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This question may be better suited to the physics stack exchange but I figure I’ll ask here as well. So a month or two ago after trying to learn to overtone sing for a couple days I finally managed to produce perceptible overtones over a fundamental. In the following week I practiced my new skill pretty obsessively. A side effect I’ve found is that now, with some effort, I am able to hear the overtones in other instruments. For example, if I sustain a middle C on my upright piano, with conscious effort I can hear the first six partials (fundamental (C4), C5, G5, C6, E6, G6). I can barely hear the sixth partial and cannot hear any trace of the seventh or above. However, the partials do not sound like sustained pitches that slowly attenuate like I would expect. Instead there is a distinct pulsating effect where, for any partial save the first, the volume oscillates between between audible and inaudible at a frequency somewhere in the range of .1Hz–10Hz. What causes this effect? It seems similar to the dissonant beating produced by two sine waves that are slightly out of tune with each other. Is this what is happening i.e. the different C-partials, E-partials etc. are slightly out of tune with each other?

Here is a recording I took: https://drive.google.com/file/d/1ulnHyYwEmGaxgT-hB5XxpaeFVwTkTWDF/view?usp=drivesdk The oscillations appear to be around 0.5 Hz

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    Is your piano in tune? That is, are all three middle-C strings in tune with each other?
    – Aaron
    Mar 20, 2022 at 0:23
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    "cannot hear any trace of the seventh or above": this is by design. The hammer is positioned to minimize the seventh harmonic.
    – phoog
    Mar 20, 2022 at 0:35
  • @Aaron It hasn’t been tuned since 2018. I forgot that most keys strike multiple strings but this is likely what is is causing the effect i.e. the different strings and therefore partials are all very slightly out of tune with each other and thus produce a slow beating effect. Mar 20, 2022 at 0:51
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    @phoog That's fascinating! Can you source it, so that if I post it as a question, it can have a documented answer?
    – Aaron
    Mar 20, 2022 at 1:17
  • @phoog, ah, thanks for your reminder about the hammer position and avoiding the 7th harmonic! :) Mar 20, 2022 at 4:18

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Based on discussion in the comments, the issue is probably that the piano is out of tune ("It hasn’t been tuned since 2018"). Most keys have multiple strings tuned to the same pitch. Middle C, for example, has 3 strings. Given the description "similar to ... dissonant beating", it seems the likely culprit that the three are not in tune with each other.

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