From a resting state, a guitar string will begin to ring as I resonate my vocal chords. If I make a fairly loud, and high-frequency "oo" sound at 440hz, then the A string on my guitar begins to vibrate and I get a nice A note.

I assume they both (vocal chord and guitar string) vibrate with the same sort of fundamental frequency (p.s. I'm self taught in comp. sci, I never went to college and never did this stuff in highschool... anywho!), however I’d more like to understand why one specifically induces the other. Is this simply due to the air distortion taking advantage of a taught string’s potential energy? If so, why doesn’t my voice resonate all strings, no matter what the note that my “oo” sound is effectively making? Why doesn't an overdriven, incredibly loud, electric guitar rupture my vocal chords whilst they are idle?

I'd like to add that my use of the word "resonate" may be less informed than most. I really mean "emits sound from vibration". Again, I am a total dilettante here.

  • $\begingroup$ The answer to your "Why?" is here: en.wikipedia.org/wiki/Resonance - However, if you also wonder "How?" - then it is through the guitar top acting as a transformer matching the acoustic impedance between the strings and air: physics.stackexchange.com/questions/452833/… $\endgroup$ – safesphere Feb 20 at 5:09
  • $\begingroup$ BTW, they are vocal cords, not chords. $\endgroup$ – PM 2Ring Feb 20 at 6:16
  • $\begingroup$ Something every physicist should do is go to a nice cathedral and listen during an organ recital. Then you will understand how your body can vibrate in resonance with the organ. $\endgroup$ – Jon Custer Feb 20 at 13:27

This is a typical resonance phenomenon. In general, a resonance is characterized by a resonance frequency, and a quality factor.

The resonance frequency tells you, at which frequency the resonance can be excited. In your case, if you produce a sound at 440 Hz with your vocal chord, you hit the resonance frequency of your guitar a-string.

The quality factor tells you, how long the resonant vibration persists, once you stop exciting it. For a high quality resonance it persists for a longer time than for a low quality one. High-quality resonances also lead to high resonant excitation amplitudes. The guitar strings, being part of a musical instrument, have quite high quality factors. Therefore, your voice can easily excite the string to visible amplitudes.

In turn, your vocal chords are made of organic tissue and muscles without a well-defined resonance frequency. We produce specific frequencies with our voice by controlling muscle tensions. But without this tension, there is no specific resonance frequency. Furthermore, even if there is, because you apply a specific muscle tension, the quality factor of this resonator is very low. This is why the sound of your guitar does not excite your vocal chords appreciably.


Is this simply due to the air distortion taking advantage of a taught string’s potential energy?

No. The "air distortion" is energy. More specifically, the sound waves in the air transport energy from your voice box to the body of the guitar, and then the body of the guitar couples that energy to the strings.

The vibration of the strings is powered entirely by you: It has nothing to do with any potential energy that was previously stored in the tension of the strings.

If so, why doesn’t my voice resonate all strings...?

The energy from your voice is transported to all of the strings, but only the one string resonates with the sound. (See @flaudemus answer for more on that topic.)


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