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Author Topic: speed of sound?  (Read 4956 times)

sharkeyandgeorge

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speed of sound?
« on: 21/04/2006 11:38:05 »
ok track this through and see if it makes sense.

now sound travels faster through a solid than a liquid and faster through a liquid than a gas due to the proximity of molecules allowing faster vibrational transfer yes?. so if we accept that as true then does it not follow that the speed of sound through atmosphere changes with air pressure and hence density i.e high air pressure is linked to sunny days so it would be harder to break the sound barrier in sunny weather than just before a thunderstorm.
 
does this make sense?

does anyone know to what degree pressure changes the speed of sound?

J.B.S Haldane on the perforated eardrums which were a consequence of his pressure experiments "the drum generally heals up; and if a hole remains in it, although one is somewhat deaf, one can blow tobacco smoke out of  the ear in question, which is a social accomplishment".


 

Offline harryneild

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Re: speed of sound?
« Reply #1 on: 21/04/2006 11:43:30 »
Also could i extend on this topic and ask if there is a maximum frequency of sound waves??

"Knowledge has to be improved, challenged, and increased constantly, or it vanishes." Peter F. Drucker
 

another_someone

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Re: speed of sound?
« Reply #2 on: 21/04/2006 12:36:42 »
quote:
Originally posted by sharkeyandgeorge

ok track this through and see if it makes sense.

now sound travels faster through a solid than a liquid and faster through a liquid than a gas due to the proximity of molecules allowing faster vibrational transfer yes?. so if we accept that as true then does it not follow that the speed of sound through atmosphere changes with air pressure and hence density i.e high air pressure is linked to sunny days so it would be harder to break the sound barrier in sunny weather than just before a thunderstorm.
 
does this make sense?

does anyone know to what degree pressure changes the speed of sound?




To my own surprise:

http://en.wikipedia.org/wiki/Speed_of_sound
quote:

In fact, assuming an ideal gas, the speed of sound c depends on temperature only, not on the pressure. Air is almost an ideal gas. The temperature of the air varies with altitude, giving the following variations in the speed of sound using the standard atmosphere actual conditions may vary.

Code: [Select]
Impact of temperature
 θ in C c in ms-1 ρ in kgm-3 Z in Nsm-3
 -10        325.4       1.341      436.5
  -5        328.5       1.316      432.4
   0        331.5       1.293      428.3
  +5        334.5       1.269      424.5
 +10        337.5       1.247      420.7
 +15        340.5       1.225      417.0
 +20        343.4       1.204      413.5
 +25        346.3       1.184      410.0
 +30        349.2       1.164      406.6

         θ is the temperature in C
         c is the speed of sound in ms-1
         ρ is the density in kgm-3
         Z is the acoustic impedance in Nsm-3
Given normal atmospheric conditions, the temperature, and thus speed of sound, varies with altitude:

Code: [Select]
Altitude Temperature                        ms-1 kmh-1 mph knots
 Sea level 15C (59F)                        340   1225 761   661
 11,000 m20,000 m
(Cruising altitude of commercial jets,
and first supersonic flight) -57C (-70F)    295   1062 660   573
 29,000 m (Flight of X-43A) -48C (-53F)     301   1083 673   585




quote:
Originally posted by harryneild

Also could i extend on this topic and ask if there is a maximum frequency of sound waves??




The following does not fully answer your questin, since it does not suggest a maximum frequency above which sound would not travel through the air, merely that above 28KHz complex sound will be distorted as different frequencies will travel at slightly different speeds.

http://en.wikipedia.org/wiki/Speed_of_sound
quote:

In a Non-Dispersive Medium Sound speed is independent of frequency, therefore the speed of energy transport and sound propagation are the same. For audio sound range air is a non-dispersive medium. We should also note that air contains CO2 which is a dispersive medium, and it introduces dispersion to air at ultrasound frequencies (> 28 kHz).
In a Dispersive Medium Sound speed is a function of frequency. The spatial and temporal distribution of a propagating disturbance will continually change. Each frequency component propagates at each its own phase speed, while the energy of the disturbance propagates at the group velocity. Water is an example of a dispersive medium.



More directly:

http://www.du.edu/~jcalvert/waves/soundwav.htm
quote:

At low pressures, absorption occurs when the wavelength becomes comparable to the molecular mean free path (about 66nm in air at STP).



Assuming a speed of sound of 331 m/s, then 66nm would equate to a frequency of about 5GHz.



George
« Last Edit: 21/04/2006 12:42:48 by another_someone »
 

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Re: speed of sound?
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