Pitch change opening thermos flask

Question

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.

geshel · Answer

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.

Answered by geshel on May 3, 2021

Pitch change opening thermos flask

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