Effect of mass and velocity on intensity of sonic booms.

[Bear_Nenno]

I’m almost positive that the mass of an object will effect the loudness of a sonic boom and that very large objects will make a louder noise than smaller objects. Is this an accurate assumption?

What about speed? Does the same object make a louder noise at MACH 1.8 than at MACH 1.2?

Also, what happens at MACH 2.0? Does the sound get twice as loud?

[mks57]

Mass isn’t the same thing as volume. Shape has a great deal to do with the intensity of a sonic boom.

[Bear_Nenno]

So size is more important, then?

[Una_Persson]

OK, it’s been a long time since I took fluid dynamics, so I’ve Googled and looked up some things.

First off, mks57 is correct - shape is one of the most important factors in sonic boom noise. However, here is what Nasa has to say on weight:

There are several factors that can influence sonic booms – weight, size, and shape of the aircraft or vehicle, plus its altitude, attitude and flight path, and weather or atmospheric conditions.

A larger and heavier aircraft must displace more air and create more lift to sustain flight, compared with small, light aircraft. Therefore, they will create sonic booms stronger and louder than those of smaller, lighter aircraft. The larger and heavier the aircraft, the stronger the shock waves will be.

~

Increasing speeds above Mach 1.3 results in only small changes in shock wave strength.

Read about this and much more here: http://www.nasa.gov/centers/dryden/news/FactSheets/FS-016-DFRC.html

[Duke_of_Rat]

A rifle bullet goes faster than most aircraft, yet I’d be willing to bet I could shoot a rifle at 30,000 ft and you’d barely (if at all) hear it on the ground, and a lot of that noise would be the report of the charge, not the sonic boom of the bullet.

[aerodave]

Shape and velocity are the two most important things. Mass only matters indirectly, as mentioned above, because in concert with shape and velocity, it dictates the nature of the flowfield necessary to maintain lift.

In general, a big, blunt object moving at supersonic speeds will plow a lot of air ahead of it, making a stronger shock wave than a smaller, more streamlined object of the same mass and velocity.

Things can get more complicated than that, because particular specialized shapes can be made that are optimized for low “boom.” NASA’s SSBD (Shaped Sonic Boom Demonstator) made some relatively minor changes to the nose of an F-5 to make big differences in boom strength.

[Bear_Nenno]

@Duke of Rat

A rifle bullet goes faster than most aircraft, yet I’d be willing to bet I could shoot a rifle at 30,000 ft and you’d barely (if at all) hear it on the ground, and a lot of that noise would be the report of the charge, not the sonic boom of the bullet.

It was actually rifle bullets that inspired this question. Through observation, I notice that .308 rounds are a lot louder wizzing passed one’s head than .223. But these bullets are bigger and faster. So I wasn’t sure what the major reason was or if they were both important.

[Duke_of_Rat]

@Bear_Nenno

It was actually rifle bullets that inspired this question. Through observation, I notice that .308 rounds are a lot louder wizzing passed one’s head than .223. But these bullets are bigger and faster. So I wasn’t sure what the major reason was or if they were both important.

The .308 is faster than the .223? I always thought the little .223 would be faster, but maybe that’s just at the muzzle. Ah, some Palma Shooting velocities.

Anyway, I was thinking of back when I was a kid. Sonic booms from aircraft happened daily, many of them were quite loud, shaking the house, rattling the windows. Most of those sonic booms were probably generated by aircraft much slower than most center fire rifle bullets. And while I have no way of knowing what speed or what aircraft caused each sonic boom, some of the booms were definitely stronger than others. But all of them were stronger than anything I’ve witnessed from a rifle bullet (but then again I don’t have a lot of experience at the muzzle end of a rifle shot). So my first guess would be that size would make more of a difference than speed.

Now look at the space shuttle. It’s large, pretty dang large for a supersonic craft. When it re-enters, it’s doing mach 25 IIRC. And one of the signatures of it’s re-entry is a multiple sonic boom. Maybe somebody else will know what causes the multiple booms on that.