Why does the noise from the motorway sometimes sound very loud and sometimes it is hardly audible?

Answer 1

The wind direction changes.

Answer 2

I'm assuming that you refer to road noise (i.e. the noise that you hear in the cabin of the car as you are driving).

This consists of two factors - wind noise and tyre/wheel noise.

Wind noise will generally be fairly static whilst in the same car (provided a constant speed is used as a basis for comparison), but will be affected by meteorological conditions to a certain extent. The most notable would be the wind itself - driving into a headwind would generate more noise than driving with a tailwind.
You would also see a noticeable effect from a physical change to the vehicle such as fitting a roof rack or towing a caravan.
Tyre noise will have much more of an effect. This is, for the most part, caused by the type of road surface that you are driving on. Generally, road surfaces consist of a composite of materials chosen for specific properties that are considered important at the time that road was commissioned. Some older surfaces resemble concrete and are quite hard wearing. However, they create a lot of road noise and do not dissipate road noise very easily. Some newer motorway surfaces are noticeably very quiet. A lot of these contain, in part, parts of used tyres that have been shredded. These have excellent water dissipation and very low road noise properties, which have been shown to reduce driver fatigue. However, there has been some suggestion that such surfaces are not sufficiently hard wearing for long term use.

Answer 3

Changes in the wind direction (which affects the sound) is the most logical one that I can think of at the moment. Also less traffic volume or smaller vehicles on the motorway i.e on a summers night (virtually no wind to assist the travel of the noise) and smallish vehicles on the motorway will result in a low noise (measured in decibels incidentally). Yet on a windy evening (the wind blowing in your direction) and larger vehicles on the motorway i.e box vans and large trucks then the noise will be louder.
It's probably a poor answer, but thats all I can think off.

Answer 4

This can depend on a lot of different things - some obvious like how much traffic there is etc. but the way sound travels can also be affected by atmospheruc conditions such as wind and temperature. On a still, cold day sound travelling up from the road can be bent over so it can be heard from further away than normal. This is caused by temperature gradients in the air causing sound, which normally travels in a straight line, to bend. A similar effect causes light to bend which is what makes mirages on hot days.

Answer 5

The reason the noise levels change is a result of Refraction and depends on the temperature outside at the time.
At night the ground tends to cool off quicker with the air temperature increasing with height.
This causes the sound waves which travel at different speeds though different air densities (thinner = hotter, thicker = colder) which causes the bottom of the sound wave to move slower in the cooler air. This causes the overall sound wave to bend down and focus to where you can hear it better.

Answer 6

I was at a party with a group of meteorologists and they told me to wait for the "temperature inversion" and the sound of the beltway would get louder. And it did, right about when they said that it would (makes going to a party of geeks actually fun -- nah, not really). The physics behind this is explained in the article at the link below. A temperature inversion usually occurs soon after the sun sets, and it is defined as when the air temperature is coolest right next to the ground and warmer as you increase in height above the ground. The article also says that echos from temperature inversions may have affected the outcome of several civil war battles.

Answer 7

Many times, cloud cover can affect how sounds travels. If there is cloud cover, the noise can bounce off the clouds back to the ground (or in various directions).

Do I have proof of this? No. My answer comes from years of casual observation - my airport is 4 miles from my house. Sometimes I can hear it, sometimes not. I've noticed the cloud cover during those times of observation.

Answer 8

I agree with the comments here. Also consider any natural or man-made 'fencing' along the motorway, which will absorb sound to a certain degree.

Answer 9

Depending on time of day ie night or day and also weather conditions. I believe that if it turns cooler the noise gets louder. It's something to do with physics but I don't know entirely why!

Answer 10

sundwaves r acrried by air.. so when there is lot of air around the sound is loud n when there is less air the sound is low