Review in Sound Absorbing Materials
X. Sagartzazua,*, L. Hervellab, and J. M. Pagaldaya
Ikerlan, Mechanical Engineering Area. Pº Arizmendiarrieta, 2. 20500 Arrasate-Mondragón. Spain. Tel: 0034 943 712400. Fax: 0034 943 796944
Universidade da Coruña, Facultade da Informática, Department of Mathematics. Campus Elviña. Spain. Tel: 0034 981167000 (ext. 1325). Fax: 0034.981167160
Corresponding autor. Email adresses: firstname.lastname@example.org (X. Sagartzazu), email@example.com (L. Hervella), firstname.lastname@example.org (J. M. Pagalday).
This article is a bibliographical revision concerning acoustic absorbing materials, also known as poroelastics. These absorbing materials are a passive medium use extensively in the industry to reduce noise. This review presents the fundamental parameters that define each of the parts comprising these materials, as well as current experimental methods used to measure said parameters. Further along, we will analyze the principle models of characterization in order to study the behaviour of poroelastic materials. Given the lack of accuracy of the standing wave method three absorbing materials are characterized using said principle models. A comparison between measurements with the standing wave method and the predicted surface impedance with the models is shown.
KEY WORDS: poroelastic material, acoustic surface impedance, poroelastic characterization, models, sound absorption
The number of products that include a low noise level design increases daily. However, apart from the design itself, it is frequently necessary to use techniques that lower the level of noise in the product or industrial application.
A variety of methods are available for noise reduction but they can be basically grouped as follows: passive and active mediums. Active mediums differ from passive mediums in that it is necessary to apply external energy in the noise reducing process. The absorbing materials, as such, are passive mediums that lower noise by disseminating energy and turning it into heat. Acoustic absorption depends on the frequency of the sound waves. In porous materials at high frequencies, an adiabatic process takes place that produces heat loss due to friction when the sound wave crosses the irregular pores. On the other hand, at low frequencies, poroelastic materials absorb sound by energy loss caused by heat exchange. This is an isothermal process. In general, poroelastic efficiency is limited to high frequencies. The absorption phenomenon differs from that of insulation or shock absorption. This process causes a vibrating movement to diminish in size with time. The origin can vary: due to friction between two surfaces, as a result of internal friction or hysteresis of the material itself, etc. Other passive mediums exist, such as resonators that reduce noise by transforming it in the vibration of the resonator itself. A noise resonator is nothing more than a system that begins to vibrate due to variations in sound pressure; the resonator begins to vibrate and produces losses in the form of heat. Resonators can be modelled as a forced and damped system with one degree of freedom, with an equivalent excitation strength, equivalent mass element, equivalent spring element and equivalent damping element. The equivalent strength is produced by the variations in sound pressure and the mass, spring and damping elements depend on the resonator to be modelled: the type of resonator, its dimensions, materials, etc. The mass consists of everything that is subject to movement and the spring to all that provides stiffness. The damping consists of everything that causes energy loss in the moving system. There are different types of resonators: Helmholtz resonators, panel resonators and punched plate resonators. Currently, active type resonators exist that vary their geometry depending on the variation of external noise to be lowered. There are also...