  A method is presented for the determination of complex-valued compression and
shear elastic moduli of polymers for ultrasound applications. The resulting
values, which are scarcely reported in the literature, are found with
uncertainties typically around 1 % (real part) and 6 % (imaginary part). The
method involves a setup consisting of a cm-radius, mm-thick polymer ring glued
concentrically to a disk-shaped piezoelectric transducer. The ultrasound
electrical impedance spectrum of the transducer is computed numerically and
fitted to measured values as an inverse problem in a wide frequency range,
typically from 500 Hz to 5 MHz, both on and off resonance. The method was
validated experimentally by ultrasonic through-transmission around 1.9 MHz.
Experimentally, the method is arguably simple and low cost, and it is not
limited to specific geometries and crystal symmetries. Moreover, by involving
off-resonance frequencies, it allows for determining the imaginary parts of the
elastic moduli, equivalent to attenuation coefficients. Finally, the method has
no obvious frequency limitations before severe attenuation sets in above 100
MHz.
