Why does newly paved asphalt sound so much quieter when driving over it?
In: Engineering
**Asphalt is Porous**
Asphalt (Hot mix asphalt or HMA) is porous since it’s basically a bunch of aggregate glued together. This allows water and air to pass into the HMA layer. In regards to road noise, this means the air is being compressed by the tire into the pavement structure rather that outward into the air. This results in less noise. Over time, the HMA will compress and become less porous (rutting as an example). *Additionally dirt and debris will also clog up the voids, causing less air to pass, which leads to more road noise.
**Asphalt dampens the load**
Pavement structures typically have a spring reaction and a dampen reaction (spring and dashpot reactions). When the asphalt is initially paved, it can be squished to reduce the impact, and almost return to it’s normal thickness. This will provide a smoother ride, and reduced noise. Over time, the asphalt layer compresses, you will have less of a dampening factor, and that will increase road noise.
**New types of asphalt**
There are new types of asphalt being placed as well that are designed specifically to be more porous to allow air/water to travel through them than the other types of asphalt typically layed down. You will really notice it on rainy days because cars won’t spray water from their tires. Instead, it passes through the HMA layer, just as air does. *These different types of asphalt will affect road noise, essentially because of the difference in gradation of the aggregates and the mixes.
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