|MadSci Network: Physics|
First I want to congratulate you on your observing skills!! Most people have never noticed that the high frequencies of sound are attenuated through air. Next time you hear thunder notice that the farther away the lightning is the more low-frequency sound you will hear from the thunder.
Speaking of thunder, what I am going to tell you is taken from an article by Arthur Few called "Thunder" which was published by Scientific American in the July 1975 edition of the magazine, but has also been included in one of Scientific American's compendia. This particular one is called "The Physics of Everyday Phenomena" and was published by W.H.Freeman in 1979.
There are two major causes of the attenuation of all sounds through the atmosphere.
The first is called "classical" attenuation and is caused by the viscosity of air, meaning that air is not a perfectly elastic medium.
The second, and more dominant, cause of attenuation is called "molecular" attenuation. "Molecular" attenuation results from interactions of the sound waves with water and oxygen molecules in which internal vibrations of the molecules are excited.
The total attenuation of sound through air has an important frequency dependence, due to the two separate causes. The amount of "acoustic viscosity" and molecular vibration is dependent on frequency, and it turns out that the attenuation increases as the square of the frequency. So, a 100 Hz wave, for instance, will travel four times as far as a 400 Hz wave to obtain the same attenuation.
There are other reasons why thunder's spectral content changes with distance, about which you can read in the article. But for even fairly close events the squared dependence on frequency alters the spectral content quite rapidly with distance.
Well, I hope this helps you! I have not actually searched elsewhere on the Web but it wouldn't surprise me if you could find some technical sites that discuss the frequency dependence of attenuation of sound through air.
John Link, MadSci Physicist
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