Road surfaces made of porous asphalt have two great advantages: a significantly reduced noise emission of vehicles and an increase in road safety by draining rain immediately. Both effects depend on the porosity of the material. It depends on the duration of the rolling process of the asphalt which value of porosity is achieved. In this paper an approach for a non-destructive determination of the porosity is discussed that can be used during the construction process. In order to determine the amount of accessible cavities measured data of the wall impedance of the asphalt are fitted with help of a phenomenological model that describes the sound field over porous absorbers. The wall impedance is measured with a PU probe. This device is carefully studied in the present work to find its limits and conditions of application. Furthermore it is compared to the device specified in the ISO 13472-1 regulations.
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