Why does newly paved asphalt sound so much quieter when driving over it?
In: Engineering
Depends on what the mix is.
A new surface from a dense mix with no voids can be quite noisy (often a high pitched whining noise as you drive over it). This will still be better than an older surface because it’s nice and smooth so you are only getting the noise from the wheels rolling, not any bumping of vibration like you get on an old surface that has been pushed out of shape or patched up.
Some types of asphalt are designed to have open pores in it. This does 2 things: firstly it lets water drain away without ponding on the surface. This is a huge safety benefit because surface water especially on high speed roads can make a massive difference to braking distances (hydroplaning really isn’t fun if your car is moving fast and the one in front is stopped). The other thing the open pores do is absorb a lot of sound because the air pressure doesn’t just reflect back, some gets baffled in the tiny holes in the mix.
Roadwork engineers have figured out a mixture of asphalt with different pebble- and grain sizes, such that the surface is ultimately flat but “raw” with lots of cavities of different sizes. The flatness reduces noise generated by the tyres, while the rawness offers sufficient grip. The cavities act like sound absorbers. The different sizes of the cavities translate to different frequencies of the noise.
As you can guess, flatness and rawness are their respective couterparts to an extent. The trick is to find a compromise that is both: quiet and grip-offering as much as possible.
Drove on special “quiet pavement” off the 520 near Kirkland. Very wealthy neighborhood, of course, but it was a pleasure to drive on.
Imagine how a drum or an acoustic guitar makes noise. There’s a hollow area for the sound to reverberate through. Old roads have a rough surface, so there’s lots of little spaces under your tires to reverberate and create noise. New pavement is smoother, so there’s less space for the sound of your tires rolling over it to propagate. This also why mud tires are louder, the deep treads give the sound room to reverberate inside the treads.
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.
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.
If you think driving on new asphalt is fun, try skating on it. I have dreams of skating on freshly paved asphalt… sooo good!
**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.
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.
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