In environmental acoustics, the ground effect is described by its acoustic impedance. By analogy with the electrical or mechanical impedance, this quantity accounts for the reaction of the support (here the ground) with a physical stress (here an acoustic wave). Many methods for measuring the acoustic impedance of materials have been developed in laboratories (Kundt tubes for example), but these "destructive" methods prove to be unsuitable for ground problems because several samples should then be taken in situ between source(s) and receiver(s).
Since the 1980s, UMRAE has been developing and using an original method for characterizing acoustic impedance based on a measurement of a pressure transfer function between two microphone positions. Then, this transfer function is compared with a theoretical transfer function in order to identify the characteristic parameters of the soil: thickness, airflow resistance, or even porosity, tortuosity, etc.
This method has been more widely disseminated and tested in recent years by works in the framework of the PLUME project, with:
- the development of a compact measuring device called MIAME (for Measurement of the Acoustic Impedance of Environmental Materials): this device is associated with a Scilab program that allows to control measurements following a large number of possibilities (signal emission, windowing, identification mode, ...);
- the organization of round robin tests to evaluate the method and the measuring device: these tests carried out by 5 laboratories made it possible to estimate the uncertainty on the determination of the acoustic characteristics of different ground categories (porous to reflective).