MECHANICAL IMPEDANCE: DIFFERENCES BETWEEN DRIVING-POINT AND TRANSFER IMPEDANCE

In 1914 the mechanical impedance was probably defined for the very first time by professor Arthur G. Webster. He realized the possibility of using the impedance concept in the study of vibrating mechanical systems. Since then, several uses of the mechanical impedance have been implemented, from biomechanics to engineering (structural health, crack detection, noise reduction, etc.). Using forced vibration techniques, structures and materials can be investigated. To this end, force is usually applied by a hammer while acceleration is measured through an accelerometer. In this study, authors focused the attention on the differences between the axial mechanical impedance (force and acceleration at the same point), here called driving-point impedance, and the non-axial mechanical impedance (force and acceleration at different points), here called transfer impedance. Asphalt concrete specimens were tested in the laboratory, using an impact hammer Brüel & Kjaer Type 8206, an impedance head Brüel & Kjaer Type 8001 (in order to derive the driving-point impedance) and a piezoelectric accelerometer Brüel & Kjaer Type 4507 (in order to derive the transfer impedance) located 2 cm far from the hitting point. Mixtures were produced as a part of the project LIFE “SNEAK” (LIFE20 ENV/IT/000181) aiming at reducing the noise of asphalt pavements with noise- and vibration- oriented studies. The influence of Crumb Rubber (CR) on the bituminous mixes was also analyzed. First results demonstrate that the driving-point impedance yields MIs lower than the transfer impedance. For the dependency on crumb rubber percentage, given the low percentages, results show that CR could cause minor effects in terms of mechanical impedance. Further studies are needed to address this topic.