Thus, in order to measure the velocity of wave propagation in the asphalt mixtures it is necessary to use acoustic equipment [3,4]. This equipment works by generating input signals, using an emitter transducer, which are received by a receiver transducer, placed on the sample surface. The signals are immediately registered by a wave analyser coupled to a computer, which processes the data. Thereby, depending on the transducer positions, three types of tests can be performed: direct, semi-direct and indirect. In this study direct test or through the sample was used, where the emitter and receiver transducers are in a straight line on opposite sides of the material to be tested. This is the most sensitive transmission since this is when the highest percentage of energy is aimed at the receiver transducer [5–7].
Mathematical description of the equation of dynamic modulus determined by ultrasonic transmission is based on the interpretation of propagation velocities (P-wave speed) and mechanical properties of the material. During the test an internal piezoelectric crystal generates a high-frequency low-energy stress wave. In turn, the transducers must be coupled to the sample surface using devices enabling the highest energy transfer possible. Thus, the wave travels
through the section and is captured by the receiver transducer, located at a distance (L) from the transmitter transducer, which at the same time converts the wave mechanical energy into electrical energy. The transmission time (t) is electronically determined (generally in microseconds) and the pulse propagation speed (VR) can be calculated using Eq. (2) below. This propagation speed is related to the physical properties of the material [1,7].