henpol747
Topic Author
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Question About Sonic Boom

Wed Jul 14, 2010 11:53 pm

Hi there,

Can the sonic boom caused by reaching Mach I be heard inside the aircraft reaching that speed?

Thanks
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sw733
Posts: 5310
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RE: Question About Sonic Boom

Thu Jul 15, 2010 12:22 am

No. The reason being that a sonic boom is a result of air compression, and when you are flying at the speed of sound, the air is also traveling along with you...as that air compresses behind the aircraft, the noise is created.

That can sound kind of confusing, but I found a good analogy. Say you are on a boat cruising along...you are creating wake, but you don't feel it, but those around and behind you do as the wake travels away from you.

Re-reading this, I have done a terrible, terrible job explaining...but in short, NO, you do not hear a sonic boom if you are on an aircraft creating one.
 
kiwiinoz
Posts: 1999
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RE: Question About Sonic Boom

Thu Jul 15, 2010 12:39 am

I always thought this:

At the point when a sonic boom is created, the vessel upon which you are travelling is now travelling faster than the speed of sound. This being the case, the sound of the sonic boom is not heard as the sound wave cannot "catch up" with the aircraft. Presumably, if the aircraft were to slow considerably immediately after the boom was created, it would be possible
 
Fly2HMO
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Joined: Sat Jan 24, 2004 12:14 pm

RE: Question About Sonic Boom

Thu Jul 15, 2010 12:43 am

Just look up on the hundreds of videos of aircraft going supersonic shot from the interior, be it fighters or concorde. There is really no discernible noise difference.
 
Noise
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RE: Question About Sonic Boom

Thu Jul 15, 2010 12:58 am

sw733 explained it correctly.
 
vikkyvik
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RE: Question About Sonic Boom

Thu Jul 15, 2010 1:54 am

Quoting KiwiinOz (Reply 2):
At the point when a sonic boom is created, the vessel upon which you are travelling is now travelling faster than the speed of sound. This being the case, the sound of the sonic boom is not heard as the sound wave cannot "catch up" with the aircraft. Presumably, if the aircraft were to slow considerably immediately after the boom was created, it would be possible

While the individual soundwaves are out-paced by the aircraft, the actual sonic boom (the near-instantaneous shock caused by the buildup of pressure waves) travels just as fast as the aircraft, no matter how fast it's going.

Quoting noise (Reply 4):
sw733 explained it correctly.

Sort of. If you have a bow shock, or a shock over the wings, or whatever, that shock starts right at the aircraft skin (or ahead of the aircraft, as may be the case with a more blunt-nosed aircraft). So you can't really say the shock is behind the aircraft, as it starts right over the aircraft at some point.

Same with a boat's wake - there's usually a large bow wave, which, for the first couple seconds, isn't actually behind the boat.

The most basic reason you can't hear a sonic boom inside the plane is simply because there is no shockwave inside the plane. There's no supersonic airflow in the airplane cabin. And since the the shockwave itself is the sound, if the shock doesn't pass over you, you don't hear it.

Quoting henpol747 (Thread starter):
Can the sonic boom caused by reaching Mach I be heard inside the aircraft reaching that speed?

And just to possibly clarify (not sure how you've learned this), the sonic boom starts once the aircraft reaches Mach 1, and stays with the aircraft until it drops below Mach 1 again. It's not an instantaneous thing - it just seems that way on the ground because it is moving at the speed of the aircraft.
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tdscanuck
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RE: Question About Sonic Boom

Thu Jul 15, 2010 2:05 am

Quoting henpol747 (Thread starter):
Can the sonic boom caused by reaching Mach I be heard inside the aircraft reaching that speed?

No.

Quoting vikkyvik (Reply 5):
if the shock doesn't pass over you, you don't hear it.

This is the key...if you're on the aircraft, you're moving with the shockwave. So there is no pressure *change* for you, and it's the change that we hear as sound. If you're on the ground (or moving relative to the aircraft), the shock passes over you and you hear the boom.

Tom.
 
ThirtyEcho
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RE: Question About Sonic Boom

Thu Jul 15, 2010 8:30 am

If you go faster than a Police car chasing you can the Police car catch up to you? No.

If you go faster than the speed of sound can a sound from behind catch up to you? No.

If you go faster than the speed of light can you see your face in a mirror? No.
 
tdscanuck
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RE: Question About Sonic Boom

Thu Jul 15, 2010 3:21 pm

Quoting ThirtyEcho (Reply 7):
If you go faster than the speed of sound can a sound from behind catch up to you? No.

Although that's true, it's not a factor in why you can't hear the boom when you're onboard. The first shockwave starts at the nose (or slightly ahead), and as long as you're inside the mach cone (which the airplane is), pressure disturbances from the shock wave reach the aircraft. The reason you don't hear anything is that the pressure's not changing for you.

Tom.
 
vikkyvik
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RE: Question About Sonic Boom

Thu Jul 15, 2010 4:32 pm

Quoting tdscanuck (Reply 8):

Not to mention (far as I remember), if you're somehow moving at Mach 2 on the ground, and an airplane passes over you at Mach 4, you certainly will hear the boom, as the sonic boom itself moves at the speed of the airplane creating it.

Quoting tdscanuck (Reply 8):
and as long as you're inside the mach cone (which the airplane is), pressure disturbances from the shock wave reach the aircraft.

Did you mean to say "pressure disturbances from the shock wave don't reach the aircraft"?
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
tdscanuck
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Joined: Wed Jan 11, 2006 7:25 am

RE: Question About Sonic Boom

Fri Jul 16, 2010 2:24 am

Quoting vikkyvik (Reply 9):
Quoting tdscanuck (Reply 8):
and as long as you're inside the mach cone (which the airplane is), pressure disturbances from the shock wave reach the aircraft.

Did you mean to say "pressure disturbances from the shock wave don't reach the aircraft"?

No, I meant "do". Pressure disturbances from a supersonic source (like the shockwave) still propagate outwards at Mach 1. If you propagate backwards from the bow shock origination point (the nose), you get to the aircraft. If this weren't true, you wouldn't have any wind noise at all when you fly supersonic, and that's obviously not true.

Tom.
 
vikkyvik
Posts: 11909
Joined: Thu Jul 31, 2003 1:58 pm

RE: Question About Sonic Boom

Fri Jul 16, 2010 9:07 am

Quoting tdscanuck (Reply 10):

Ohhh, gotcha, I clearly didn't grasp what you meant.

Sorry about that!  
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Maverick623
Posts: 4651
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RE: Question About Sonic Boom

Fri Jul 16, 2010 10:55 pm

Quoting sw733 (Reply 1):
Say you are on a boat cruising along...you are creating wake, but you don't feel it, but those around and behind you do as the wake travels away from you.

This is the most correct analogy, because a supersonic shock wave and a boat wake are exactly the same type of shock wave, formed in nearly the same way.

Quoting ThirtyEcho (Reply 7):
If you go faster than the speed of light can you see your face in a mirror? No.

Completely different set of rules. Suffice it to say that, if indeed you did travel faster than light (or even if the mirror did), you would still see your reflection, just passing in the opposite direction. And extremely distorted. If you're holding the mirror, nothing would look out of place, because the speed of light is the same relative to any particular frame of reference.
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MingToo
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RE: Question About Sonic Boom

Mon Jul 19, 2010 4:20 pm

Quoting ThirtyEcho (Reply 7):
If you go faster than a Police car chasing you can the Police car catch up to you? No.

If you go faster than the speed of sound can a sound from behind catch up to you? No.

If you go faster than the speed of light can you see your face in a mirror? No.

Mmmm, that last one is not such a simple answer. Firstly, faster than light travel is not possible within current understanding, so it isn't possible to determine what the answer would be. If it is possible, it is also time travel.

But if you were to travel very close to light speed, the effect of looking in a mirror would not be as you would expect. Intuition might tell you that the light from you face would take much longer to reach the mirror than usual because the mirror would be moving forward at near light speed and the light would be trying to catch it up.

But that is not the case. The light will take exactly the same time to reach the mirror as if you were stationary. The reason being that there is no stationary ... everything is relative. The fact that you are moving while perhaps someone else sees you travel by doesn't matter. You are no more special than they are .. you could equally be the one who is stationary and them the one that is moving. There are no grid lines in space to compare to, only the relative movement between things.

So with you holding the mirror a metre away you will see that the light takes 1/300,000,000 of a second since light travels at 300,000,000 metres per second.

The freaky part is when that other person looks as you whizz by them at near the speed of light. They see you going near to the speed of light and the light coming off your face. So how fast so they see that light going ? Your speed + the speed of light ? Nope, that's not allowed, that's not how the universe works. They also see it travelling at the speed of light. But since they see the mirror travelling close to that speed, they do see the problem of the light having to catch up with the mirror. So to them, it will take much longer than 1/300,000,000 of a second for the light to reach the mirror.

What seemed quick to you will seem much slower to them which is relativity.

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