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Guest chris_kirk

Calculation of speed of sound in air.

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Hi all...A question for all you mathematicians, physicists and accomplished aviators! During a PPL ground brief today a discussion arose between my instructor, myself and four other students regarding the calculation of the speed of sound in air. I asked why the equation called for the local air temperature, but not the atmospheric pressure. I received an explanation that was extensive but left me feeling somewhat unsatisfied!I decided to research the matter online this evening and am now more confused than ever! The following quote appears to make sense:

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Excellent question. I found this site that lets you compute different air paramters at different altitudes and temperatures. Indeed I first tried sea level and then 5,000 meters - speed of sound seems to be identical provided temperatures are equal. The calculator requires you to input the air pressure (altitude) ONLY to compute temperature. We both learned something today ..http://www.digitaldutch.com/atmoscalc/index.htmMichael J.WinXP-Home,AMD64 3500+,Abit AV8, Radeon X800 Pro,WD 36GB Raptor,1 GB PC3200 http://www.reality-xp.com/community/nr/rsc/rxp-higher.jpg

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Hi Chris:I am no mathematician or physicist,but have played with the speed of sound and temp as a audio engineer.Sound waves and pressure-when something vibrates in air it creates a series of wave cycles.with each cycle,the air particles will move back and forth,alternately raising and lowering air pressure.think of the bee`s wings.the wings move back and forth,or oscillate,creating waves in the air.If its your average-size bee,the wings may oscillate at 250 times per second.In other words,the frequency of the bee`s wings is 250 Hz.As the wing moves out from the body,it compresses the air being pushed away,increasing the air pressure of those particles.The air behind the wings decompresses,or rarefies.When the wings return,they compress and rarefy air particles in the opposite direction as before.These pressure changes create sound waves,in the case of the bee,the waves travel through the air at 250 Hz and reach our ears.at that point,the ear drum,and eventually in turn cilia(tiny hairs in the inner ear),are stimulated 250 times per second,and our brain interprets this stimulation as sound.....If at 0c ,speed travels at 1087fps, add 2 feet for each degree of rise in temperature.15c would be approx 1117fps....30c would be approx 1147 fps....hope this helps......Brent

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>Hi Chris:I am no mathematician or physicist,but have played>with the speed of sound and temp as a audio engineer.Sound>waves and pressure-when something vibrates in air it creates a>series of wave cycles.with each cycle,the air particles will>move back and forth,alternately raising and lowering air>pressure.think of the bee`s wings.the wings move back and>forth,or oscillate,creating waves in the air.If its your>average-size bee,the wings may oscillate at 250 times per>second.In other words,the frequency of the bee`s wings is 250>Hz.As the wing moves out from the body,it compresses the air>being pushed away,increasing the air pressure of those>particles.The air behind the wings decompresses,or>rarefies.When the wings return,they compress and rarefy air>particles in the opposite direction as before.These pressure>changes create sound waves,in the case of the bee,the waves>travel through the air at 250 Hz and reach our ears.at that>point,the ear drum,and eventually in turn cilia(tiny hairs in>the inner ear),are stimulated 250 times per second,and our>brain interprets this stimulation as sound.....If at 0c ,sound>travels at 1087fps, add 2 feet for each degree of rise in>temperature.15c would be approx 1117fps....30c would be approx>1147 fps....hope this helps......Brent...ps correction== If at 0c(should be SOUND not Speed) travels at 1087fps....sorry doh!

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Thank you both for your replies. I had a couple of phone conversations with my instructor subsequent to my ground brief (great to have an instructor you get on well with... pity he's about to bu***r off to "proper job"; isn't that always the way?!) and had managed to get us both in a mess over this!!I think I'm getting a handle on it now, so I'll have to give HIM a brief to undo the damage I did!Thanks again,Chris Kirk

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