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Comment by Retric | Hacker News Reader

The article is correct, their explanation is poor. When you hear a sound with your eyes closed you can normally locate where it’s coming from. As in close your eyes and snap your fingers. Now suppose someone sets off a bomb some distance from you. You see the explosion or lightning flash etc but it takes a while for sound to show up. For stationary objects it doesn’t really matter you can still locate direction just fine.

Aircraft in level flight are also loud enough to be heard at distance sufficient to notice a delay. If the aircraft is flying by you hear sound from exactly one instant, but it like the explosion it was produced in the past. So if you close your eyes and try to locate the aircraft by sound you will point to wherever it was when it produced that sound not where it is right now.

The same is true of every sound you hear, but normally distances are short enough and speed are low enough it just doesn’t matter.

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It's just completely unrelated to why the aircraft passes you before you hear it.

They never suggested the first time you would hear it was the aircraft was already past you.

However, the maximum difference in angle between it’s current location and the location the sound comes from is just after it flew past you.

The title is "How far behind a plane is its noise?" It's not. It's ahead of the aircraft.

The noise is always pointing behind the aircraft or any moving object due to lag.

Light also encodes the direction to an objects past location, even though light is always moving faster than the object.

> The noise is always pointing behind the aircraft or any moving object due to lag.

I don't know what a pointing noise is.

> Light also encodes the direction to an objects past location, even though light is always moving faster than the object.

Yet the light is never outrun by the object, much like the subsonic plane never outruns its noise. However, here, the (fast) noise arrives after the (slow) plane. In your analogy, that would correspond to the (slow) object arriving before the (fast) light.

The direction of propagation is the way sound and light point to something. It’s why people listen to music in stereo rather than mono which sounds weird and also why telescopes work. Again simplified, but when you hear a sound with your right ear before the left that tells you something.

> arrives after the (slow) plane

That has nothing to do with the article, just your misunderstanding of what was described. At every instant in time the sound you hear corresponds to something produced by the aircraft at a specific moment in the past. Thus when someone is listing right now they hear a specific sound not the aircraft taking off.

Light from an aircraft also takes time to arrive. It might not seem relevant because of the speeds and distances involved, but the exact same thing is happening and it is relevant for light in other contexts.

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