The accuracy of some models for the airflow resistivity of nonwoven materials

The airflow resistivity is a key parameter to consider when evaluating the acoustic performance of a fibrous material. The airflow resistivity is directly linked to a fibrous materials acoustic properties which allows for the non-invasive measurements of the fibre diameter and material density from acoustical data. There are several models that relate the airflow resistivity to the acoustic behaviour through the material's density and fibre diameter. It is not always obvious how accurately a model represents the true value of the flow resistivity of a nonwoven material with a fibre size variation. Therefore, the scope of this paper is to compare the performance of several theoretical and empirical models applied to a representative range of nonwoven fibrous media composed of blends of different fibre sizes and types. Being able to understand the performance of these models in application to fibre blends will enable users to characterise these types of fibrous media more precisely. From this work, it was concluded that the Miki model (Miki, 1990) is the most accurate model to invert the airflow resistivity from acoustical surface impedance of a wide range of nonwoven blends

An application of Kozeny–Carman flow resistivity model to predict the acoustical properties of polyester fibre

Modelling of the acoustical properties of polyester fibre materials is usually based on variations of the Bies and Hansen empirical model [1], which allows the calculation of the air flow resistivity of a porous material. The flow resistivity is the key non-acoustical parameter which determines the ability of this kind of materials to absorb sound. The main scope of this work is to illustrate that an alternative theoretical model based on the KozenyCarman equation can be used to predict more accurately the flow resistivity from the fibre diameter and bulk material density data. In this paper the flow resistivity is retrieved from the acoustic absorption coefficient data for polyester fibre samples of different densities and fibre diameters. These data agree closely with the flow resistivity predicted with the proposed KozenyCarman model

On the relationship of the observed acoustical and related non-acoustical behaviours of nanofibers membranes using Biot- and Darcy-type models

There is a general lack of publications on the acoustical and related non-acoustical properties of nanofibrous media. This work attempts to contribute to this gap and to highlight problems associated with acoustic and related non-acoustic characterisation of these materials. The work, presumably for the first time, applies Biot- and Darcy-type mathematical models to explain the observed acoustical and related non-acoustical behaviours of the nanofibres. It identifies theoretical gaps related to the physical phenomena which can be responsible for the observed acoustical behaviours of nanofibrous membranes and it presents recommendations to fill these gaps. The novelty of this work is in the use of a robust theoretical model to explain the measured acoustical behaviour of thin nanofibrous membranes placed on a foam substrate. With this model the actual flow resistivity of nanofibers is estimated from acoustical data. It is demonstrated that a classical model for the flow resistivity of fibrous media does not work when the Knudsen number becomes greater than 0.02, i.e. then the diameter of nanofibres becomes comparable with the mean free path

Noise suppression using local acceleration feedback control of and active absorber

A popular approach for Active Noise Control (ANC) problems has been the use of the adaptive Filtered-X Least Mean Squares (FXLMS) algorithm. A fundamental problem with feedforward design is that it requires both reference and error sensors. In order to reduce the size, cost and physical complexity of the control system a feedback controller can be utilized. In contrast with FXLMS a feedback controller utilizes local velocity measurements of a sound-absorbing surface instead of global pressure measurements. Most control problems, including ANC, can be formulated in the General Control Configuration (GCC) architecture. This type of architecture allows for the systematic representation of the process and simplifies the design of a vast number of controllers that include H-infinity and H2 sub optimal controllers. Such controllers are considered ideal candidates for ANC problems as they can combine near optimal performance with good robustness characteristics. This paper investigates the problem of reflected noise suppression in acoustic ducts and the possibilities and trade-offs of applying H2 control strategies. Hence, by controlling locally the reflecting boundary structure, a global cancelation of the undesired noise can be accomplished. In the paper the H2 local feedback control strategy and performance are investigated using an experimental pulse tube facilit

What Is The Actual Influence Of A Nano-Fibrous Membrane On The Acoustical Property Of Porous Substrate?

There is a general lack of publications on the acoustical and related non-acoustical properties of nano-fibrous media. Existing publications typically present scanning electron microscope images of these media together with the acoustic absorption coefficient or transmission loss data (e.g. [1,2]). These works often discuss the nano-fibre production process, quote data on the fibre diameter and surface density for nano-fibrous membranes produced as a result of the reported process. However, little or no information is usually presented on the material pore structure, membrane thickness or bulk density. No effort is made to explain the observed acoustical performance of nano-fibrous membranes using a valid theoretical or semi-empirical model [1,2]. This paper describes the problems associated with acoustic and related non-acoustic characterisation of these materials. It attempts, probably for the first time, to use a Biot- and Darcytype mathematical models to explain the observed acoustical and related non-acoustical behavior of nano-fibres. It identifies theoretical gaps related to the physical phenomena which can be responsible for the observed acoustical behaviour of nano-fibrous membranes and it makes recommendations to fill these gaps

Noise suppression using local acceleration feedback control of an active absorber

A popular approach for active noise control problems has been the use of the adaptive filtered-X least mean square algorithm. A fundamental problem with feedforward design is that it requires both reference and error sensors. In order to reduce the size, cost and physical complexity of the control system, a feedback controller can be utilised. In contrast to filtered-X least mean square, a feedback controller utilises local acceleration measurements of a sound-absorbing surface instead of global pressure measurements. Most control problems, including active noise control, can be formulated in the general control configuration architecture. This type of architecture allows for the systematic representation of the process and simplifies the design of a vast number of controllers that include Formula and controllers. Such controllers are considered ideal candidates for active noise control problems as they can combine near-optimal performance with good robustness characteristics. This article investigates the problem of reflected noise suppression in acoustic ducts and the possibilities and trade-offs of applying Formula control strategies. Hence, by controlling locally the reflecting boundary structure, a global cancellation of the undesired noise can be accomplished. In this article, the Formula local feedback control strategy and performance are investigated using an experimental pulse tube. The Formula design was chosen because it was able to provide consistently a stable response in contrast to the design

Acoustic impedance matching using loop shaping PID controller design

For several decades Proportional-Integral-Derivative control (PID) has been successfully used for a wide variety of industrial processes and remains the most used method. Recent work concerning the tuning of PID control coefficients has been proven to provide both robust and near-optimal performance using a Frequency Loop Shaping (FLS) procedure. The FLS tuning method minimizes the difference between the actual and the desired target loop transfer function. Such a control design procedure is ideal for problems in which the desired closed loop frequency response is predetermined over a specific frequency band. This paper explores the possibilities and trade-offs of applying the FLS control strategy in Active Noise Control (ANC) problems. The use of the FLS design is ideal for the problem of noise suppression in ducts, because the required acoustic impedance for the elimination of reflecting sound waves in the one-dimensional case is well defined. Hence, by controlling locally the reflecting boundary structure, a global cancelation of the undesired noise can be accomplished