The sound of a train blowing it’s whistle coming towards you is different than when the train is going away from you. I guess when it is coming towards you the air is compressed and just the opposite when it is going away. What about a sound that is being blowing by the wind. Would there be a different pitch up wind from down wind? Hope I made myself clear.
The example of the whistle is the Doppler Effect. The link provides a good explanation as to how it works. Whether or not wind affects sound in a similar way, I do not know.
A sound wave moving through air is a travelling, repeating pattern of compressions and rarefactions (the opposite of compressions) in the molecules that make up the air.
When a noisy object like a whistle on a moving train moves toward you, the compressions in the pattern arrive at your ear at a faster rate, raising the perceived frequency (pitch) of the sound. The opposite occurs if the noise is moving away from you.
As for a windy day, the actual molecules carrying the wave pattern are being blown around, so the pattern may lose all coherency before it ever gets to you.
It seems to me it would. The sound wave moves with the air mass and if the air mass is moving with respect to you as when the wind is blowing, the rate at which pressure changes pass you, *i.e. *the frequency, should be increased or decreased.
I should add that you wouldn’t have any way of knowing this because you don’t know what the original frequency is. With a train, for example, when the train passes you the frequency changes and the apparent frequency changes.
No, a constant wind will not cause a doppler effect…the peaks and troughs will reach you at the same frequency as they are emitted, but they’ll overall reach you faster or slower, depending on the direction of the wind.
Your’e absolutely right. The peaks and troughs are still the same distance apart. I could have thought of that if I’d just taken a little time to do it.
While we’re on the subject of sound and wind.
You’re all familiar with the fact that sound seems to travel much better down-wind, than up-wind.
Naïvely one might think that this is because the sound waves ‘get help’ from the wind, so that the speed of sound adds to the wind speed.
WRONG! The wind speed is normally on the order of 10m/s, whereas the speed of sound is 340m/s. An extra 3-5% makes no discernable difference.
Instead it’s the fact that the air moves slower closer to the ground than a little bit up. If you’re down-wind of the sound source, the parts of the sound waves that would otherwise have passed right over your head will be deflected downwards, as the upper part of the waves travels faster, and thus causes constructive interference closer to the ground.
The converse is also true. If you’re up-wind of the sound source, the bulk of the sound energy will be deflected upwards into the air, as the sound travels slower in the higher trajectories!
They aren’t the same distance apart - but they are the same time apart, which is what’s necessary for the frequency to be unchanged.