1 In situ assessment of the normal incidence sound absorption coefficient of asphalt mixtures with a new impedance tube Freitas E. 1 , Raimundo I. 1, Inácio O. 2, Pereira P. 1 1 Universidade do Minho efreitas@civil.uminho.pt , isaac.raimundo@ressonantia.com , ppereira@civil.uminho.pt 2 InAcoustics, Lda. octavio.inacio@inacoustics.com Abstract Normal incidence sound absorption coefficient of materials is usually calculated with the use of an impedance tube following the ISO 10534-2 international standard. The same is applied in the evaluation of the sound absorption characteristics of pavement mixtures. In this research an impedance tube which was specifically designed to be used in field conditions has been developed. In order test the tube six asphalt slabs were constructed in laboratory and its sound absorption coefficient measured with the new impedance tube. Then, several road sections, with different types of surface, among dense asphalt layers and thin layers, namely with rubberized asphalt were tested. In this paper, some considerations are made regarding the performance of the new impedance tube and the sound absorption coefficient of each asphalt mixtures. Keywords: Kundt-tube, asphalt, absorption. 1 Introduction To evaluate the sound absorption characteristics of materials, several techniques are usually employed. Most popular among them are the reverberation room method, described in ISO 354:2003, which returns the value of the sound absorption coefficient in diffuse sound field. The drawback of this technique relates to the fact samples to be assessed must be present in the reverberation room, a fact sometimes proves to be impossible, or at least undesirable. To solve this problem other techniques were developed. The spot method and the impedance tube method are examples of those techniques, which permit the in situ evaluation or the use of small samples of the material to be tested. Invited Paper