One of the main environmental problems of the industrialised countries is the noise, which can be defined as an unwanted or unpleasant outdoor sound generated by transport, industry and human activities in general. When it is not possible to reduce the emission of noise acting on the source, the reduction of noise levels in its transmission phase using acoustic screens (AS) seems appropriate; such screens are in common use to reduce noise levels and have been extensively studied since the middle of the 20th century. Over the last decades, various acoustic screen designs have been investigated to increase the screening effect. The research carried out focuses on both the reduction of diffraction at the top edge of the screen by varying the shape at the top or adding absorptive materials to the noise screen, but all these screens are basically formed by a continuum rigid material with a superficial density high enough, to reduce transmission of noise through the screen, in accordance with the mass law. At the end of the nineties, another type of screen based on arrangements of isolated scatterers embedded in air, emerged. Among other interesting properties, these screens provide new mechanisms to control the noise based on the Bragg law. First, a Sonic Crystal Acoustic Screen (SCAS) was presented, where the scatterers are arranged following crystalline patterns. After that, a new prototype of AS based on sonic crystals appears, which increases the attenuation capabilities using arrangements based on fractal geometries. The screens designed in this way have been referred to as Fractal-based Sonic Crystal Acoustic Screens (FSCAS) in this chapter. In both the cases, the mechanism that prevents the transmission of noise, and therefore increases the noise attenuation, is the destructive Bragg interference due to a multiple scattering process. Finally, a new concept of AS based on a periodic arrangement of scatterers, with a slit dimension between them that is smaller than the wavelength is introduced. This latest screen is called Subwavelength Slit Acoustic Screen (SSAS) which presents a Wood anomaly and Fabry-Perot resonances, being the destructive interferences among the scattered waves, responsible for the attenuation capabilities of these screens. This new kind of AS (SCAS, FSCAS and SSAS) presents interesting properties and can be considered as a real alternative to the classical AS, which are formed by a continuum rigid material. The aim of this chapter is to present these open AS, and it is organised as follows. In Section 1, an introduction about classic acoustic screens is presented. Numerical models and experimental set-up for the screens are introduced in Section 2. Then, in Section 3 the transmission properties of Sonic Crystals are explained, and the research advances in this field related to the design of a screen based on the new mechanism of noise control are highlighted. The definition and development of the Fractal-based Sonic Crystal Acoustic Screen are shown subsequently. The Subwavelength Slit Acoustic Screen is developed in Section 4. Finally, in Section 5 the main results and conclusions of the work are presented.
Part of the book: Advances in Noise Analysis, Mitigation and Control
The term fractal was coined in 1975 by Benoit Mandelbrot. Since then, fractal structures have been widely used by the international scientific community. Its range of applications includes multiple areas, such as optics, physics, cryptography, medicine, economics, and so on. The application of fractal structures to modulate light beams in the field of optics has been extensively studied, and it has been shown that in some cases these new fractal lenses improve the response of traditional lenses. Fractal lenses are able to provide beamforming capabilities, and allow the optimization of the optical beam according to the specific requirements. In some applications, it may be necessary to improve the focus in a certain area, while in others it may be critical to obtain a sharp attenuation by means of destructive interference. It may even be required a beam profile with multiple focus and a certain control over them. This work investigates the application of fractal structures based on Polyadic Cantor sets as ultrasonic lenses, analyzing how the relation between the different design parameters and the performance of the lens. It is shown that the working frequency becomes a precise control mechanism that can modify dynamically the focus position of the lens.
Part of the book: Fractal Analysis