Can the speed of sound or light be slowed down?

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gitgeezer
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Re: Can the speed of sound or light be slowed down?

Post by gitgeezer » Tue Jul 19, 2016 5:54 pm

John Ross wrote:
gitgeezer wrote:its speed through air is influenced only by temperature, not by density.
Not so, pressure counts, because air is not an ideal gas. So sound would travel more slowly in higher levels of the atmosphere even if there were no temperature difference.
I was speaking of open atmospheric air. Most discussions of atmospheric air I have seen treat it as an ideal gas. Any deviations from ideal are considered so slight as to be safely disregarded. NASA has this to say: “The speed of sound is a constant within a given gas and the value of the constant depends on the type of gas (air, pure oxygen, carbon dioxide, etc.) and the temperature of the gas.”

Science Daily says: “Humidity has little effect on the speed of sound, nor does air pressure by itself. Air pressure has no effect at all in an ideal gas approximation. This is because pressure and density both contribute to sound velocity equally, and in an ideal gas the two effects cancel out, leaving only the effect of temperature. Sound usually travels more slowly with greater altitude, due to reduced temperature.”

John Ross
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Re: Can the speed of sound or light be slowed down?

Post by John Ross » Tue Jul 19, 2016 7:19 pm

gitgeezer wrote:This is because pressure and density both contribute to sound velocity equally, and in an ideal gas the two effects cancel out, leaving only the effect of temperature. ”
Right, I remember now.

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fast eddie
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Re: Can the speed of sound or light be slowed down?

Post by fast eddie » Tue Jun 13, 2017 8:38 pm

Sound speed in air varies with temperature. In materials it also varies with the Bulk Modulus. It is true that light also slows down in glass, water (and other liquids). The speed of sound in air is about 350 meters/s while in water is approximately 1500 m/s.
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