On the other hand, we have the methods that use ultrasound, using the relationship between the acoustic impedance, propagation velocity and density.
Thus, in a single measurement, at each point in the surface of the analysed specimen the propagation velocities, the thickness and the acoustic impedance can be obtained with high accuracy, allowing us to calculate additional parameters related to the mechanical properties of materials such as the density.
For this reason, a different kind of calibration equipment is necessary, which could provide the possibility to measure the AET characteristics using different types of waves and to evaluate the characteristics of these transducers under different acoustic impedance
The acoustic impedance
is determined by the density p and of the acoustic velocity v of the material (2):
For example, at the boundary of two media with acoustic impedances
The amount of seismic energy transmitted and reflected depends on the acoustic impedance
contrast between the two formation beds.
As a longitudinal wave normally travels from medium 1 to medium 2 with acoustic impedances
Once a sound wave reaches an interface of different acoustic impedance
, the wave is partially reflected and partially transmitted.
The next step is to define the acoustic properties of the acoustic medium and specify the boundary conditions, which are applied as surface velocities, pressures, acoustic impedances
, and simultaneous velocities and acoustic impedances
In both cases [lambda]/4 layers were used for matching materials with different acoustic impedances
in order to achieve more efficient transmission of the ultrasonic wave through the waveguide-fluid interface .
The composite samples were suspended freely in air and their electrical impedances measured over the frequency range 300 kHz to 50 MHz where the total impedance at resonance is given by the sum of the electrical impedance and the acoustic impedances
In fact, the acoustic impedance
upstream and downstream of the flame is used as an input in the feedback loop model developed by Baade (1978, 2004; Baade and Tomarchio, 2008).