Sound is a kind of wave, which is the vibration of air. When the air flows, the propagation of sound wave can be decomposed into the superposition of two kinds of motion, the flow of air and the motion of sound wave relative to the air.
In addition to conditions such as temperature and density, factors that affect sound propagation include "wind speed gradients." Wind speed gradients can change the path of sound through the air, or wind can cause sound to bend. This is the most interesting place.
We take the typical sound velocity of 340m/s, and there is a breeze of 5m/s in the air. The sound intensity brought by the downwind due to the wind is ≈1.03 times that of when there is no wind, which is really too small. But if the wind is particularly strong, the listener may not even be able to hear the spoken voice clearly because the wind is stronger than the human voice.
For sound waves with long travel paths (greater than 50 meters), when the wind is opposite to the direction of sound propagation (that is, against the wind), the sound waves will be refracted upwards. This is because the rough surface (big trees, buildings, wheat, etc.) will hinder the wind propagation and form turbulence Q, so that the wind speed near the ground is slightly lower than the upper air, or that the upper air speed is faster (near the surface The wind speed generally increases with distance from the ground), and this difference in wind speed creates a wind speed gradient. As mentioned above, the gradient will in turn affect the speed of sound. In the reverse zone, the greater the wind speed, the slower the sound wave propagation speed (actual sound speed = sound speed minus wind speed), so the sound will bend upward.
Theoretically, for a 6-level strong wind with a wind speed of 13 kilometers per hour, the average reduction of a few decibels per 100 meters can be reduced, but according to actual measurements, a loss of more than 20 decibels can be observed in a relatively long distance.
By the same token, when the wind is in the same direction as the sound travels (tailwind), the sound is refracted towards the ground, so conditions are favorable for the sound to travel to the ground and bounce off the ground, increasing the intensity of the sound (sound level).
Wind speed gradients cause refraction of sound, which makes it easier to hear sounds from sources (even from behind trees) downwind (downwind) and harder to hear upwind (upwind).
Of course, if the listener is close to the sound source and upwind, the sound will actually be heard stronger.
This is because reflections from the ground are in play. But as long as the distance is farther (300 meters), there will be a phenomenon of rapid sound attenuation (indicated by the shaded area in the figure below).
The above are theoretical analysis, the following is the actual measured data, the ordinate value should be the correction amount caused by the wind factor.
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In summary, the wind not only affects the speed of sound propagation, but also affects the direction of sound propagation.
