The characteristic impedance and propagation constant of porous materials are discussed from a viewpoint of physical realizability, and new regression models based on experimental data by Delany and Bazley are derived. Modifications from their original models are so made that the impedance function satisfies the positive-real property and that the propagation constant written in terms of complex variables becomes a regular function in the right half-plane. The new models are shown to be useful for the prediction of acoustical behavior of porous materials, especially in the double-layer case, even outside the frequency range of validity of the original models.

https://www.jstage.jst.go.jp/article/ast1980/11/1/11_1_19/_pdf

Acoustical Properties of porous materials : Modifications of Delany-Bazley models

Acoustic characterization of natural fibers for sound absorption applications

Sound-absorbing materials absorb most of the sound energy striking them, making them very useful for the control of noise. They are used in a variety of locations - close to sources of noise, in various paths, and sometimes close to receivers. Although all materials absorb some incident sound, the term "acoustical material" has been primarily applied to those materials that have been produced for the specific purpose of providing high values of absorption. The major uses of absorbing materials are almost invariably found to include the reduction of reverberant sound pressure levels and, consequently, the reduction of the reverberation time in enclosures, or rooms. A wide range of sound-absorbing materials exist. In the 1970s, public health concerns helped change the main constituents of sound-absorbing materials from asbestos-based materials to new synthetic fibers. Although, these new fibers are much safer for human health, more recently, issues related to global warming may increase the use of natural fibers instead of synthetic ones.

Recent Trends in Porous Sound-Absorbing Materials

A method is proposed that provides an approximation of the acoustic energy decay (energy–time curve) in room impulse responses generated using the image-source technique. A geometrical analysis of the image-source principle leads to a closed-form expression describing the energy decay curve, with the resulting formula being valid for a uniform as well as nonuniform definition of the enclosure’s six absorption coefficients. The accuracy of the proposed approximation is demonstrated on the basis of impulse-response simulations involving various room sizes and reverberation levels, with uniform and nonuniform sound absorption coefficients. An application example for the proposed method is illustrated by considering the task of predicting an enclosure’s reflection coefficients in order to achieve a specific reverberation level. The technique presented in this work enables designers to undertake a preliminary analysis of a simulated reverberant environment without the need for time-consuming image-method simulations.

/pdf/prediction-of-energy-decay-in-room-impulse-responses-207w0wj2j8.pdf

Prediction of energy decay in room impulse responses simulated with an image-source model

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

https://www.math2market.de/Publications/1970Delany-Bazley.pdf

E.N. Bazley

Papers

Acoustical properties of fibrous absorbent materials