Sonic booms and air pressure

  1. Correct me if I'm wrong I'm only 15. But doesn't air pressure affect the speed at which is needed to make a crack/boom. If so it would be easier at 50k feet to go faster, but you would not created a cone until you reach a higher speed. And vice versa with say. 5k feet.
     
  2. jcsd
  3. Filip Larsen

    Filip Larsen 985
    Gold Member

    Mach speed do indeed vary with altitude. In the standard atmosphere [1], mach speed at 50k feet altitude is around 87% of the sea level value, i.e. the speed of sound is slower and not faster at 50k.

    For further information I recommend you search for "mach speed at altitude" which should give you references like speed of sound at wikipedia [2] and tables of mach speed [3].

    [1] http://en.wikipedia.org/wiki/International_Standard_Atmosphere
    [2] http://en.wikipedia.org/wiki/Speed_of_sound
    [3] http://www.aerospaceweb.org/question/atmosphere/q0112.shtml
     

  4. Thank you!
     
  5. russ_watters

    Staff: Mentor

    Note that most factors tend to roll-up into one: temperature. Since a shock wave is air molecules bouncing off each other, their speed depends on temperature. At higher altitude, pressure is lower, but then so is temperature. The wiki link explains it and includes a graph showing it.
     
  6. While true, this is commonly misunderstood. Many people think that the speed of sound is slower due to the lower density of the air at altitude, and this is incorrect. In reality, as russ said, the only really significant factor is temperature. The speed of sound scales as the square root of the temperature, and since it is colder at high altitudes, the speed of sound is correspondingly lower.
     
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