Dear Madam, the tyre rubber as additive to asphalt mixture changes the properties of asphalt layer. There are many factors that may affect on decreasing of sound level from moving of vehicles. One of that is to use smaller size of aggregates. Rubber from tyres can be used as additive to asphat mixture in the form of aggregate (dry process) or like part of the bitumen (wet process). The effect of damping and sound absorption can be explained from interaction of tire and pavement and elactic properties of ruberized asphalt mixture. I am not a specialist in tests of soung absorption but I think there are some papers in literature. 

Tire rubber inclusions in an asphalt concrete reduce the overall stiffness of the material, dampening the impact of the tire tread as it comes into contact with the pavement.  The tire rubber inclusions impede the propagation of vibration energy through the mix.  Tire rubber inclusions can allow the asphalt concrete design to use more overall asphalt binder, which contributes to reduced stiffness and brittleness over time.  I have an esteemed friend who developed a method to predict or determine noise dampening properties of asphalt mixtures using impedance testing of mix samples.  

https://scholar.google.com/citations?user=-SjQmwYAAAAJ&hl=en  

https://krishnaprapoornabiligiri.weebly.com/  

I agree with Marek