A method for evaluating the acoustical properties of homogeneous and isotropic porous materials that may be modeled as fluids having complex properties is described here. To implement the procedure, a conventional, two-microphone standing wave tube was modified to include: a new sample holder; a section downstream of the sample holder that accommodated a second pair of microphone holders and an approximately anechoic termination. Sound-pressure measurements at two upstream and two downstream locations were then used to estimate the two-by-two transfer matrix of porous material samples. The experimental transfer matrix method has been most widely used in the past to measure the acoustical properties of silencer system components. That procedure was made more efficient here by taking advantage of the reciprocal nature of sound transmission through homogeneous and isotropic porous layers. The transfer matrix of a homogeneous and isotropic, rigid or limp porous layer can easily be used to identify the material’s characteristic impedance and wave number, from which other acoustical quantities of interest can be calculated. The procedure has been used to estimate the acoustical properties of a glass fiber material: good agreement was found between the estimated acoustical properties and those predicted by using the formulas of Delany and Bazley.

A transfer-matrix approach for estimating the characteristic impedance and wave numbers of limp and rigid porous materials

A tunable high-static–low-dynamic stiffness vibration isolator

Variable stiffness material and structural concepts for morphing applications

In this paper, the question of the acoustical determination of macroscopic thermal parameters used to describe heat exchanges in rigid open-cell porous media subjected to acoustical excitations is addressed. The proposed method is based on the measurement of the dynamic bulk modulus of the material, and analytical inverse solutions derived from different semiphenomenological models governing the thermal dissipation of acoustic waves in the material. Three models are considered: (1) Champoux–Allard model [J. Appl. Phys. 20, 1975–1979 (1991)] requiring knowledge of the porosity and thermal characteristic length, (2) Lafarge et al. model [J. Acoust. Soc. Am. 102, 1995–2006 (1997)] using the same parameters and the thermal permeability, and (3) Wilson model [J. Acoust. Soc. Am. 94, 1136–1145 (1993)] that requires two adjusted parameters. Except for the porosity that is obtained from direct measurement, all the other thermal parameters are derived from the analytical inversion of the models. The method is appl...

Acoustical determination of the parameters governing viscous dissipation in porous media.

https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/89122/1/j.jsv.2007.11.045.pdf

Semi-active vibration isolation system with variable stiffness and damping control

A method for measuring the characteristic impedance and propagation constant of porous materials is described in this paper. Measurements were performed based on a surface impedance method that required a set of distinct acoustic impedances derived at the material surface. This requirement is satisfied by arbitrarily changing the air space depth behind the material, and then a new formulation is derived so that a recently developed method of determination, called the transfer function method, can be applied. An appropriate set of air space depths is also discussed. Glass wool and porous aluminum were used to assess the usefulness of the present method. The normal acoustic impedance and normal absorption coefficient of the test materials with arbitrary thicknesses or with an arbitrary air space depth behind them were calculated from the obtained characteristic impedance and propagation constant and were compared with the measured values that were obtained directly by using the transfer function method. The good agreement achieved suggests that the present method is reliable and effective enough to measure the characteristic impedance and propagation constant over a broadband frequency range.

http://pcfarina.eng.unipr.it/Public/Standing-Wave/Bulk%20Properties_1.pdf

Transfer function method for measuring characteristic impedance and propagation constant of porous materials

https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/128768/1/j.jsv.2010.06.021.pdf

Optimum tuning of series and parallel LR circuits for passive vibration suppression using piezoelectric elements

Most passive vibration isolation systems are composed of springs and dampers. Although it is possible to improve the isolation performance by active vibration control, the complexity, power requirements and cost of such a system have restricted its use. A vibration isolation system with variable damping is practical and has good performance in the high frequency region, but it was found not to improve the responses in the low frequency region. On the base of a damping on-off control method, a stiffness on-off control method and a combination of damping and stiffness on-off control method were proposed. Comparison of the responses among the proposed methods and the conventional methods showed that the damping and stiffness on-off control method had the best isolation properties in the whole frequency region. A new system with controllable dampers of two Voigt elements in series was used to achieve the proposed idea.

https://www.jstage.jst.go.jp/article/jsmec/48/2/48_2_305/_pdf

Vibration isolation by a variable stiffness and damping system

This paper deals with reducing floor impact vibration and sound by using a momentum exchange impact damper. The impact damper consists of a spring and a mass that is contact with the floor. When a falling object collides with the floor, the floor interacts with the damper mass, and the momentum of the falling object is transferred to the damper. In this works a computational model is formulated to simulate dynamic floor vibration induced by impact. The floor vibration is simulated for various sized damper masses. A proof-of-concept experimental apparatus was fabricated to represent a floor with an impact damper. This example system consists of an acrylic plate, a ball for falling object, and an impact damper. A comparison between simulated and experimental results were in good agreement in suggesting that the proposed impact damper is effective at reducing floor impact vibration and sound by 25% and 63%, respectively.

/pdf/reducing-floor-impact-vibration-and-sound-using-a-momentum-3ctjx0x6mt.pdf

Hideo Utsuno

Papers

Semi-active vibration isolation system with variable stiffness and damping control

Transfer function method for measuring characteristic impedance and propagation constant of porous materials

Optimum tuning of series and parallel LR circuits for passive vibration suppression using piezoelectric elements

Vibration isolation by a variable stiffness and damping system

Reducing Floor Impact Vibration and Sound Using a Momentum Exchange Impact Damper