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Item type:Publication, Access status: without files , Helmholtz resonator with two resonance frequencies by coupling with a mechanical resonator(2023-09-01); ; ; This contribution presents the acoustical properties and corresponding theoretical descriptions of a Helmholtz resonator with an integrated cantilever inside the Helmholtz resonator neck. The advantage of this resonator design is a second resonance frequency, which leads to a supplemental frequency region with high sound absorption properties. An analytical model is derived to calculate the resonance frequencies and the absorption coefficient of the resonator system. This model has been verified with finite element simulations for the resonance frequencies, taking into account several geometric parameter variations. It has been validated experimentally for the calculated absorption coefficients by impedance tube measurements. In conclusion, it has been found a high influence of the geometric parameters on the resonance frequencies and a broader sound absorption performance of the coupled resonator system.Publicationtype: Journal Article143 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Impact of surface roughness on acoustic resonators made of closed-cell lightweight foams(Curran Associates, Inc., 2021-07); ; ; ; Acoustic resonators, such as Helmholtz resonators or slit resonators, are known as a highly efficient noise control mean for narrow frequency band applications. Their simple passive structure can be built from many different materials. If a noise control treatment is highly restricted by the additional weight, those resonators could be built out of a closed-cell lightweight foam. Because of both, manufacturing reasons and the cellular microstructure of the foam, the surface of such resonators is not perfectly smooth. Especially when openings of the resonators are milled in the foam, the surface roughness inside the opening might be relatively high. This leads to a certain damped behavior of the resonator that cannot be captured by current models. In this contribution, the effect of surface roughness on the airflow resistance and its impact on the acoustic behavior of slit resonators, like the absorption coefficient or the transmission loss for wall integrated means, is studied experimentally.Publicationtype: Conference Paper58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Aircraft side wall with improved low-frequency sound insulation using a lining panel with Helmholtz resonators(International Institute of Acoustics and Vibration (IIAV), 2019); ; ; The continuous development of aircraft engines aims to reduce fuel consumption by increasing the bypass ratio. However, an increased bypass ratio changes the sound emission of the engine, such that the sound spectrum inside the cabin can be dominated by low frequency tones with high sound pressure levels, penetrating more efficiently the current cabin wall than sound of higher frequencies. Thus, the comfort level of the passengers could be adversely affected. This contribution presents a new side wall panel design that is capable to significantly increase the transmission loss of low frequency tones. The rear side of the lining is fully covered by a resonator system of a cantilever integrated into a Helmholtz resonator. With this approach, the lining with resonators can be tuned with respect to two resonance frequencies to improve the sound transmission loss of the cabin wall. A transfer matrix for the Helmholtz resonator panel is derived and the transmission loss of the proposed side wall design is determined. The analytically calculated transmission loss is compared to numerical and experimental results obtained under diffuse field conditionsPublicationtype: Conference Paper145 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , An efficient analytical model for baffled, multi-celled membrane-type acoustic metamaterial panels(2018-03-17); ; A new analytical model for the oblique incidence sound transmission loss prediction of baffled panels with multiple subwavelength sized membrane-type acoustic metamaterial (MAM) unit cells is proposed. The model employs a novel approach via the concept of the effective surface mass density and approximates the unit cell vibrations in the form of piston-like displacements. This yields a coupled system of linear equations that can be solved efficiently using well-known solution procedures. A comparison with results from finite element model simulations for both normal and diffuse field incidence shows that the analytical model delivers accurate results as long as the edge length of the MAM unit cells is smaller than half the acoustic wavelength. The computation times for the analytical calculations are 100 times smaller than for the numerical simulations. In addition to that, the effect of flexible MAM unit cell edges compared to the fixed edges assumed in the analytical model is studied numerically. It is shown that the compliance of the edges has only a small impact on the transmission loss of the panel, except at very low frequencies in the stiffness-controlled regime. The proposed analytical model is applied to investigate the effect of variations of the membrane prestress, added mass, and mass eccentricity on the diffuse transmission loss of a MAM panel with 120 unit cells. Unlike most previous investigations of MAMs, these results provide a better understanding of the acoustic performance of MAMs under more realistic conditions. For example, it is shown that by varying these parameters deliberately in a checkerboard pattern, a new anti-resonance with large transmission loss values can be introduced. A random variation of these parameters, on the other hand, is shown to have only little influence on the diffuse transmission loss, as long as the standard deviation is not too large. For very large random variations, it is shown that the peak transmission loss value can be greatly diminished.Publicationtype: Journal Article113 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Perforated membrane-type acoustic metamaterialsThis letter introduces a modified design of membrane-type acoustic metamaterials (MAMs) with a ring mass and a perforation so that an airflow through the membrane is enabled. Simplified analytical investigations of the perforated MAM (PMAM) indicate that the perforation introduces a second anti-resonance, where the effective surface mass density of the PMAM is much higher than the static value. The theoretical results are validated using impedance tube measurements, indicating good agreement between the theoretical predictions and the measured data. The anti-resonances yield high low-frequency sound transmission loss values with peak values over 25 dB higher than the corresponding mass-law.Publicationtype: Journal Article72 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Enhancing the low-frequency noise reduction of a double wall with membrane-type acoustic metamaterials(2016-08-21); ; In the framework of the research project Comfortable Cabin for Low-emission https://tore.tuhh.de/submitAircraft (COCLEA), a lowfrequency noise shield concept has been developed consisting of a double wall with integrated so-called membrane-type acoustic metamaterials (MAMs). MAMs are composed of small unit cells with a thin membrane layer and small rigid masses attached to it. These structures exhibit tunable narrow frequency bands with transmission loss values considerably larger than the corresponding mass law. This can possibly be exploited to improve the otherwise poor low-frequency performance of a standard double wall. Analytical and numerical studies of the noise shield concept indicated substantial improvements in the low-frequency regime. Subsequent laboratory measurements, however, showed that some important practical aspects need to be considered in a technical realization of the concept. The present contribution summarizes the most important findings of this research activity. Based upon the analytical, numerical, and experimental results, the relevant design parameters of the concept are identified. On this basis, an advanced design of the noise shield concept has been realized and validated in laboratory measurements.Publicationtype: Conference Paper42 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A membrane-type acoustic metamaterial with adjustable acoustic properties(2016-07-07); ; ; The proposed design distinguishes itself from other realizations by a stacked arrangement of two MAMs which is inflated using pressurized air. The static pressurization leads to large nonlinear deformations and, consequently, geometrical stiffening of the MAMs which is exploited to adjust the eigenmodes and sound transmission loss of the structure. A theoretical analysis of the proposed inflatable MAM design using numerical and analytical models is performed in order to identify two important mechanisms, namely the shifting of the eigenfrequencies and modal residuals due to the pressurization, responsible for the transmission loss adjustment. Analytical formulas are provided for predicting the eigenmode shifting and normal incidence sound transmission loss of inflated single and double MAMs using the concept of effective mass. The investigations are concluded with results from a test sample measurement inside an impedance tube, which confirm the theoretical predictions.Publicationtype: Journal Article113 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Analytical model for low-frequency transmission loss calculation of membranes loaded with arbitrarily shaped massesAn analytical model for the transmission loss calculation of thin rectangular and circular membranes loaded with rigid masses of arbitrary shape, the so-called membrane-type acoustic metamaterials, is presented. The coupling between the membrane and the added masses is introduced by approximating the continuous interaction force with a set of discrete point forces. This results in a generalized linear eigenvalue problem that is solved for the eigenfrequencies and eigenvectors of the coupled system. The concept of the effective surface mass density is employed to calculate the low-frequency transmission loss using the obtained eigenpairs. The proposed model is verified using numerical data from a finite element model and the convergence behavior of the point matching approach is investigated using Richardson extrapolation. Finally, a method based upon the grid convergence index for estimating the error that is introduced due to the point matching approach is presented.Publicationtype: Journal Article35