Deyu Li

On the design of long T-shaped acoustic resonators

In this work we present a more general mathematical model for the calculation of resonance frequencies for long, T-shaped acoustic resonators. The method is based upon wave propagation and, unlike previous theories, no constraints on the geometry of the resonator are imposed. In addition, a new end-correction model based upon Rayleigh’s end corrections is proposed and evaluated. The theory is used to develop a plane-wave multimodal-based design theory, which permits higher-order 1-dimensional modes of the T-shaped acoustic absorber to be used for absorbing high-frequency noise within enclosures. A series of experiments are conducted on round and square cross section resonators to validate the theory, evaluate the end correction models, and demonstrate design examples.

Investigation of the Sound Transmission into an Advanced Grid-Stiffened Structure

The noise transmission behavior of an advanced grid-stiffened (AGS) composite structure has been investigated by combining numerical and experimental methods. Structural-acoustic coupling was found to be light, permitting separate analysis of the structure and acoustic cavity. Finite element analysis permitted the resonant frequencies of acoustic cavity and structure to be calculated, which play an important role for noise transmission through the structure. Acoustic mode shapes permitted internal coincidence frequencies to be estimated and provided insight into modal pressure distributions, when considering payload location. Experimental structural and acoustic modal analysis permitted the resonant frequencies and damping ratios for the structure and cavity to be determined, which in turn were used to corroborate the FEA model. Finally, direct measurement of the noise transmission was performed based on noise reduction spectrum (NRS), which is calculated front spatial averages of the RMS acoustic pressures inside and outside of the shell. It was found theft the NRS was dominated by acoustic resonances, which were marked by sharp dips in the NRS curve. Internal coincidence of the axial wavenumbers was also found to be a significant mechanism for noise transmission. External coincidence and ring frequencies were found to provide less of an impact on the overall NRS for the structure.

Effect of internal resistance of a Helmholtz resonator on acoustic energy reduction in enclosures.

The effect of internal resistance of a Helmholtz resonator on acoustic energy reduction in an enclosure and the multimodal coupling-based Helmholtz resonator design are investigated. Using the analytical solution of a resonator-enclosure interaction model, an energy reduction index is defined in a frequency band to optimize the resonator resistance. The dual process of energy dissipation and radiation of the resonator is quantified. Optimal resistance of the resonator and its physical effect on the resonator-enclosure interaction are numerically evaluated and categorized in terms of frequency bandwidths. Predictions on the resonator performance are confirmed by experiments. Comparisons with existing models based on different optimization criteria are also performed. It is shown that the proposed model serves as an effective design tool to determine the internal resistance of the resonator in order to achieve sound reduction in the frequency band enclosing acoustic resonances.

Noise control in enclosures: Modeling and experiments with T-shaped acoustic resonators

This paper presents a theoretical and experimental study of noise control in enclosures using a T-shaped acoustic resonator array. A general model with multiple resonators is developed to predict the acoustic performance of small resonators placed in an acoustic enclosure. Analytical solutions for the sound pressure inside the enclosure and the volume velocity source strength out of the resonator aperture are derived when a single resonator is installed, which provides insight into the physics of acoustic interaction between the enclosure and the resonator. Based on the understanding of the coupling between the individual resonators and enclosure modes, both targeted and nontargeted, a sequential design methodology is proposed for noise control in the enclosure using an array of acoustic resonators. Design examples are given to illustrate the control performance at a specific or at several resonance peaks within a frequency band of interest. Experiments are conducted to systematically validate the theory and the design method. The agreement between the theoretical and experimental results shows that, with the help of the presented theory and design methodology, either single or multiple resonance peaks of the enclosure can be successfully controlled using an optimally located acoustic resonator array.

Noise Transmission Control Studies on a Chamber Core Composite Cylinder

The vibroacoustic behavior and sound transmission properties of a mock-scale chamber core composite cylinder were studied, and the feasibility of the active structural acoustic control and passive control was also investigated. A box-beam model of the chamber core cylindrical shell was used for calculating the critical frequency and the ring frequency. The coupling problems between structural and acoustic modes were investigated, and the structural and acoustic modal parameters were identified from measured data. The sound transmission into the chamber core cylindrical structure was measured with and without fill materials in its wall chambers. The structural stiffness-, cavity resonance-, coincidence-, and mass-controlled zones were identified and verified.Copyright

A three-dimensional model for T-shaped acoustic resonators with sound absorption materials

Recent development in noise control using T-shaped acoustic resonators calls for the development of more reliable and accurate models to predict their acoustic characteristics, which is unfortunately lacking in the literature. This paper attempts to establish such a model based on three-dimensional theory for T-shaped acoustic resonators containing sound absorption materials. The model is validated by experiments using various configurations. Predictions on fundamental and high-order resonance frequencies are compared with those obtained from the one-dimensional model and finite element analyses, and the effects of the physical and geometric parameters of the absorption materials on the resonance frequencies and Q-factor are also investigated numerically and experimentally. Limitations and applicability of existing one-dimensional models are assessed. The proposed general three-dimensional model proved to be able to provide an accurate and reliable prediction on the resonance frequencies for T-shaped acoustic resonators with or without absorption materials. This can eventually meet the requirement for resonator array design in terms of accuracy.

Design and resonant frequency prediction for long T‐shaped acoustic resonators

The use of acoustic resonators is an effective way to control cavity resonances in small enclosures. One popular device is the long, T‐shaped acoustic resonator which consists of three branches. Two branches (referred to as ‘‘Branch‐1’’ and ‘‘Branch‐2’’) are co‐axial and both have one open end and one closed end, and the third branch (referred to as ‘‘Branch‐3’’) is perpendicular to the co‐axis and has two open ends. In practical cavity noise control, the optimal position of Branch‐3, i.e., the length of Branch‐1 or Branch‐2 is determined by the mode shape of the controlled cavity mode, and the length of Branch‐3 is typically chosen to be as short as possible to minimize the occupied space of the resonator. If the cross‐sectional areas are given, the only design parameter is the length of Branch‐1 or Branch‐2. In this study, three new models are developed to calculate the end corrections for the three branches. The novel theory is also used to design long T‐shaped acoustic resonators for control. In addit...

Noise Control of a Chamber Core Cylinder Using Cylindrical Helmholtz Resonators

Previous investigations have determined that the noise transmission into a finite cylindrical structure at low frequencies is dominated by the cavity resonances. Therefore, noise control at the first several cavity resonances for a Chamber Core cylinder can significantly reduce the noise level at low frequencies inside the cylinder. This work explores the feasibility of noise control for the Chamber Core cylinder using cylindrical Helmholtz resonators. The targeted frequencies are the first four cavity resonances. Detailed considerations of the resonant frequency calculation, resonator design, and experimental verification are presented. The effects on the noise reduction spectrum of two closely spaced resonators are experimentally studied. The optimal position of the resonators is also discussed. The noise control results indicate that the Helmholtz resonators can significantly attenuate the noise level at the targeted frequency bands.Copyright

Investigation of the sound transmission behavior of a chamber core cylinder

Several kinds of novel composite structures, such as advanced grid stiffened (AGS) and chamber core (CC) structures have been designed, fabricated, and investigated for both civil and military applications. The chamber core composite is a novel advanced sandwich‐type structure that is created by filament winding an inner shell onto a cylindrical mandrel, arranging previously fabricated U‐shaped channels around the perimeter of this shell to form the inner chamber walls, and filament winding an outer shell followed by a co‐cure process. In this study, the structural/acoustic behavior of a normal composite chamber core cylinder is investigated both theoretically and experimentally. Lightly coupled structural and acoustic modal parameters are identified using experimental modal analysis techniques. The properties of sound transmission loss (TL) of the cylinder are also investigated experimentally. The effect of the structural/acoustic natural frequencies and the damping on the sound transmission loss is anal...

Theoretical investigation of noise transmission into a finite cylinder

A new mathematical model for characterizing noise transmission into a finite elastic cylindrical structure with application to a ChamberCore composite cylinder is presented. A plane wave obliquely impinges on the structure, the external sound field is approximated by the solution for an infinite cylinder, and the internal sound field is solved with the structural and acoustic modal interaction method. The noise reduction spectrum for characterizing noise transmission into the cylinder is defined, and the analytical model for the calculation of the noise reduction spectrum is developed. The analytical results show that the cavity resonances dominate the noise transmission into the finite cylinder, and the longitudinal acoustic modes play an important role in the noise transmission at the low frequencies. These results are matched with experimental results.