Why does newly paved asphalt sound so much quieter when driving over it?
In: Engineering
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.
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.
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