How do environmental conditions affect the way we hear traffic noise?

[Larry Bishop]

I'm curious as to why I can hear traffic noise sometimes and not others. The busy hwy is about a mile away across a field. Sometimes it sounds like it is at my door but other times I can't hear it but still see a lot of traffic. How does environmental conditions (temp, no wind, etc) affect the sound?

Thanks

 

[tech99]

I'm curious as to why I can hear traffic noise sometimes and not others. The busy hwy is about a mile away across a field. Sometimes it sounds like it is at my door but other times I can't hear it but still see a lot of traffic. How does environmental conditions (temp, no wind, etc) affect the sound?

Thanks

The sound waves traveling to you closely over the ground are probably subject to some attenuation due to obstructions along the path. Sound waves traveling slightly upward will be stronger but will normally pass over you. In some conditions, however, they can be bent down to Earth by refraction. For this to happen, sound must travel slightly faster as you go higher. Refraction can arise if the wind blows from the road towards you, or if the air is warmer higher up i.e. there is an inversion. This might happen, for instance, after Sunset when the ground cools rapidly.

 

[sophiecentaur]

For clarification: Sound travels at different speeds for different air temperatures. It goes faster in warmer air. Refraction can easily occur when there is a gradient of temperature with height. With colder air near the ground, the sound can be bent downwards and focussed at a distant point. If there is a colder layer above, the sound can be bent upwards and away from the ground.
This effect can also happen when there is a wind. It gives the impression of blowing the sound 'towards you' because it is traveling slower near the ground ( drag) and leads to refraction and bending of the sound energy down towards the ground.