Sonic boom

[Splat]

Do you know any sites that CLEARLY explain how sonic boom happens, and also how it is perceived on the ground. i.e. why do people only hear it once rather than constantly?

Yours boomingly...

Splat

[Tonic Please]

They would hear it once? It makes two booms just incase you didnt know.

You can learn a lot about sonic booms by looking at the wakes boats leave in the water.

If you toss a pebble in a pond, little waves will form in concentric circles and propagate away from the point of impact. If a boat travels through the pond at 3 to 5 miles per hour, little waves will propagate in the same way both ahead of and behind the boat, and the boat will travel through them.

If a boat travels faster than the waves can propagate through water, then the waves "can't get out of the way" of the boat fast enough, and they form a wake. A wake is a larger single wave. It is formed out of all the little waves that would have propagated ahead of the boat but could not.


When an airplane travels through the air, it produces sound waves. If the plane is traveling slower than the speed of sound (the speed of sound varies, but 700 mph is typical through air), then sound waves can propagate ahead of the plane. If the plane breaks the sound barrier and flies faster than the speed of sound, it produces a sonic boom when it flies past. The boom is the "wake" of the plane's sound waves. All of the sound waves that would have normally propagated ahead of the plane are combined together so at first you hear nothing, and then you hear the boom they create.

It is just like being on the shore of a smooth lake when a boat speeds past. There is no disturbance in the water as the boat comes by, but eventually a large wave from the wake rolls onto shore. When a plane flies past at supersonic speeds the exact same thing happens, but instead of the large wake wave, you get a sonic boom.


Smooth skies,

Dan :O

[Splat]

Thanks Dan.

Do you know of any web sites with the info?

Thanks

S

[Tonic Please]

http://www.sciencenet.org.uk/database/Physics/Original/p00122d.html



http://www.madsci.org/posts/archives/feb98/886430559.Ph.r.html



http://www.physlink.com/Education/AskExperts/ae53.cfm


http://newton.dep.anl.gov/askasci/phy00/phy00143.htm


:)

Dan

[PAXboy]

Am I correct in understanding that, whilst each person hears only one (or two!) 'booms', due to the continued passage of the a/c, it leaves a constant wake of 'boom'? Hence people not liking SST overflying as it leaves this sonic footprint at all times that it is above M1?

[Notso Fantastic]

The Pocket Rocket (Concorde) leaves a horizontal cone-shaped sonic boom behind it (with Concorde at the apex). This is spreading out at the speed of sound (in a pressure front). It will spread down and reach the ground in a hyperbolic shape (viewed from above) that is moving at the same speed as the supersonic aeroplane. Any point on the ground in the carpet of the sound will hear a double boom as the pressure wave passes caused by the leading edge and trailing edge of the machine.

Is it something to get excited about? Supersonic ops are conducted away from populated areas to minimise nuisance, so it's not a problem, the peripheries of affected areas only receive a weak 'boom' (either end of that hyperbolic pressure wave moving over the ground). Ever stood on a pavement and heard a loud lorry moving past? Had one of those awful Jap motorbikes with a 'tweaked' exhaust accelerate past you? Worked with noisy machinery, or been troubled by it? Had trash pickups at 4 am? (favourite Australian trick). There are far more important sources of sound pollution, but Concorde always made a glamorous target for complaints!

[BOAC]

"Had trash pickups at 4 am?" - yes, and you can have her phone number if you want :cool:

[jungly]

Paxboy......answer: Yes!

[cwatters]

> Am I correct in understanding that, whilst each person hears
> only one (or two!) 'booms', due to the continued passage of
> the a/c, it leaves a constant wake of 'boom'?

Yes. It's a bit like a planes shadow. You only see the shadow as it passes over you on the ground but it's always got one. It also travels at roughly the same speed as the plane.

[DrSyn]

Some good answers here to Splat's question. The design (shape) and altitude of the craft also affects the perceived sound at ground level. I agree with Notso's post, in particular and add a couple of personal observations on the hype of Concorde's noise footprint that ultimately helped to screw it commercially.

One lazy weekend afternoon in Masirah Island (Oman), back in the '70s, I was sitting alone on the jetty, soaking up a bit of sun and dangling a hook in a vague hope of catching a suitably edible treat to take back to the mess kitchen. The water was almost calm and you could have heard a pin drop. Suddenly, there was a "distant" double-boom. If I had been having a conversation with someone, we would probably not have noticed it. My first thought was range activity on the mainland many miles away, but realised that was not a weekend option! I looked up and slightly north was a fast-moving contrail heading out across the Indian Ocean. It was the BA/SIA Concorde enroute from BAH to SIN. It would have been above FL500 by then. In even a small town, I doubt anyone would have heard it.

Conversely, the Shuttle, which has all the aerodynamic properties of a breeze block (may she soon return to service) initially approaches KSC descending through FL450 @ M2.0 / 15,000 ft/min and produces the most magnificent double boom, that echoes around the area for many seconds afterwards, rattling windows and provoking cries of (typically) "Good morning Florida!"

(Lest there be any doubt the last para is not intended to be a criticism)

[Splat]

Thanks for the replies.

Splat

[Notso Fantastic]

The shuttle sonic boom was a wonderful window rattler. On vacation watching on TV in Orlando, they announced the boom passing downtown. I got the kids out in the garden and told them to wait, and when it rolled over Kissimmee, they jumped! I was once alone in a yacht cockpit halfway back to Portsmouth from Cherbourg. Just done a 360 degree scan- absolutely empty, nobody in sight horizon to horizon. The double boom that hit us made me jump- never saw the source, but the channel is full of danger areas for sonic flying.
The effect is fun at airshows where somebody shows off approaching at high TAS. You see them coming, but can't hear them- then when they go past, the sound 'crashes' on you. Great fun- the Reds do it, but too gently (to save on coronaries)!

[CBLong]

This fantastic photo actually shows you the shock wave responsible for the sonic boom - although in this case it's being generated by the land-speed-record-breaking Thrust SSC (Supersonic Car):

http://freespace.virgin.net/john.coppinger/SSC%20Shock%20Wave%20copy.JPG

More here (http://freespace.virgin.net/john.coppinger/thrustss.htm)

[Tonic Please]

Id like to know what a speed camaera would think of that :ooh: