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Old 02-08-2019, 03:29 AM   #1
xoxoxoBruce
The future is unwritten
 
Join Date: Oct 2002
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Sound Travels

My buddy was down here gathering firewood left behind by the new sewer line they ran through my property. A neighbor stopped to talk to him and told him he and his wife really enjoyed when I had the band rehearsing here. They would mix drinks and sit outside grooving on the tunes. News to me.

When we make noise, loud party or tuning an engine, we usually try to take neighbors into consideration, especially in late or early hours. But it's hard to figure how far and how much of the sound travels. It gets attenuated by distance, building, foliage, etc. Because there is no neighbors there we don't think about up.

I came across this today...

Quote:
Glaisher wrote on his memoirs of balloon flight, Travels in the Air:
I find that the intensity of various sounds emitted at the surface of the earth is carried up to very great heights in the atmosphere. The whistle of a locomotive rises to near 10,000 feet, the noise of a railway train to 8,200 feet, the barking of a dog to 5,900 feet; the report of a musket is heard to about the same height; the shouting of men and women can be heard sometimes as high as 5,000 feet, and at this altitude the crowing of a cock and the sound of a church bell are audible.

At a height of 4,550 feet the roll of a drum and the music of an orchestra are distinctly heard. At 3,255 feet in altitude, a man's voice may make itself heard; the rolling of a cart on the pavement can be distinguished somewhat higher; and in the stillness of the night the course of a river, or even that of a small stream, produces at this elevation almost the effect of a high waterfall. At a height of 3,000 feet the croaking of frogs in a morass is heard in all its intensity, and even the sharp note of the mole-cricket is distinguished easily at an altitude of 2,500 feet.

What Glaisher and his companions experienced was the effect of humidity on sound level. It has been observed that as humidity increases the sound level also rises. Clouds and fog being more humid “collects sound with such intensity,” explains Glaisher, “that whenever, in passing through a cloud, we have heard a band playing in a town beneath us, the music always seemed to be close at hand.”
“Lower humidity absorbs more sound, especially at higher frequencies, because of "molecular relaxation" in the gases in the air (a level of 10% humidity absorbs the most),” explains NPS. “A substantial change in atmospheric pressure, equivalent to thousands of feet of elevation gain, has a small influence on noise level for most sources, but substantially affects the received levels of those sounds.”
Sound carries better in humid air? I'd have thought it was the other way round. Mother Nature is sneaky.
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