed of sound in the exhaust stream. The inputs of the algorithm include measurements of temperature, pressure, and relative humidity of the ambient intake air, specification of the gasoline/ethanol fuel blend, and a direct measurement of the exhaust gas temperature. Comparisons are made between sound speed approximation calculations based on air to calculations obtained by the algorithm.

Commentary by Dr. Valentin Fuster
2008;():79-89. doi:10.1115/NCAD2008-73036.

This is the first of a two-part paper that lays out a theory of broadband noise from rough surfaces and uses it to interpret recent experimental data. The analysis in Part 1 is based on an application of Rapid Distortion Theory to an incident field of correlated micro-velocities defined upstream of the roughened region. Those velocity disturbances are linked to the frequency-wavenumber spectrum of wall pressures from a standard model of turbulent boundary-layer flow. The field of incident microvelocities distorts as it is convected irregularly around and over the roughness elements and thereby generates the predicted broadband sound. Computed results in the Part-2 paper will gauge the role of a boundary layer’s mean shear in the noise-production process relative to the rapid distortions carried by an artificially irrotational mean flow (for reasons to be described in Part 2, shear effects turn out to be insignificant for the application of immediate interest). Calculations in support of recent measurements will be presented for a range of operating conditions and for their associated set of dimensionless scaling parameters.

Commentary by Dr. Valentin Fuster
2008;():91-100. doi:10.1115/NCAD2008-73039.

This paper describes a technique for the investigation of noise sources correlated to tip clearance flows in a low-speed axial fan. A detailed experimental acoustic study is carried out examining the chord-wise evolution of a rotor flow field in the proximity of the blade tip in a low-solidity impeller. The experiment is performed by keeping the rotor “frozen” inside an anechoic chamber. The Mach number, Reynolds number, and blade incidence angle are set in the static frame of reference, reproducing the flow field in the rotating frame. The ‘frozen’ fan rotor is mounted in an anechoic chamber. The near-field pressure perturbations are measured using a chord-traversed microphone. Near-field pressure data is then compared with theoretical predictions, experimental data, and numerical simulations. In this way the validity of the developed experimental scheme is assessed. The purpose of the present program of work is to identify the change in near-field noise as a result of the chord-wise turbulent structures that are located close to the blade surface at the tip. The objective is to study the evolution of turbulent flow structure paths along the chord, and thus to provide insights into their acoustic significance. The present program of work is facilitated by the existence of a detailed pre-existing experimental database on the fan studied.

Commentary by Dr. Valentin Fuster
2008;():101-110. doi:10.1115/NCAD2008-73044.

It has been proposed that microphones mounted flush on the ground surface should measure reduced wind noise levels since the microphone does not disturb the flow and the wind velocity at the ground is small. However, there are pressure fluctuations induced on the ground by the interaction of the turbulent fluctuations above the ground with the average wind velocity gradient. Model calculations of the pressure spectrum under different wind velocity profiles have been developed based on the theoretical work of Kraichnan.1 Kraichnan calculated the turbulence-shear contribution to pressure fluctuations under boundary layer turbulence with an exponentially varying flow velocity profile. The calculations presented here incorporate measured outdoor spectra of the turbulence and different models of the velocity profile to investigate pressure fluctuation measurements outdoors at the ground surface. The predicted power spectral densities of the pressure are compared to each other and to measurements. The ultimate purpose of this research is to develop means to minimize wind noise interference in acoustic measurements. [Research supported by the U.S. Army TACOM-ARDEC at Picatinny Arsenal, NJ.]

Commentary by Dr. Valentin Fuster
2008;():111-118. doi:10.1115/NCAD2008-73052.

The objective of this study was to experimentally investigate the broadband trailing edge noise generated by a sharp trailing edge geometry and an asymmetric blunt edge. The flow field in the vicinity of the sharp trailing edge was found to be equivalent to that of a flat plate turbulent boundary layer. The interaction of the two boundary layers with the edge was responsible for broadband noise generation. The blunt trailing edge geometry exhibited additional complexity, with turbulent boundary layer separation and sound generated by vortex shedding. The measurement program included hot-wire anemometry, unsteady surface pressure, and radiated sound utilizing two microphone arrays. The boundary layer parameters and wall pressure spectra were used to compute the radiated sound from existing scattering theory. These calculations agreed very well with the array data, with differences typically within 2dB over the frequency range considered valid for the theory.

Topics: Noise (Sound)
Commentary by Dr. Valentin Fuster
2008;():119-128. doi:10.1115/NCAD2008-73055.

This paper describes the development of a new method for measuring pass-by sound from trucks and other vehicles using 2-dimensional arrays. The approach provides 2-dimensional quantitative maps “images” of the cross-range and elevation distribution in the vehicle side view. The method is an application and extension of an array technology that was originally used for the characterization of static aeroacoustic sources in wind tunnels. The focus of this work is on identifying and rank-ordering the important contributing sources of passby noise. This development includes two phases: developmental testing at a test track site, and road-side testing at two California State highway sites. The acquisition post-processing allows the “observer” to track the vehicle cross-range in order to create a time sequence of source maps that may be interpreted as both level relationships and directivity patterns. The processing applies both range and approximate Doppler adjustments to spectra as a function of time during pass-by or, equivalently, to vehicle position relative to the array’s center. An image demodulation scheme is shown to clarify the images. The initial phase of this work occurred at a test track using known “cooperative” truck sources. This experience permitted the verification of the method and the definition of a final measurement approach that was viable at a highway site. Subjects were all trucks that varied in model, vehicle speed, tread, and the presence of a trailer. The array beamformer’s ability to localize and the measurement system’s ability to track were validated using both stationary and moving sources. Following validation at the test track site, the instrumentation was transferred to two California highway sites. There, acoustic calibration was used to align the array with the road track and to provide a spatial reference for mapping the “images”. Both light and heavy vehicles at these sites were “uncooperative” with arrivals and speeds randomly determined by traffic flow. This work was funded by the California Department of Transportation.

Topics: Noise (Sound) , Roads , Trucks
Commentary by Dr. Valentin Fuster
2008;():129-133. doi:10.1115/NCAD2008-73071.

This paper deals with the effects of atmospheric absorption on the propagation of high-speed jet noise. The common practice for determining the far-field jet noise spectra at a distance far from the jet exit (>100D, where D is the nozzle exit diameter) involves extrapolating data that is typically obtained between 35D and 100D from the nozzle exit. The data is extrapolated along a radial line from the nozzle exit by accounting for the effects of spherical spreading and atmospheric absorption. A previous paper discussed far-field measurements that were obtained for a twin engine aircraft at three locations along a radial line in the peak noise radiation direction. The authors were unable to extrapolate the spectra from the nearest location to either of the further locations and the observed differences were attributed to nonlinear effects in the jet noise signal. It is the purpose of this paper to show that the common extrapolation practice is valid for high speed jets, except in the peak radiation direction and its surrounding angles. Mach wave radiation is present at these locations and the common practice will yield unsatisfactory results, similar to those observed in the previous paper. The data used in this paper is taken from experiments carried out at 1/5th-scale and full scale and the experimental conditions of these high-speed jets are quite similar to those of the previous paper.

Topics: Noise (Sound)
Commentary by Dr. Valentin Fuster
2008;():135-144. doi:10.1115/NCAD2008-73081.

Sound transmission in a finite-length lined annular duct with mean swirling flow is investigated using mode-matching methods. The main application of this work is the acoustics of aeroengine duct systems, especially the prediction of sound transmission behind a fan/rotor stage where the swirl velocity could be comparable to the axial velocity. First, a spectral collocation technique is used to determine the eigenmodes of three-dimensional linearized Euler equations, then two mode-matching schemes are utilized for calculating the sound transmission in ducts. The two schemes are compared with each other and also with the finite element method. The modified matching scheme is believed to deal with the impedance discontinuity more accurately at the interface. A sound power definition for high frequencies in the presence of mean swirling flow is used for the analysis of sound transmission characteristics. The modified matching scheme is then used to analyze the effect of mean swirling flow on the sound power transmission loss and conduct liner optimization in the impedance plane, compared with the uniform axial mean flow case. Finally, sound attenuation due to inner wall lining, outer wall lining, and combined inner and outer wall lining, respectively, is investigated.

Topics: Sound , Ducts , Swirling flow
Commentary by Dr. Valentin Fuster
2008;():145-151. doi:10.1115/NCAD2008-73084.

In this study, effects of windscreen material property on wind noise reduction are investigated at different frequencies of incoming wind turbulence. The properties of porous materials used for the windscreen are represented by flow resistivity. Computational techniques are developed to study the detailed flow around the windscreen as well as flow inside the windscreen that uses a porous material as the medium. The coupled simulation shows that for low-frequency turbulence, the windscreens with low flow resistivity are more effective in noise reduction. Contrarily, for high-frequency turbulence, the windscreens with high flow resistivity are more effective.

Commentary by Dr. Valentin Fuster
2008;():153-165. doi:10.1115/NCAD2008-73091.

This Part-2 paper applies Part 1’s theory of sheared rapid distortions to compute broadband noise from flow over large roughness elements, and compares those calculations to recent wind-tunnel measurements. The calculations suggest that shear effects are subdominant in the sound-production process. Post-processing of computed results brings out key features of the theory’s non-equilibrium distorting turbulence. A follow-up analysis makes possible the physical interpretation of the measured acoustic spectral densities in terms of the kinematics of the spatially non-uniform carrier flow.

Commentary by Dr. Valentin Fuster

Active and Passive Noise Control

2008;():167-174. doi:10.1115/NCAD2008-73018.

This paper addresses the construction, measurement, and analysis of a double-panel active partition (DPAP) and its accompanying analog feedback controllers. The DPAP was constructed by attaching an aluminum cone loudspeaker at each end of a short segment of a circular duct. Two analog feedback controllers were designed and built using the measured frequency response function of each panel. Two independent (decoupled) feedback controllers were then used to minimize the vibration amplitude of each panel in the presence of an acoustic disturbance. A normal-incidence transmission loss measurement system was used to assess the performance of the DPAP and of a single panel passive partition. Error signal attenuations show that it is both feasible and effective to simultaneously control both panels with decoupled feedback controllers, and that simultaneously controlling both panels of the DPAP has a distinct advantage over controlling a single panel. The reduction in vibration amplitude across the surface of the transmitting panel was confirmed with scanning laser vibrometer measurements. Transmission loss results were obtained for two passive and three active configurations. The average normal incidence transmission loss over the active measurement bandwidth (50–1,000 Hz) for the active double-panel was 60 dB. This is an average of 39 dB more transmission loss than a passive single panel partition.

Commentary by Dr. Valentin Fuster
2008;():175-185. doi:10.1115/NCAD2008-73022.

This study contrasts two modeling techniques proposed to accurately predict the influence of sintered fibrous metal (a non-woven structure of metallic fibers attached to one another by sintering processes), as a liner substitute, on sound attenuation performance and resulting noise emission for conventional aftermarket dissipative mufflers. Predicted values are compared to sound measurement data from stationary engine exhaust tests of a commercially available single-cylinder 450cc off-road motorcycle. The performance prediction techniques rely on the appropriate application and combination of pre-existing silencer design, engine exhaust and gas flow performance models as an economic alternative to more complex and expensive modeling programs that are typically beyond the reach of most small to medium-sized businesses in the motorcycle aftermarket industry. With respect to test results that showed approximate acoustical parity between mufflers containing the two different liner types, application limitations on the most suitable prediction technique are presented along with suggestions for further model refinement or additional physical testing. Further research is also invited to explore the impact of this liner substitution on muffler backpressure and its consequential impact on realized engine power.

Commentary by Dr. Valentin Fuster
2008;():187-194. doi:10.1115/NCAD2008-73024.

Noise Induced Hearing Loss is the most common occupational disease in the U.S. and of paramount importance in the mining industry. According to data for 2006 from the Mine Safety and Health Administration (MSHA), Continuous Miner operators accounted for 30.2% of underground mining equipment operators with noise doses exceeding the Permissible Exposure Limit (PEL). This figure becomes more significant considering that 49% of the 2006 national underground coal production was extracted using continuous mining methods. Thus, there is a clear need to reduce the sound radiated by Continuous Mining Machines. The first step towards efficient noise control of a Continuous Mining Machine requires identification of the various noise sources under controlled operating conditions. To this end, a 42-microphone phased array was used in conjunction with 4 reference microphones to sample the acoustic field of a machine in the Hemi-anechoic chamber of the Pittsburgh Research Laboratory. These data were processed using a frequency-domain beamforming algorithm to obtain acoustic maps of 5 sides of the machine. The focus of the test was on the conveyor noise since previous studies showed that operation of the conveyor is the most important contributor to the sound radiated by the machine. From the acoustic maps, the following potential areas for noise control were identified, and included: chain-tail-roller interaction, chain flight tip-side board interaction, and chain-upper deck interaction.

Commentary by Dr. Valentin Fuster
2008;():195-200. doi:10.1115/NCAD2008-73025.

Normally, small differences in day-to-day and laboratory-to-laboratory sound absorption measurements do not have large consequences because most noise control applications are not highly sensitive to small changes in sound absorption. However, in the automotive industry, materials are not purchased unless they meet strict sound absorption targets. As a result, decisions worth millions of U.S. dollars are made based on acoustic measurements. As material sound absorption moves closer to target values, the consequences of small measurement variations, such as those which might be caused by changes in ambient temperature and humidity during the course of a test, become more critical. The purpose of the work presented in this paper is to investigate which materials used for vehicle sound absorption are sensitive to temperature and humidity. Measurements are made using an impedance tube. It is discovered that typical materials used as absorbers in automotive applications are not sensitive to small temperature changes, and only a few materials are sensitive to changes in humidity.

Commentary by Dr. Valentin Fuster
2008;():201-206. doi:10.1115/NCAD2008-73027.

The consumer today places greater demands upon the vehicle acoustical engineer than in the past. Product quality has always been associated with a quiet ride. Automotive engineers recognize that the predominant sources of vehicle interior noise are wind, tire-road or rolling noise, and the powertrain. This paper suggests a test protocol for measuring wind and rolling noise using a chassis rolls dynamometer and road tests. Automotive engineers are frequently confronted by customer complaints concerning wind noise. Usually, engineers resort to using wind tunnels to address these concerns and to conduct diagnostic studies to remedy wind noise problems. Unfortunately, wind tunnels are expensive to rent and difficult to schedule. As an alternative, the engineer can learn a great deal about the wind noise of a vehicle by using a chassis rolls dynamometer along with road tests [1,2]. If the chassis rolls surface texture closely matches that of the road surface, the tire-road or rolling noise signal in both situations can be assumed to be equivalent. The powertrain noise source can be minimized by shifting the vehicle into neutral and coasting. Wind noise is a source for the road measurements, but not for the chassis rolls. Hence, the wind noise can be calculated by measuring the cab interior noise for both operating conditions, and subtracting the rolling noise measured on the chassis rolls. The two vehicles tested in this study included a pickup truck and a sport utility vehicle. The acoustical data revealed significantly different rolling and wind noise characteristics. The pickup truck had significantly louder rolling noise, and the wind noise was dominated by low frequency sound. The sport utility vehicle was much quieter overall and was significantly quieter for rolling noise than the pickup. The wind noise of the sport utility vehicle also was dominated by high frequency components. Both vehicles showed that rolling and wind noise trends increase linearly with speed. However, the slope of wind noise data for the sport utility vehicle was much steeper than the pickup, which suggested that it was more sensitive to wind noise as speed increased. Exterior noise data from both vehicles showed that the tire-road signal from the road differed significantly from that of the chassis rolls dynamometer. Rolling & wind noises will become even more critical as the motor vehicle industry adopts hybrid electric and, in the future electric fuel cell vehicles, because powertrain noise sources in the vehicle will likely be reduced. The procedure suggested here provides an inexpensive simple approach to assessing rolling and wind noise in the vehicle.

Commentary by Dr. Valentin Fuster
2008;():207-214. doi:10.1115/NCAD2008-73034.

This paper presents a passive method to control the broadband noise inside a small enclosure using multiple, optimally located, T-shaped acoustic resonators. The resonators consist of two mutually perpendicular tubes: one short and one long. The significance of the T-shaped acoustic resonator is its large aspect ratio, which makes it possible to be integrated into host structures to reduce the space requirement in implementation. When the resonator is introduced into a noisy enclosure, the re-radiation from its aperture, excited by the primary source, interacts with the existing sound to effectively attenuate the noise level inside the enclosure. A general model is developed, representing the acoustic interaction between the enclosure and multiple resonators, which is used to evaluate the acoustic performance of the resonators. Based on the understanding of the coupling between resonators and enclosure modes, both targeted and nontargeted, a sequential design method is proposed to determine the optimal location of the resonator. Numerical simulations are carried out to demonstrate the implementation procedures of noise control in several resonance peaks of a right parallepiped enclosure. Experiments are also conducted to validate the theory and the design method. The agreement between the theoretical and experimental results shows that, with the help of the presented theory, noise levels within a relatively broad frequency band in an enclosure can be successfully attenuated using optimally located acoustic resonators.

Commentary by Dr. Valentin Fuster
2008;():215-224. doi:10.1115/NCAD2008-73046.

In extension of previous methods to identify military impulse noise in the civilian environmental noise monitoring setting by means of a set of computed scalar metrics input to artificial neural network structures, Bayesian methods are investigated to classify the same dataset. Four interesting cases are identified and analyzed: A) Maximum accuracy achieve on training data, B) Maximum overall accuracy on blind testing data, C) Maximum accuracy on testing data with zero false positive detections, D) Maximum accuracy on testing data with zero false negative rejections. The first case is used to illustrative example and the later three represent actual monitoring modes. All of the cases are compared and contrasted to illuminate respective strengths and weaknesses. Overall accuracies of up to 99.8% are observed with no false negative rejections and accuracies of up to 98.4% are also achieved with no false positive detections.

Commentary by Dr. Valentin Fuster
2008;():225-231. doi:10.1115/NCAD2008-73062.

FIR filter for a adaptive filter algorithm, is mostly used for an active noise control system. However, FIR filter needs to have more large size of the filter length than it of IIR filter. Therefore, the control system using FIR adaptive filter has slow calculation time. In the active noise control system of the short duct, the reference signal can be affected by the output signal, so IIR filter for ARMA system can be more suitable for the active noise control of the short duct than FIR filter for MA system. In this paper, the recursive LMS filter, which is adaptive IIR filter, is applicated for the active noise control inside the short duct. For faster convergence and more accurate control, a variable step size algorithm is introduced for this recursive LMS filter (R-VSSLMS filter). Using this algorithm and considering the secondary path, the filtered-u R-VSSLMS is conducted successfully on the real experiment in the short duct. The performance of the active control using the filtered-u R-VSSLMS filter, is compared with the performance of the active control using a filtered-x LMS filter.

Commentary by Dr. Valentin Fuster
2008;():233-242. doi:10.1115/NCAD2008-73073.

Fuel economy, international competition and socioeconomic factors have forced manufacturers to develop lighter automotive vehicles. However, such vehicles are typically more susceptible to noise and vibration problems. The internal sound field in the passenger compartment is affected significantly by the acoustic modal characteristics of the cavity, by the dynamic behaviour of the surrounding structure, and by the nature of the coupling of these two dynamic systems. The purpose of this research is to develop and analyse a new vibroacoustic model containing the main compartment cavity and the luggage compartment cavity. Special attention is placed on the effect of a double walled partition between the main compartment cavity and the luggage compartment. The system is studied using ANSYS finite element (FE) code. The modelling involved shell finite elements for the structure and three-dimensional (3D) acoustic elements for the cavities. The 3D FE modal analysis produced results visualizing the complex picture of acoustic-structure coupling in the lower frequency range (30–200 Hz). It was found that strong coupling between the thin walled structure and the acoustic enclosures exists in the vicinity of any acoustic resonance. The key noise reduction principle examined is the passive application of a characteristic impedance mismatch. Using an FE model, numerical simulations are conducted to study the effect of various design parameters on acoustic transmission. The results show that at least 8 dB reductions in sound pressure level may be achieved with a modest level of vacuum in the double partition between the main cabin and the luggage compartment cavity.

Topics: Noise control
Commentary by Dr. Valentin Fuster
2008;():243-247. doi:10.1115/NCAD2008-73077.

Noise in large high voltage induction motors (500Hp–18000Hp) may be airborne or magnetic in nature. Usually, large high voltage induction motors are custom built and tailored to meet customer’s demand. Since every motor is unique in its design, it is imperative to predict accurately the magnetic noise generation during design phase, this way avoiding expensive rework cost and not loosing the customer confidence. Stator – rotor mechanical design, along with careful electrical coil design, can significantly cut down magnetic noise in an induction motor. This paper discusses the various causes and control of magnetic noise in large induction motors. Theoretical noise predictions in large induction motors, along with measured experimental noise data, are presented.

Commentary by Dr. Valentin Fuster
2008;():249-259. doi:10.1115/NCAD2008-73086.

In this paper we develop a data based model, design a high performance robust controller, and apply the controller in real-time to reduce narrowband acoustic noise from a cooling fan. A custom, portable enclosure houses the cooling fan. One end of the enclosure is fitted with four speakers and four microphones, inside of a short duct, connected to a data acquisition system and a personal computer. Passive materials mounted at the other end of the enclosure reduce backside noise. The frequency of the narrrowband noise is assumed unknown and therefore a control design that can be updated in realtime is needed. The control design that is presented uses a nominal model and a nominal controller. The nominal controller is enhanced by using closed loop signals and taking into account the modeling error. The end result is a data-based method for updating a nominal controller to improve performance.

Commentary by Dr. Valentin Fuster
2008;():261-269. doi:10.1115/NCAD2008-73088.

Sound transmission in hydraulic lines is of great importance in many engineering applications. Sound produced from hydraulic pumps may be radiated to the environment, and transmitted between components through flexible hoses, often modelled as shell-type structures. Noise in hydraulic lines filled with flowing fluid is generated through complex fluid-structure interactions. In this project, a conceptual muffler configuration consisting of a set of alternating shell segments was investigated. By varying parameters such as material properties and the hose dimensions, both fluid and structural waves in the hose are attenuated through the creation of stop bands at the operating frequency. In this paper, thick- and thin-shell theories were investigated. It was found that, for low frequencies or long wavelengths, consistent results were obtained from both theories. The transfer matrix method was used in conjunction with Floquet theory in the analysis of the periodic shell system. Preliminary results showed that numerous stop bands appear and substantial attenuation can be achieved. The first two natural frequencies of a shell with and without fluid loading were computed. Their values agree with similar results from other researchers. Finally, several parameters were varied to study their effects on the natural frequencies. These results will be used later in the design of the shell attenuator.

Commentary by Dr. Valentin Fuster

Structural Acoustics

2008;():271-275. doi:10.1115/NCAD2008-73009.

The Vincent Circle principle may be stated as follows. If a structure is excited harmonically, the response at another position at a particular frequency will trace a circle in the complex plane as a result of a dynamic stiffness modification between two points. As either the real or imaginary part of an introduced dynamic stiffness is varied from plus and minus infinity, the structural or acoustic response will map a circle in the complex plane. This paper summarizes the basis for this little known principle. Two numerical simulations are included to demonstrate how the principle can be applied. In the first example, a cantilevered plate is used to confirm that the principle is amenable to noise problems. A similar analysis is then performed on a construction cab to illustrate the applicability of the method if the structure is excited at multiple locations. The results suggest that the principle can be used in place of or in conjunction with more sophisticated numerical optimization schemes.

Topics: Noise (Sound)
Commentary by Dr. Valentin Fuster
2008;():277-280. doi:10.1115/NCAD2008-73015.

The inverse boundary element method (BEM) is a numerical procedure whereby sound pressure measurements in the near field are used to predict the vibration on the vibrating surface. After the vibration (or particle velocity for an opening) is determined, the sound pressure in the far field can be predicted using a forward BEM analysis. This paper will examine a particular example where the far field sound pressure was predicted for a generator set. The results indicate that the vibration predicted by the inverse BEM can be used to accurately predict the sound pressure as far away as 7 meters from the source.

Commentary by Dr. Valentin Fuster
2008;():281-294. doi:10.1115/NCAD2008-73017.

This paper examines numerical techniques to compute the resonant frequencies and coupled mode-shapes of general structural-acoustic systems using the finite element method. This information is useful in quantifying the key frequencies and vibration patterns of elastic structures coupled to bounded or unbounded acoustic fluid volumes. This paper reviews and evaluates three finite element solution techniques that deal with the computational difficulties encountered in structural-acoustic eigen-analysis. One of the difficulties stems from the fluid-structure coupling in the finite element equations, which depending on the variable used to discretize the acoustic fluid, either introduces non-symmetric area coupling terms in the mass and stiffness matrices or adds symmetric area coupling terms in the damping matrix. The other difficulty is related to the application of a radiation boundary condition in those structural-acoustic problems involving unbounded acoustic domains. The radiation boundary condition introduces damping terms in the finite element equations that lead to a complex eigen-analysis to determine the resonant frequencies and coupled mode-shapes. The finite element techniques evaluated in this paper consist of subspace projection employing an augmented-symmetric form of the fluid-structure equations as well as new extensions of component mode synthesis (CMS). Basic examples of simplified structural-acoustic systems are used to compare the solution accuracy between the three finite element techniques. Examples consist of simplified one-dimensional (1-D) and two-dimensional (2-D) elastic structures coupled to closed and open acoustic spaces.

Commentary by Dr. Valentin Fuster
2008;():295-300. doi:10.1115/NCAD2008-73026.

An equivalent-acoustic finite element method is developed for modeling sound absorbing materials, such as seats and interior trim in an automobile passenger compartment. The equivalent-acoustic method represents the sound absorbing material using acoustic finite elements with frequency-dependent material properties determined from the measured acoustic impedance of sound absorbing material samples. Solution of the equivalent-acoustic model within the Nastran computer capability, and coupling of the model with an acoustic finite element model of an enclosure, such as the passenger compartment, are developed. The accuracy of the equivalent-acoustic method is assessed for modeling a sound absorbing material in a one-dimensional impedance tube, a foam layer in a rectangular box enclosure, and the seats in an automobile passenger compartment.

Commentary by Dr. Valentin Fuster
2008;():301-309. doi:10.1115/NCAD2008-73028.

Current modeling of the static and dynamic characteristics of fluid film bearings typically employs a single impedance matrix to represent the force transfer between a bearing and journal centerlines. A numerical method has been proposed that distributes the bearing impedances around the circumference of the fluid film to allow for more accurate modeling of higher order circumferential modes. In order for this method to be used with confidence, its results must first be validated. For this purpose, an experimental test method and apparatus capable of measuring these distributed bearing impedances has been developed. This paper will present the preliminary bearing displacement and pressure measurements collected from the journal bearing test apparatus and will compare these experimental results to those calculated numerically. Discrepancies between the data sets will be discussed and future steps will be outlined.

Commentary by Dr. Valentin Fuster
2008;():311-318. doi:10.1115/NCAD2008-73030.

An approach dealing with the responses of and acoustic radiation from temporally and spatially stochastic shell structures to non-Gaussian random excitations is presented in this paper. It employs the stochastic central difference method developed earlier by the author and his associate. The emphasis of the presentation is, however, on the responses of stochastic shell structures with large spatial variations and under non-Gaussian nonstationary random excitations. The shell structures are discretized by the mixed formulation, lower order triangular shell finite elements developed by the author and his associate in 1994. As a demonstration of the relative ease of use of the approach, computed results for a temporally and spatially stochastic, clamped spherical cap subjected to a central point force treated as a non-Gaussian nonstationary random excitation are included. It is concluded that a simple and relatively very efficient approach is available for analysis of responses of temporally and spatially stochastic shell structures perturbed by non-Gaussian nonstationary random excitations.

Commentary by Dr. Valentin Fuster
2008;():319-329. doi:10.1115/NCAD2008-73048.

Coupling loss factor is one of the most important parameters used in the statistical energy analysis (SEA). It regulates the energy flows between two subsystems. Theoretically, the coupling loss factors are usually calculated from the wave transmission coefficients derived using two semi-infinite systems. While the final system equation in the SEA is based on the powerful energy conservation principle, in the process the coupling loss factors are assumed to be unaffected by the configuration changes. In other words, the coupling loss factors which are experimentally or analytically determined under some ideal conditions are considered to be unchanged by the more complicated coupling conditions under a system configuration. The validity of this treatment needs to be carefully studied because there is a belief that the SEA method can be readily extended to lower frequencies so long as the coupling loss factors can be somehow satisfactorily determined. In this study, wave transmission coefficients are obtained by calculating the energy flows for a three-beam system coupled together in T-shape. Separately, the wave transmission coefficients are estimated by using energy flows calculated for each possible pairing of two beams. By comparing the results from these two different approaches, some insightful information has been obtained regarding the characteristics of the coupling loss factors in determining power flows in structures.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2008;():331-334. doi:10.1115/NCAD2008-73051.

A new algorithm for mode-based frequency response analysis, which takes into account frequency-dependent material properties, is proposed. First, the projection subspace is determined by computing the eigenmodes of the system. If the AMLS-type eigensolver is used and the frequency-dependent material is confined to a limited area (often less than 1% of the whole model), eigenmodes are computed only in the region with the frequency-dependent material. Next, during the frequency response analysis portions (corresponding to the frequency-dependent material) of the stiffness, viscous damping, and structural damping operators are computed and projected onto the modal subspace. The original contribution of this paper is the algorithm, which augments the projected operators (stiffness, viscous damping, or structural damping) by the contributions from the area with the frequency-dependent material properties without the need to recompute the operator over the whole domain. This algorithm was successfully implemented in a commercial finite element code, Abaqus 6.8. The results for a vehicle body-in-prime model show good agreement with a direct-solution frequency response analysis. In the addition, the cost of the proposed algorithm is a fraction of the directsolution analysis.

Commentary by Dr. Valentin Fuster
2008;():335-338. doi:10.1115/NCAD2008-73053.

This paper proposed an approach to construct the acoustic cloak by a network of subwavelength Helmholtz resonators. Based on transmission line model to describe the acoustic wave propagation inside such effective anisotropic medium, we derived the acoustic parameters such as effective density and compressibility. Our simulation demonstrates the propagation of acoustic waves can be bent and excluded from an object inside the cloak with no perturbation of exterior field, which may have great potential application in ultrasound noise control.

Commentary by Dr. Valentin Fuster
2008;():339-347. doi:10.1115/NCAD2008-73057.

Enclosures with diffuse reflection boundaries are modeled with an energy-intensity boundary element method using uncorrelated broadband directional sources. Input power is assumed to enter the enclosure through the walls, which are also absorptive. An absorption-based perturbation analysis is used to analyze the spatial variation of the acoustic field, which is shown to obey certain scaling laws. A series expansion in terms of spatial-average absorption gives separate boundary integral problems at each order. For the primary spatial variation, the effects of the relative distributions of absorption and input power are linear and uncoupled. These distributions can be expressed in terms of constituent spatial modes corresponding to the ways absorption and input power can be distributed. These amplitudes can be adjusted to tailor the spatial variation subject to system constraints. Examples include how to distribute absorption to minimize sound levels in one location, or how to achieve a uniform interior field.

Commentary by Dr. Valentin Fuster
2008;():349-356. doi:10.1115/NCAD2008-73059.

The circular cylinder pipe is extensively used for the supplement of the gas. The leakage of this gas induces the catastrophic problem when it leases into open area in the city without any monitoring. A correlation method has been mostly used for the detection of the leakage. It is needed a good coherence and an efficient energy transmission to the external sensors for the reliable estimation of the correlation. This paper investigated theoretically the propagation of the acoustic wave of the circular cylinder pipe containing the gas in a pipe for the development of the leakage monitoring system. The acoustic wave is propagated through the waveguide of the circular pipe with the characteristics acoustically coupled by the gas contained in a cylinder and the shell. However, as a special case, the acoustic waves in a metal pipe containing gas are corresponded closely to the uncoupled in-vacuo shell waves and to the rigid-wall duct fluid waves. In this case, the dominant acoustic energy can be estimated at the frequencies in which coincidence between the shell modes and the acoustic modes occurs. In the paper, the characteristics of the dominant waves are theoretically investigated and analyzed experimenttally with a long steel pipe. The measured data is clearly analyzed by the continuous wavelet transform and by spectral density analysis.

Commentary by Dr. Valentin Fuster
2008;():357-361. doi:10.1115/NCAD2008-73063.

In the high frequency limit, a vibrating panel subject to spatially-random temporally-broadband forcing is shown to have broadband power and directivity properties that can be expressed in simple analytical terms by a limited set of parameters. A lightly-loaded fixed-fixed membrane with a distribution of broadband uncorrelated drive points is analyzed. The theory is developed using classical modal methods and asymptotic modal analysis, assuming small damping. The power and directivity of the radiated pressure field are characterized in terms of structural wave Mach number, damping ratio, and dimensionless frequency. The relatively simple directivity pattern that emerges can be shown to arise from edge radiation. From the point of view of edge radiation, assuming a lightly damped reverberant structure, the same radiation formula and directivity pattern can be derived in a much simpler manner. Broadband radiation from structures with subsonic and supersonic flexural wave speeds is discussed and characterized in terms of a simple interpretation of the surface wavenumber spatial transform. The results show that the physical idea of interpreting edge radiation in terms of uncancelled volumetric sources is not correct, and the effect of higher order edge singularities is in fact very significant. The approach implies a relationship between radiation and structural power flow that is potentially useful in energy-intensity based prediction methods, and can be generalized to more complex structures with application to vehicle interior noise prediction.

Commentary by Dr. Valentin Fuster
2008;():363-371. doi:10.1115/NCAD2008-73066.

A theory based on cross-sectional averaging is developed to analyze quasi-one-dimensional acoustic propagation in hybrid ducts with two propagation media in the cross-section. Specifically, ducts lined with a thick layer of porous material are considered. The porous material makes the duct wavenumber complex, changing the phase speed and introducing attenuation. To lowest order, the wavenumber depends only on the ratio of cross-sectional areas and the properties of the constituent media, and surprisingly not on the material configuration in the cross-section. High frequency accuracy can be improved by using a small correction that includes shape coefficients that depend on the cross-sectional configurations. If the propagation wavenumber is measured experimentally in a hybrid duct, the complex effective sound speed and density, fundamental porous material properties, can be extracted relatively easily. Experimentally, open cell foam samples line the sides of a tube closed at one end, and the complex wavenumber is determined from standing wave measurements. The cross-sectional averaging theory is then used to determine the acoustic properties of the open-cell foam. Results are compared for various lining configurations to assess the accuracy of the method. Another application of this work is the theoretical and experimental study of the propagation of quasi one-dimensional acoustic waves through a duct with spatially periodic area changes. This configuration exhibits stop-band and pass-band behavior, with substantially reduced sound transmission in stop bands, but little effect in pass bands. The regions of the duct with larger cross-sectional area are partially filled with an annular region of porous material to provide pass-band attenuation, leaving a constant area passage for airflow. Predictions and measurements for hybrid ducts with periodic area changes are presented. A muffler designed to place engine harmonics in targeted stop-bands is described.

Commentary by Dr. Valentin Fuster
2008;():373-382. doi:10.1115/NCAD2008-73069.

Sound transmission through multi-element flexible barriers is studied. Configurations analyzed include panels with different thickness and elastic modulus in different regions and layered structures connected by an elastic suspension. The purpose of the research is to see if flexibility and controlled resonant behavior can be used to block sound transmission even when structural damping is very low. Strategies are considered to alter vibrating surface wavenumber spectra to reduce coupling between the structure and the acoustic field. Another approach that can be employed is the utilization of structural wave cut-off with multi-element materials. Finally, multiple differentially tuned subsidiary elements acting as resonators can be used to greatly reduce the structural response. Examples of acoustic transmission loss through panel barriers using different strategies are presented, and the potential advantages and possible shortcomings of various approaches are evaluated.

Commentary by Dr. Valentin Fuster
2008;():383-388. doi:10.1115/NCAD2008-73070.

The ubiquity of porous materials in engineering applications has driven a large body of work in the development of predictive and analytical models for their behavior, as well as the numerical implementation of these models. Here, the implementation of a specific class of models is described: materials for which an equivalent-fluid adequately captures the dynamic behavior. These materials include limiting cases where the solid matrix is either so stiff that it is relatively immobile, or so compliant that its motion has only a damping effect on the fluid motion. A wide class of automotive trim materials, acoustic insulation, fabrics, and aerospace materials fit this description. Several material models have been implemented recently in the commercial finite element code, Abaqus. These include the models of Craggs, Delany-Bazley, Miki, and the generalized model of Kang & Bolton. All of the models share an implementation using frequency-dependent material properties. In Abaqus, these properties are assigned to standard acoustic finite elements. Frequency-domain solution is significantly more efficient through the use of a distributed-memory parallel sparse solver, and through projection onto the space of real-valued modes. Results from the new implementation are compared to established benchmarks, and performance is discussed.

Commentary by Dr. Valentin Fuster
2008;():389-396. doi:10.1115/NCAD2008-73080.

A New finite element sandwich plate is presented. It is based on discrete displacement approach and allows for both symmetrical and unsymmetrical configurations. The validity and accuracy of the presented element is assessed through comparisons with both tests and classical FE modeling. The tests consist of various configurations of sandwich panels in a coupled plate-cavity system. A parametric study, using the developed element, is finally presented to highlights the effects of skin and core properties on the vibration and radiation of such structures under both airborne and structure-borne excitations.

Commentary by Dr. Valentin Fuster
2008;():397-413. doi:10.1115/NCAD2008-73089.

The normal surface velocities of highly a non-spherical object are reconstructed based on the measurement of field acoustic pressures using Helmholtz equation least-squares (HELS) method. The objectives of this study are to numerically examine the feasibility and accuracy of reconstruction and the impacts of various parameters involved in reconstruction of vibro-acoustic responses using HELS. The vibrating object is a simply-supported and baffled thin plate. The reasons for selecting this object are that plate is the most challenging source geometry for HELS method, and it represents a class of structures that cannot be exactly described by the spherical Hankel functions and spherical harmonics, which are primarily embedded in the HELS formulation, yet the analytic solutions to vibro-acoustic responses of a baffled plate are readily available so the accuracy of reconstruction can be checked in detail. The Rayleigh integral is used to generate the input field acoustic pressures for reconstruction. The Euler’s equation is employed to establish the system model of reconstruction of vector velocities. Regularization associated with the truncated singular value decomposition is utilized to compromise the resultant accuracy and stability of the vector velocity reconstruction. The reconstructed normal surface velocities are validated against the benchmark values, and the out-of-plane vibration patterns at several natural frequencies are compared with the natural modes of a simply-supported plate. The impacts of various parameters such as the measurement points, measurement distance, the location of origin of coordinate system, microphone spacing, and ratio of measurement aperture size to the area of source surface of reconstruction on the resultant accuracy of reconstruction are examined.

Topics: Equations
Commentary by Dr. Valentin Fuster
2008;():415-421. doi:10.1115/NCAD2008-73090.

Noise is one of major annoyances in modern life. Studies have shown that the most pervasive sources of noise in our environment today are those associated with transportation, among which highway traffic noise is a dominant one. Traditional highway noise barriers are solid obstructions built along the sides of the highway, at an average cost of 1 million dollars per mile. In this paper, a new design was proposed, based on the phononic band gap phenomenon. The new design uses either a hollowed wall structure or an array of discrete columns in place of a solid wall. It is anticipated that such new designs will provide more effective noise shielding at the target frequency range, and yet reduces the structural requirement for the foundation support of the barrier wall. Preliminary simulation results are presented for a number of different configurations of the wall designs, and their relative advantages and shortcoming are compared.

Commentary by Dr. Valentin Fuster

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