Technology continued
Sonic crystals are artificially fabricated materials designed to control, direct, and attenuate sound formed by periodic variation of the acoustic properties of the material. They arise from well known solid state theories and gain their name from atomic crystal structures, which are unique arrangements of atoms or molecules in a crystalline liquid or solid. Such a crystal structure is composed of a pattern, a set of atoms arranged in a particular way. It is the crystal's structure and symmetry play a role in determining many of its physical properties.
The first realisation of a sonic crystal, although actually older than the sonic crystal concept, is the discovery that a minimalist sculpture by Eusebio Sempere, comprising a two-dimensional periodical arrangement of cylinders, manipulated the transmission of sound passing through it.
The ability for a sonic crystal to manipulate sound can be found in the interference of sound waves as they are scattered by the cylinders. Because of the periodic nature (analogous to an atomic structure) this interference constructive or destructive interference, dependant on the frequency of the incoming sound wave. When this interference is constructive, the incoming sound energy is reflected back and the wave cannot propagate through the sonic crystal.
Constructive interference occurs when the path difference between the interfering waves is equal to an integer number of their wavelength. Since the path difference is determined by the spacing of the crystal, a, it is clear that constructive interference occurs when the spacing of the crystal is comparable to the wavelength, λ of the incoming sound. This means that a sonic crystal can be tailored to attenuate noise in any frequency range by adjusting the crystal’s parameters.
To gain additional performance over conventional sonic crystals, the solid scattering objects are replaced by broad band sound attenuating devices, developed by Loughborough University physicists. This incorporates an additional attenuation mechanism of resonance to block out an increased range of frequencies related to the dimensions of the resonators, not the spacing between them.