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When a tuning fork is struck I hear two tones. From a distance I can hear a high octave frequency of the pitch of the tuning fork. Though, if I listen to it closely (closer to my ears), I also hear a lower octave frequency of the pitch of the tuning fork. I want to clarify that what I'm hearing is correct.

Also, is there something about “pure tones” that produces this effect, and if so what are the harmonics of a pure tone/tuning fork? Just from brief reading on the topic, I’ve seen it said that tuning forks' overtone(s) (I'm assuming there is only one overtone) ring out quickly. Then why am I hearing a high octave version of the fork's pitch and a low octave version of the forks pitch until it rings out?

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    Tiiiiiiinnnnnngggggggggggggg
    – AakashM
    Oct 16, 2023 at 9:34

3 Answers 3

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Although tuning forks produce very pure tones compared to other instruments, they do not produce actual pure tones. One reason that you can hear the overtones easily, despite how quiet they are, because human ears are very sensitive in the frequency range that they exist in (2-5kHz). The frequency separation between the overtones and fundamental also prevent the fundamental from masking the overtones effectively.

The reason these overtones exist at all is because the tuning fork has several modes of vibration- different ways in which the metal can bend and produce periodic oscillations. Each mode sounds a different frequency. You suspect that there is only one overtone, but that is not exactly right- there are many overtones, but one (the "clang mode") tends to ring longer and louder than the rest. This clang mode isn't necessarily a harmonic of the fundamental, and it is totally unrelated to the fork's use as a musical tool.

A similar question has been asked on physics stackexchange, and there is some more technical detail on the overtones produced here. If you want to find the overtones of your particular tuning fork, you can record it and analyze the recording with a software spectrogram/spectrograph/spectrum analyzer.

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    The relationship of the clang mode to the fundamental mode can vary between tuning fork designs. You can't rely on it being a fifth.
    – Edward
    Oct 14, 2023 at 21:24
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    Yes, it's most likely that the high tone you hear is the clang mode. The lower tone is the fundamental mode.
    – Edward
    Oct 14, 2023 at 21:24
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    @Lecifer what is the purpose of a 221.23 Hz tuning fork?
    – phoog
    Oct 15, 2023 at 9:28
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    @phoog -- you really do NOT want to know. frequencyheals.com/product/221-23-hz-frequency-of-venus Oct 15, 2023 at 16:51
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    @phoog uh oh Sweetwater set me up! (Jk)… But honestly I assumed it wasn’t a huge deal as I never perfectly tune my guitar strings. Maybe I should pay more attention to that. I’m gonna get a 440hz fork soon as well.
    – Lecifer
    Oct 16, 2023 at 4:17
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The tone the tuning fork is tuned to is generated by transversal waves in the tips of the metal fork. It needs amplification by a resonance body (or as you did: close listening). This is the low tone.

Depending on how you hit the tuning fork (eg with a hard object, rather on the top than sideways), also longitudinal waves can be generated. These are not the swing of the fork tips, but they travel along the fork with the speed of sound in the metal. This is the high tone.

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    Longitude modes are unlikely in a traditional tuning fork.
    – fraxinus
    Oct 15, 2023 at 14:05
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I'm guessing that the higher tone is an overtone the first of which is the same note an octave higher (the first overtone of an A is an A for example). However, I don't what's causing the lower note. It's possible that the lower note is fundamental pitch note or that something else entirely is happening.

I would recommend you ask the Physics stack exchange, this seems like more an interesting physics question than a music one.

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  • I actually posted it on there first, but I thought maybe I was posting it in the wrong place lol! I’ll repost it on there again. And what you said is my speculation. If the fork is closer to your ear, you hear the Low A and the high A. If it is further away, you only hear the high A (the low A is still ringing but only perceivable up close which I’m assuming has to do with size of the fork). The high A also rings out earlier than the Low A.
    – Lecifer
    Oct 14, 2023 at 20:28
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    Ideophones such as tuning forks do not have harmonic overtones, so the overtone is unlikely to be found in the harmonic series.
    – phoog
    Oct 15, 2023 at 9:30
  • @phoog I guess the tuning fork not being classified as an instrument (idiophone), is because it hasn’t really been used as one because I don’t see it anywhere en.wikipedia.org/wiki/Hornbostel%E2%80%93Sachs
    – Lecifer
    Oct 15, 2023 at 18:24
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    @Lecifer it is evident from the definition of idiophone and the nature of tuning forks that a tuning fork is an idiophone. Also see Warren Burt - Music For Tuning Forks (1987) (Full Album) on YouTube.
    – phoog
    Oct 15, 2023 at 20:55
  • @phoog Thanks I checked it out, amazing! Very interesting composition. & I found a couple of books (through Wikipedia sources & references), that actually do list the tuning fork as an Idiophone. amazon.com/gp/aw/d/1884365086/ref=ya_aw_od_pi?ie=UTF8&psc=1
    – Lecifer
    Oct 16, 2023 at 21:33

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