Why does newly paved asphalt sound so much quieter when driving over it?
In: Engineering
When it’s first paved it is nice and flat. But due to a mic of cars using it and the weather damaging it the asphalt become more bumpy and gets more cracks.
When new pavement is put down, the paving machine smooths it out with a roller. Freshly paved roads have a very smooth surface with no bumps or irregularities. That’s why there’s less road noise. The noise is caused by your wheels hitting things on the road surface. Over time, the asphalt surface will start to wear out in places, or settle in places, and you’ll start hearing more road noise.
The asphalt actually remains liquid and gradually flows downward as it ages, exposing the rock aggregate and shifting the surface profile from what starts as flat to more pebbly. The gaps between the pebbles cause extra noise because tires expand into and out of them as they roll on over.
Fun fact: roads near exposed sand like beaches or deserts tend to remain quiet because the sand getting blown on fills in those gaps as they form.
Fun fact #2: asphalt is near 100% recyclable. Just lift it up and heat it enough and the liquid will flow freely again and the whole mix can then be reapplied.
Newly paved roads make the surface, called a roadway, smooth and easier than roll over.
Run your finger nails over corduroy the run your fingernails across a bed sheet. You’ll notice a difference in the sound.
If you cover the corduroy with a few layers of smooth material, the no’s will become quieter with each layer.
The roadway is actually made of smaller pieces of rock than the base or the sub-base which creating the smoother top coat.
The base is better at holding its shape and supporting weight because it is made or larger materials .
The sun base is better still at holding its shape and made of even larger material that has irregular edges which lock in place by pressing against one another.
Over time, the top coat will become stressed and possibly worn but most importantly the weight of the vehicles will push the top coat into the rougher material of the base.
So, when repaving, the top coat (roadway) is removed and replaced with a smooth(ish) and level coat.
If you think driving on new asphalt is fun, try skating on it. I have dreams of skating on freshly paved asphalt… sooo good!
Independent of the other responses here of “new road smoother”, it is also possible your newly paved roads make use of porous asphalt which is a newer (~1980) special kind of asphalt. One of its advantages is absorbing some of the tire noise.
Asphalt is essentially made of three things rock, sand, and oil. The sand helps fill the space between the rock and the oil binds everything together.
When initially laid down and compacted an asphalt surface can be fairly smooth. However with time the sand and oil wear away leaving more exposed rock and a rougher surface that causes more noise when driven on.
Source: I worked on an asphalt road crew for 2 years.
**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.
The old surface is most likely being replaced with newly developed [quiet asphalt](http://asphaltmagazine.com/noise-reducing-pavements-get-loud-acclaim-in-us-and-europe-understanding-the-issue/). I remember reading about this a while back: while developing new road surfaces designed to drain water faster to prevent puddles/hydroplaning, they accidentally developed a quieter road surface. As someone who lives close to a freeway, this is pretty awesome. In other news, there’s an asphalt magazine.
Asphalt doesn’t make any sound, if you are hearing asphalt you may have synesthesia. Synesthesia is a condition in which your brain provides a different sensation in a different sense than the one that your senses are actually experiencing. In this instance you are “hearing” something because you saw it.
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