Sound Scattering by Lattices of Heated Wires

Abstract

[EN] The aim of this work is to demonstrate theoretically and experimentally how acoustic wave propagation can be controlled by temperature gradients. Starting with the simplest case of two hot wires in air the study extends over periodic structures known as Sonic Crystals (SCs). The Finite Elements Method (FEM) has been employed to perform numerical simulations in order to demonstrate collimation and focusing effect of acoustic waves in two-dimensional (2D) SC whose filling fraction is adjusted by temperature gradients. As a part of the research, Bragg reflection and Fabry-Perot type of acoustic effects are investigated for the proposed type of SC. As example, a SC with desired transmittance can be tailored. Also, gradient index (GRIN) 2D sonic lenses are studied. Using parallel rows of heated wires whose temperatures vary according to a prefixed gradient index law a GRIN lens can be designed with a given performance. Moreover, by changing the temperature of the wires a change in the filling fraction inside the GRIN SC can be achieved. Thus, the local refraction index, which is directly related to the filling fraction, is changed too and an index gradient variation inside the GRIN SC is obtained. This GRIN SC is a direct analogy of gradient media observed in nature. Like their optical counterparts, the investigated 2D GRIN SC lenses have flat surfaces and are easier for fabrication than curved SC lenses. The bending of sound waves obtained by GRIN acoustics structures can be used to focusing and collimating acoustic waves. Another aspect of this work is about tuning some SC properties as effective refractive index, effective mass density, etc. in order to obtain a SC with prefixed properties. Since active tuning of the phononic band gaps is certainly desirable for future applications with enhanced functionalities, few attempts have been made to develop tunable SCs thus far. By controlling the incident angle or operating frequency, a GRIN SC can dynamically adjust the curved trajectory of acoustic wave propagation inside the SC structure. Among the last studies of tunable SCs, the filling fractions were tuned either by direct physical deformation of the structure or external stimuli. The former is impractical for most applications and the latter often requires very strong stimuli to produce only modest adjustment. In this work another way to tune the SC properties is proposed. Hot and cold media have different density, speed of sound, refractive index, etc. in comparison with the same properties at normal conditions, so inserting temperature gradients inside the medium can be used to tune the SC properties in certain limits. The proposed way to obtain temperature gradients inside SC is by wires made of Nicrom which are heated by electrical currents. There are some important advantages of this method. First, changing the electrical current intensity through the wires the SC properties can be changed dynamically. Second, it is relatively easier to change the filling fraction simply by adjusting the current intensity…