MovieChat Forums > The Sound Barrier (1952) Discussion > Would they have heard it?

Would they have heard it?

posted 12 years ago by Jason_Radley
1 reply | jump to latest

Been reading up on Wikipedia about the speed of sound and sonic booms and noticed this:

Depending on the aircraft's altitude, sonic booms reach the ground two to 60 seconds after flyover. However, not all booms are heard at ground level. The speed of sound at any altitude is a function of air temperature. A decrease or increase in temperature results in a corresponding decrease or increase in sound speed. Under standard atmospheric conditions, air temperature decreases with increased altitude. For example, when sea-level temperature is 59 degrees Fahrenheit (15 °C), the temperature at 30,000 feet (9,100 m) drops to minus 49 degrees Fahrenheit (−45 °C). This temperature gradient helps bend the sound waves upward. Therefore, for a boom to reach the ground, the aircraft speed relative to the ground must be greater than the speed of sound at the ground. For example, the speed of sound at 30,000 feet (9,100 m) is about 670 miles (1,080 km) per hour, but an aircraft must travel at least 750 miles (1,210 km) per hour (Mach 1.12, where Mach 1 equals the speed of sound) for a boom to be heard on the ground.[1]

[–] ZurichGnome 8 years ago

I think your conclusion is wrong. Hearing the boom has nothing to do with the speed of sound on the ground. The boom occurs when the negative pressure zone behind the plane gets filled quickly with super high pressure air generated at the front of the plane. Once the boom occurs, its sound will travel until dissipated, and it has nothing to do with sound speed at any altitude.

Even if it takes longer to reach the ground from a higher altitude, if it reaches the ground it will be heard, even though the sound is no longer propagating at anywhere near the higher speed of sound at the higher altitude at which it was generated.

In the 1950s we used to hear sonic booms quite frequently from planes that were so high up that they weren't even visible. If a sonic boom is generated below about 40,000 feet, it will be softer than one generated at a higher altitude. Also, if the plane is flying far enough above Mach 1, no boom will be heard at all.

Some other bits of info re: sonic booms: the crack of a supersonic bullet passing overhead or the crack of a bullwhip are examples of a sonic boom in miniature.

Any sufficiently advanced technology is indistinguishable from magic. - Arthur C. Clarke