If an aircraft were to accelerate through sub-sonic to multiplle mach speeds, would there be a sonic boom for each mach reached?
For example, if a plane accelreated from sub-sonic to Mach 5, would it generate 5 sonic booms?
For some reason I was under the impression it would, but I’m not sure if I’ve remembered silly.
IIRC a sonic boom is caused when something goes at the speed of sound, because the noise it causes for a while arrives all at once (or, alternatively, because it’s moving along the ‘crest’ of a sound wave) so mach>2 wouldn’t have an effect.
Anyone know for sure?
If the vehicle is travelling at a speed considerably greater than the speed of sound, it should be leaving its own sound behind and (I think) the stationary observer should hear more of a whoooOOOOOOMPHHHHhhhhh…
Thinking about it again, maybe you do get another boom - if you’re getting ahead of the sound, you’ll reinforce every other peak… agh, physics is only getting me so far. Sorry. Cite, anyone?
As I recall, with any supersonic object travelling overhead, you’ll hear two loud bangs in quick succession. They’re leading and trailing edges of a shockwave caused by the supersonic transit of the aircraft.
Ok,
Time to cut the disinformation campaign. SpectBrain is on the right track. Once an aircraft, or anything else travels supersonic, it generates a ** constant ** series of sonic booms along its path, not just one. Multiples of Mach have nothing to do with it. Once the aircraft exceeds Mach 1, then the sonic booms are constantly generated because air ahead of the aircraft simply cannot move out of the way in time.
Shagnasty is right on, and I’d like to aid in the fight against the common misconception of sonic booms. The problem stems for the term itself; “boom” implies a short transient phenomenon. This is an observer-based description because the shock wave moves over a stationary observer and sounds like a boom. If you could see the shock waves created by the leading edges of aircraft, you would see them sweeping back behind the craft at an oblique angle and where they hit the ground is where the sound is heard. They move with the aircraft. There is not a single “boom” as the aircraft breaks the sound barrier, but a continuous shock wave generated by the supersonic vehicle and moving with the vehicle. The shock wave is simply a (near) discontinuous change in pressure, which is perceived by the ear as a boom.
An analogy might help. Throw a pebble in a pond and it generates a wave moving out in a growing circle. If you are standing in the water, you feel the “boom” as the wave moves past you, but the wave still exists. It only seems like a brief event to a stationary observer.
As the aircraft speed increases to higher and higher Mach numbers, the shock waves from the leading edges are swept at higher and higher angles. This means that for a stationary observer, the “boom” will occur further behind the aircraft because the shocks at a sharper angle intersect the ground much further behind the aircraft than if a slower aircraft travelled at the same altitude. There are multiple shocks coming off the vehicle (nose, wing, tail, etc.) but for most vehicles these are so close together they are indistinguishable. The shuttle is long enough that the shocks are spaced far enough to hear distinct booms, but most supersonic aircraft are not.
Ah - thanks for that - much appreciated. I remembered some comment in New Scientist about multiple “booms”, which referred to mulitple parts of the aircraft, but wasn’t sure whether it referred to multiple machs.
The mach angle is easy to calculate. If you want to sketc it draw a circle with a radius that represents the speed of sound. Draw a line from the center with length proprotional to the mach number, i.e. 2x the radius for mach 2. Now draw an angle from the end of the line tangent to the circle on both sides. Calculate the angle from centerline with arcsin(1/mach) This gives the angle of the shockwave to the flight path. Double it for the included angle of the cone.
Another analogy: Ever been water-skiing? If a boat passes you at speed, then the wake will only hit you once. But there’s always a wake being produced by that boat, and you can see it approaching you before it hits and moving away afterwards. The sonic boom produced by an airplane is fairly similar to a wake from a boat, except it’s sound waves instead of water waves, and it’s three-dimensional.
Excellent analogies, but if you want to actually see an actual sonic boom, look here
That’s a little misleading in this context. What’s being visualized in that photo is the transonic regime when there is a normal shock perpendicular to the flow. Most vehicles accelerate through this point as quickly as possible because of the stress on the airframe. In a fully supersonic flow, the shocks are oblique (swept at an angle, not perpendicular) and attached to the nose/leading edges.
Just a follow-up to explain what I meant by “misleading”. The condition in that photograph is typically transient because the aircraft continues to accelerate and the shock configuration changes. The swept shocks are weaker than normal shocks (i.e. the pressure and density changes across the shock are not as great) so they are less likely to exhibit the condensation shown in that photo. Sonic booms are continuous phenomena as explained above, but pictures like this are typically labelled in such a way to make it seem like this single point when the speed of sound is exceeded is the cause of the boom. This reinforces the misconception that the boom is a single event rather than a moving wave.
This page has pictures of a sonic boom taken using a unique Schlieren setup: http://www.dfrc.nasa.gov/Newsroom/FactSheets/FS-033-DFRC.html
paperbackwriter, what the heck kind of camera was that guy using? Assuming that the airplane is no more than 30m long and travelling at least Mach 1, and judging from the sharness of the picture, I estimate that the exposure couldn’t have been more than about a millisecond. Isn’t that awfully fast for a camera?
Trivia: this effect happens just under Mach 1, when the vehicle itself is subsonic but the accelerated flow regions around the aircraft are supersonic leading to normal shocks standing on the body. As the vehicle itself goes supersonic, the normal shocks move forward until they’re on the leading edges and then become oblique. That of course has very little effect on your calculation of camera requirements.
Not even particularly fast. 1/1000 sec top shutter speed is standard for all 35mm film cameras and digital with some having shutter speeds as fast as 1/8000 second. Heck, even the ancient Speed Graphic press camera of Weegee fame had a focal plane shutter capable of 1/1000 second exposures. Leaf shutter cameras typically top out at 1/500 second but even with slow shutter speeds a sharp picture can be taken with careful panning though the background will be have motion blur. Video cameras often have a 1/10,000 second virtual shutter speed and can take very sharp still frames, particularly with progressive scan cameras.
FWIW the picture paperback writer linked to doesn’t really show the transonic/supersonic shock wave but water vapor reacting to the pressure change. AFAIK it’s actually in a reduced pressur area behind the wave. You can often see this forming above wing leading edges at high angles of attack at subsonic speeds, another reduced pressure area. She shape of the vapor cone indicates a shock wave but is not the shock wave itself.
One thing you could do is check out the Discovery channel and/or the Discovery Wings channel when they play their shows on flight. Severial of the shows dealing with supersonic flight tend to review how sonic booms are made and why. They also include CGI animations that show how they’re created.
Well, the craft will emit Cerenkov radiation once it passes c…
