Low-frequency Acoustic Resonances Research Articles

Cylindrical Acoustic Gas Thermometry

We review recent determinations of the Boltzmann constant kB and the differences T − T90 that used cylindrical acoustic gas thermometry (c-AGT). These determinations measured the acoustic resonance frequencies of argon gas enclosed by metal-walled, cylindrical cavities. (Here, T is the thermodynamic temperature and T90 is the temperature measured on the International Temperature Scale of 1990, ITS-90.) In the range 234–303 K, the standard uncertainty of c-AGT ranges from 1.9 × 10−6T to 2.6 × 10−6T. This uncertainty is much smaller than the errors in ITS-90; therefore, c-AGT can help improve ITS-90. Moreover, we are extending c-AGT up to 1358 K. With increasing temperatures, c-AGT becomes advantageous relative to AGT based on quasi-spherical cavities because long cylindrical cavities (1) naturally fit into cylindrical heat pipes or multi-shelled thermostats; (2) provide the immersion required by transfer temperature standards, such as long-stemmed platinum resistance thermometers; and (3) have more useful, low-frequency acoustic resonances. In preparation for high-temperature c-AGT, we identified suitable materials for fabricating cylindrical cavities and we developed techniques for measuring acoustic resonance frequencies using sources and detectors outside the high-temperature thermostat. We also considered alternative test gases and optimal dimensions of cavities.

Suppression of low-frequency acoustic resonances in integrated optic lithium niobate modulators

The influence of low-frequency acoustic vibrations on optical signals in integrated optic lithium niobate modulators is considered. It is shown that low-frequency acoustic vibrations can significantly contribute to the modulator transfer function. Characteristics of the acoustic vibrations have been observed. It is shown that these vibrations correspond to excitation of standing Lamb waves. Methods for acoustic resonance suppression have been suggested and verified.

State Estimation Using Dependent Evidence Fusion: Application to Acoustic Resonance-Based Liquid Level Measurement.

Estimating the state of a dynamic system via noisy sensor measurement is a common problem in sensor methods and applications. Most state estimation methods assume that measurement noise and state perturbations can be modeled as random variables with known statistical properties. However in some practical applications, engineers can only get the range of noises, instead of the precise statistical distributions. Hence, in the framework of Dempster-Shafer (DS) evidence theory, a novel state estimatation method by fusing dependent evidence generated from state equation, observation equation and the actual observations of the system states considering bounded noises is presented. It can be iteratively implemented to provide state estimation values calculated from fusion results at every time step. Finally, the proposed method is applied to a low-frequency acoustic resonance level gauge to obtain high-accuracy measurement results.

The Effectiveness of the Suppression of Low Frequency Acoustic Resonances with Porous Sound Absorbing Structures of Multifunctional Upholstery Materials of Car Body Interior

The article presents the results of researches on the effectiveness of the technical method of suppression of low frequency acoustic resonances occurring in the air volumes of passenger and luggage areas of the car body by increasing the absorption degree of sound wave energy by multifunctional panel and volumetric elements of a car body interior. The research reviews a sliding shutter and integral carpeting of a luggage compartment, rear wheel arch trim, backrest upholstery and car door trim as the modified multi-functional elements of the car body interior. The results of experimental researches show that putting the modified multifunctional car body interior upholstery, characterized by high sound absorbing properties, into the studied prototype of a car allows to reach the effect of reduction the overall sound levels up to 9 dBA on the mode of intensive acceleration movement and sound pressure levels, which are registered at a frequency of the second motor harmonic of the engine 2n / 60 - 16 dB.

Large amplitude free vibration of a flexible panel coupled with a leaking cavity

This paper addresses the effect of leakage on the natural frequencies of a large amplitude vibrating panel backed by a cavity, which has not been considered in many other related studies. The structural-acoustic governing equations are employed to study this nonlinear problem. An elliptical integral method, which was recently developed for the nonlinear panel cavity problem, is introduced here to solve for the structural-acoustics responses. The present results agree reasonably well with those obtained from the classical harmonic balance method. Modal convergences of the nonlinear solutions are performed to verify the proposed method. The effects of vibration amplitude and leakage size are studied and discussed. It is found that (1) the edge leakages in a panel cavity system significantly affect the natural frequency properties, and (2) the edge leakages induce a low frequency acoustic resonance.

Low frequency acoustic resonances in urban courtyards

Urban courtyards can be regarded as open cavities in the urban area, in which resonances can be excited by waves generated in the neighboring streets. The aim of the present work is to experimentally and numerically investigate low frequency resonance phenomena in these configurations. Experiments are carried out in a scale model and a numerical study is performed with a coupled modal-finite elements method. The method enables the three-dimensional modeling of the acoustic field and thus to take into account the interactions between the courtyard and the street canyon that occur above the roof level, a particular characteristic of wave propagation in urban areas. The attention is focused on two aspects, the amplification of the sound level inside the courtyard and the acoustic attenuation in the street due to resonances. Experimental and numerical results are in good agreement and show a strong resonant behavior of these configurations.

Fairing Noise Control Using Tube-Shaped Resonators

Abstract : The potential for noise mitigation in composite Chamber Core fairings is investigated by using the walls of the fairing structure itself as acoustic resonators. This is the first documented application of long cylindrical tube-shaped resonators for fairing noise control. The theory and modeling of tube-shaped resonators for controlling fairing acoustic resonances is presented. The potential for noise mitigation in composite Chamber Core fairing using the walls of the fairing structure itself as acoustic resonators is investigated. Design criteria such as geometry damping, spatial coupling, and robustness are considered for a variety of tube resonators. The results showed that a small number of tube resonators reduced the amplitude of low-frequency acoustic resonances by 10-12 dB in the test system and provided nearly 6 dB of reduction over the bandwidth from 0 to 400 Hz.

Acoustic rhinometry measurements in stepped-tube models of the nasal cavity

Stepped-tube models with a constriction in the anterior section were used to evaluate the effects that nasal valve passage area and nasal cavity shape have on acoustic rhinometry (AR) measurements. The AR-determined cross-sectional areas beyond a constriction of small passage area were consistently underestimated, and the corresponding area–distance curves showed pronounced oscillations. Also, the AR technique did not accurately reproduce abrupt changes in passage area. The results suggest that, regardless of the particular shape of the nasal cavity model, AR does not provide reliable information about cross-sectional areas posterior to a severe constriction. The experimental results are discussed in terms of theoretically calculated acoustic input impedance for the models studied, the physical limitations of AR, and assumptions made in AR algorithms. The study demonstrated that energy losses and sound wave attenuation due to air viscosity do not significantly affect AR measurements. It was also shown that passage area beyond a severe constriction is underestimated because the barrier created by the constriction reflects most of the incident sound power. The results also indicate that the oscillations in area–distance curves are due to low-frequency acoustic resonances in the nasal cavity model.

Effects of the nasal valve on acoustic rhinometry measurements: a model study.

The influence of nasal valve on acoustic rhinometry (AR) measurements was evaluated by using simple nasal cavity models. Each model consisted of a cylindrical pipe with an insert simulating the nasal valve. The AR-determined cross-sectional areas beyond the insert were consistently underestimated, and the corresponding area-distance curves showed pronounced oscillations. The area underestimation was more pronounced in models with inserts of small passage area. The experimental results are discussed in terms of theoretically calculated "sound-power reflection coefficients" for the pipe models. The reason for area underestimation is reflection of most of the incident sound power from the barrier at the front junction between the pipe and the insert. It was also demonstrated that the oscillations are due to low-frequency acoustic resonances in the portion of the pipe beyond the insert. The results suggest that AR does not provide reliable information about the cross-sectional areas of the nasal cavity posterior to a significant constriction, such as pathologies narrowing the nasal valve area. When the passage area of the nasal valve is decreased, the role of AR as a diagnostic tool for the entire nasal cavity becomes limited.

Effects of paranasal sinus ostia and volume on acoustic rhinometry measurements: a model study.

We used pipe models to investigate the effects of paranasal sinus ostium size and paranasal sinus volume on the area-distance curves derived by acoustic rhinometry (AR). Each model had a Helmholtz resonator or a short neck as a side branch that simulated the paranasal sinus and sinus ostium. The AR-derived cross-sectional areas posterior to the ostium were significantly overestimated. Sinus volume affected the AR measurements only when the sinus was connected via a relatively large ostium. The experimental area-distance curve posterior to the side branch showed pronounced oscillations in association with low-frequency acoustic resonances in this distal part of the pipe. The experimental results are discussed in terms of theoretically calculated "sound-power reflection coefficients" for the pipe models used. The results indicate that the effects of paranasal sinuses and low-frequency acoustic resonances in the posterior part of the nasal cavity are not accounted for in the current AR algorithms. AR does not provide reliable information about sinus ostium size, sinus volume, or cross-sectional area in the distal parts of nasal cavity.

Excitation of Thermoacoustic Instabilities by Interaction of Acoustics and Unstable Swirling Flow

Unstablethermoacousticmodeswereinvestigatedandcontrolledinanexperimentallow-emissionswirlstabilized combustor, in which theacousticboundary conditions weremodie ed to obtain combustion instability. Theacoustic boundary conditions of the exhaust system could be adjusted from almost anechoic (ree ection coefe cient jj rjj < 0:2) to open-end ree ection. Several axisymmetric and helical unstable modes were identie ed for fully premixed and diffusion-typecombustion. Theseunstablemodeswereassociated with e ow instabilities related to therecirculation wake-like region on the combustor axis and shear-layer instabilities at the sudden expansion (dump plane). The combustion structure associated with the different unstable modes was visualized by phase-locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, whereas the helical modes showed variations in the radial location of maximal heat release. The axisymmetric mode was the dominant one during unstable combustion. It was obtained by forcing a longitudinal low-frequency acoustic resonance. Helical modes could only be obtained when the axisymmetric mode was suppressed by using a nonree ectingboundarycondition.Aclosed-loopactivecontrolsystemwasemployedtosuppressthethermoacoustic pressure oscillations and to reduce NO x and CO emissions. Microphones were used to monitor the pressure oscillations during thecombustion process and provide input to thecontrol system. An acousticactuation was used to modulate the aire ow and thus affected the mixing process and the combustion. Upstream excitation modie ed the shear-layer structure and was shown to be superior to downstream excitation, which combined less effective shear-layer excitation with noise cancellation. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant 24% reduction of NO x emissions were obtained in premixed combustion using an acoustic power of less than 0.002% of the combustion power. The control of the diffusion e ame was less effective, and NO x emissions increased at the phase that was most effective in suppressing the pressure oscillations. The differences between the behavior of the control system in the two combustion modes was caused by different levels of interaction between the combustion process and the shear layer.

5723049 Treatment of a formaldehyde-containing waste stream

The article presents the results of researches on the effectiveness of the technical method of suppression of low frequency acoustic resonances occurring in the air volumes of passenger and luggage areas of the car body by increasing the absorption degree of sound wave energy by multifunctional panel and volumetric elements of a car body interior. The research reviews a sliding shutter and integral carpeting of a luggage compartment, rear wheel arch trim, backrest upholstery and car door trim as the modified multi-functional elements of the car body interior. The results of experimental researches show that putting the modified multifunctional car body interior upholstery, characterized by high sound absorbing properties, into the studied prototype of a car allows to reach the effect of reduction the overall sound levels up to 9 dBA on the mode of intensive acceleration movement and sound pressure levels, which are registered at a frequency of the second motor harmonic of the engine 2n / 60 - 16 dB.

Low-frequency acoustic resonance level gauge

A new level-measurement method is presented. It is based on the acoustic resonance of a waveguide. This method can be successfully implemented for fluid-level measurements in industrial plants, where problems of residues, deposits, foams, etc., can be expected. An experimental gauge has been built and tested under artificially induced residues, foams, deposits, etc. The resonance gauge proves to be insensitive under extreme operating conditions. The proposed method represents an alternative to ultrasonic as well as to some other level-measurement methods.

Low-frequency acoustic resonance level detector with neural-network classification

This work presents a new level-detection method. It is based on the acoustic resonance of a waveguide. This method can be successfully implemented for fluid-level detection in industrial plants, where problems of residues, deposits, foams, etc., can be expected. The performance of the detector is additionally improved by the use of neural networks. Neural networks enable the detector to learn from the reference detection cycles; therefore, such a system is flexible and can be easily optimized (trained) for specific operating conditions. The detector has been tested in a highly viscous medium, Styrofoam granulate and under artificially induced residues and deposits in the resonance waveguide.

Helium gas purity monitor based on low frequency acoustic resonance

Monitoring gas purity is an important aspect of gas recovery stations where air is usually one of the major impurities. Purity monitors of Katherometric type are commercially available for this purpose. Alternatively, we discuss here a helium gas purity monitor based on acoustic resonance of a cavity at audio frequencies. It measures the purity by monitoring the resonant frequency of a cylindrical cavity filled with the gas under test and excited by conventional telephone transducers fixed at the ends. The use of the latter simplifies the design considerably. The paper discusses the details of the resonant cavity and the electronic circuit along with temperature compensation. The unit has been calibrated with helium gas of known purities. The unit has a response time of the order of 10 minutes and measures the gas purity to an accuracy of 0.02%. The unit has been installed in our helium recovery system and is found to perform satisfactorily.

Active low frequency acoustic resonance suppressor

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