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Comment by karaterobot | Hacker News Reader

> It’s not to say that trees don’t crack—but rather that spooky noises long attributed to trees may emerge from the night sky itself.

Well, it seems like he demonstrated that the night sky itself can make sounds under certain conditions, not that these sounds are always the night sky.

By the way, I don't recall ever hearing the supposed tree cracking sound in an area where there were no trees. If it's always just the sky, you'd expect to hear it at least occasionally on the plains, or coming from 250' in the air above you when you're on a frozen lake.

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I think that framing comes from the article rather than the Aurora researchers (I skimmed some of their papers & didn't see it mentioned), but the article claims:

> Indeed, these loud cracking sounds are often attributed to large pressure splits in tree trunks, caused by sap freezing and expanding inside the tree’s interior. But while freezing sap in trees has been found to produce sounds at ultrasonic frequencies, outside of the range of human hearing, scientists have found no evidence this phenomenon might make sounds that are audible to the human ear.

Though if you told me you'd heard a tree make a groan or a crack, I'd be inclined to believe it, it doesn't strike me as outlandish.

That quote seems odd to me. Specifically,

1. The sound is attributed to trees cracking

2. The cracking is caused by sap freezing and expanding

3. Sap freezing produces sound in the ultrasonic range

4. So it's not the trees

Point 4 is not a valid conclusion from 1-3, because it was never stated that the sap freezing is what is being heard. Rather, it's the trees cracking, which is _caused_ by the sap freezing... but its own thing with its own sound.

I think you missed the last bit:

> scientists have found no evidence this phenomenon might make sounds that are audible to the human ear.

Which I take to mean they’ve measured ultrasounds but no audible sounds.

"Found no evidence" and "didn't even try to measure" isn't really the same, is it?

I'm not sure why you say I missed that. I didn't. Can you expand on what you meant by your reply.

To me, your reply actually highlights what I was talking about; because your use of "this phenomenon" is _somewhat_ ambiguous.

1. "this phenomenon" can be the sound of "large pressure splits in tree trunks, caused by sap freezing and expanding" (presumably audible to humans)

2. "this phenomenon" can be the sound of "freezing sap in trees" (presumably not audible to humans)

Seems like they would've recored both in the field, no? If they were recording sap freezing in the field, presumably the mics would pick up on other parts of the tree undergoing stresses and making audible sounds.

For that to have not been the case, either they would've had to freeze sap in the lab, or they would've had to go way out of their way to isolate recordings of just the sap in the field without the rest of the tree (is that even possible with normal recording tech?)

Point 4 seems to have been made by you, not the article.

I guess maybe it depends on how you read it? To me, it read like

> This sound is often attributed to X. But <some component of X> doesn't make sound that humans can here.

The "people say it's this, but actually ..." reads, to me, like they're saying it's _not_ this. Which was point 4.

> or coming from 250' in the air above you when you're on a frozen lake.

If you're on a frozen lake, you much prefer the crack coming from above than below.

And then of course, there are the "ice making" sounds that do come from below. And different lakes and ponds even seem to have somewhat different voices in that way.

And something equally chilling -- the sound of a glacier cracking (more of a deep thud sound).

Depends whether the landscape is prone to inversions. They happen most often in wide and enclosed valleys.

Maybe trees are affecting the rate and intensity of Inversion.

Well thunder and lightning comes from the sky and lightning is caused by static effects of ice crystals in thunder clouds. So maybe it's something similar.

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