Is it the grooves cut in the surface or just something in the concrete? It's not just bridges; it's concrete surfaced roads too, but bridges are usually louder. I've also noticed in some states they are louder than others, and have different tones, but in general make a high-pitched screeching noise. Also, in Georgia (which has the loudest bridges I have observed) you can hear it on asphalt shortly before you even make contact with the bridge. Anybody know why that is? I'm wondering if I'm actually hearing other cars that are ahead of me already on the bridge.
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$\begingroup$ Could be PAVEMENT GROOVING. $\endgroup$– Keith McClaryCommented Nov 14, 2017 at 5:39
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$\begingroup$ Note that your tires generate noise on other surfaces as well, though usually these sounds are less obnoxious than grooved concrete. $\endgroup$– AsherCommented Nov 14, 2017 at 5:58
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$\begingroup$ -1. Not clear what you are asking. You have not included enough information about the "concrete surface" for us to identify the cause of the noise, nor its pitch. Did the sound come from Rumble Strips? Or from transverse grooving? See also city-data.com/forum/urban-planning/…. $\endgroup$– sammy gerbilCommented Nov 15, 2017 at 12:45
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$\begingroup$ Could the bridge structure be amplifying the sound? $\endgroup$– DeepCommented Nov 17, 2017 at 3:54
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$\begingroup$ Purely concrete surface. Not rumble strips. Rumble strips make lower noises. Probably transverse grooving, although. I'll try to find a video. $\endgroup$– SagierianCommented Nov 17, 2017 at 10:26
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1 Answer
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As the threads of your tires roll into the grooves of a Rumble Strip, the air is pushed out creating a wave of pressure. The base harmonic of this sound would be equal the number of groves rolled over by a wheel per second. Consider your speed is 30 meters per second (108 km/h or 67.5 mph) and the grooves are 3 cm apart (about 33 per meter). This makes the base sound frequency $33*30\approx 1 kHz$. With multiple grooves acting at once, there are many harmonics making the sound feel high pitched. The distance between the grooves however may vary dramatically depending on the type of the strip.
There is a wealth of information on rumble strips in Wikipedia.
EDIT: Per the OP comment, he refers to the entire bridge surface grooved rather than just rumble strips. While the physics of grooves and wheels making a sound is the same, the purpose of grooving bridges is to reduce hydroplanning and other water related hazards. Due to the proximity of water and higher relative humidity, the probability of condensation on bridges is higher. In fact, a while ago, I had an accident when the wheels of my car spinned at the top of a non-grooved bridge and the car flipped over.
This specific safety technique is called bridge deck grooving:
"Bridge deck grooving is an effective method for reducing weather-related bridge hazards. By cutting patterned groves into the concrete deck, water is displaced, thus improving traction, vehicle control, and overall safety." Source
And sometimes grooves also make music :)
answered Nov 14, 2017 at 5:16
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$\begingroup$ Not talking about rumble strips. Just general bridge surfaces. I know how rumble strips work $\endgroup$ Commented Nov 17, 2017 at 11:27
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$\begingroup$ @Sagierian Thank you for the clarification. I have updated my answer to reflect your specific inquiry. $\endgroup$ Commented Nov 17, 2017 at 17:07
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$\begingroup$ Sorry, I figured most people would know what I was talking about. Maybe these aren't as common as I thought. $\endgroup$ Commented Nov 18, 2017 at 10:46