ABSTRACT
High damping properties or defined scattering effects are very interesting for many applications, especially when high frequencies occur. Previous studies on elastomer materials filled with active fillers such as carbon black or silica show increasing damping behavior for high temperatures at high frequencies. Furthermore, it has been reported that inactive fillers e.g. glass spheres of certain size show scattering effects in addition to (hysteretic) damping when they are embedded in an elastomer matrix (Oprisoni 2011). The first mathematical description for the scattering of an electromagnetic wave by spherical particles with small radius was derived by Rayleigh and forms the basis of the classical theory for scattering of waves in inhomogeneous media (Strutt 1945). Scattering of ultrasound by a spherical particle strongly depends on the ratio of its dimension to the wavelength of the incident ultrasonic wave λ. A measure for this ratio is the scattering parameter x = k ⋅ r, where r is the radius of the particle and k = 2π/λ is the wave number. Rayleigh scattering occurs when the size of the diffuser particles is much smaller than the wavelength of the ultrasonic wave (r<<λ) (Shutilov 1988) and usually starts when the scattering center is 1/100 to 1/50 of the wavelength (Cranknell 1980). In our case a rough estimation leads to a wavelength of 3 mm at a sound velocity of approx. 1500 m/s and a frequency of 0.5 MHz. This corresponds to a particle size of 30 μm to 60 μm as scattering limit.
