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If I return to my (closed) thermos flask for some more coffee, I hear a ringing sound as I open the flask (it is all metal), but the pitch of the ring increases as the sound fades - duration on the order of a second.

Is this easily explainable? There is likely a temperature equalisation and a pressure equalisation of the air, but the latter would be too fast, surely.

The thermos is a double-skin stainless steel type, and has kept the coffee too hot to drink straight away.

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    $\begingroup$ Any chance it's a consequence of the human ear not having a flat frequency response? That is, perhaps as the amplitude of the oscillations decrease your hear can no longer hear the lower frequencies as well as the higher? The frequency response of the human ear tends to peak in the few kHz range, I think. $\endgroup$ Oct 1 '20 at 17:37
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To fully answer we'd want to look at the frequency content of the sound versus time - eg a spectrogram. However my hunch is that the ringing sound is not a pure tone, instead it is composed of multiple overtones. Some overtones will attenuate faster than others, depending on the shape, material, and boundaries of the flask. If there is a mechanism which causes the lower overtones to attenuate sooner, it would cause the effect you describe.

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