PhD Defence by Diana Maria Garza Agudelo

Title: Acoustic absorbing metamaterials for multidirectional incident waves

Supervisors
Principal supervisor: Associate Professor, Vicente Cutanda Henriquez, Department of Electrical and Photonics Engineering, DTU, Denmark
Co-supervisor: Associate Professor, Cheol-Ho Jeong, Department of Electrical and Photonics Engineering, DTU, Denmark

Evaluation Board
Associate Professor, Finn T. Agerkvist, Department of Electrical and Photonics Engineering, DTU, Denmark
Associate Professor, Luís Manuel, Department of Civil Engineering of the University of Coimbra, Portugal
Assistant Professor, Elke Deckers, KU Leuven, Belgium

Master of the Ceremony
Associate Professor, Jonas Brunskog, Department of Electrical and Photonics Engineering, DTU, Denmark

Abstract
Sound absorbing materials are used in a wide variety of engineering applications. They are commonly used to control the amplitude of waves reflected by room surfaces. For instance, they can be found as surface treatment in classrooms to increase speech inteligibility or in large spaces such as open plan offices or large industrial facilities to limit noise levels. Sound absorbing metamaterials are engineered arrangements of unitary elements, often resonators, that have been shown to have the ability to absorb frequencies with wavelengths that are more than two orders of magnitude larger than their thickness. This makes them especially useful for reflection control at low frequencies where wavelengths can be several meters long. Nevertheless, few works have studied and developed sound absorbing metamaterials for high absorption over a wide range of wave incidence angles. This ability is especially relevant for applications in room acoustics, where waves reach surfaces from many different directions. In this PhD project, a systematic study of the angle-dependent behavior of metasurfaces realized as periodic arrangements of Helmholtz resonators is first presented. Their limitation to achieve high absorption over a wide range of wave incidence angles in the region approaching grazing incidence is highlighted. Then, a novel metasurface design is proposed. It combines the periodic arrangement of resonators with a sonic crystal. Specific designs considering single and multifrequency excitation are obtained by optimization of the dimensions of the constitutive elements. In both test cases, it is found that the added sonic crystal significantly improves the performance of the surface of resonators in the neargrazing incidence region. The theoretical absorption coefficient of one of the designs is compared to the measured performance of a 3D printed sample. The agreement between both results is very good. The studies conducted in this project highlight the potential of sound absorbing metamaterials as multidirectional absorbers.