Course:Phys341 2020/Voice and instruments compared
Sprawled across my bed softly chatting with my roommate, I hear a low ambient sound...almost melodic, echoing the pace of my speech. Pressed into the mattress between my roommate and I is my guitar, face up, our voices vibrating the air above and around it… a fact of which it seems aware as our words gently excite its strings. How is this occurring?
In exploring the concept of resonance, my initial research provided the example of a tuning fork, struck and moving close to another one which, without being touched, begins to vibrate itself; similar to my guitar making noise without being plucked. The rather endearing term that can describe this: Sympathetic Resonance.
What is required for sympathetic resonance to occur?
Instruments have natural frequencies at which they vibrate. A pipe organ, for example, has pipes of which each will naturally amplify a specific frequency. Some instruments have specific “Sympathetic Strings” whose purpose is to create sound via resonance, without actually being plucked. The Indian sarangi, for example, has only a few “gut strings” which are played, while other sections of multiple strings called “tarabs” exist only to resonate, contributing a certain depth and mood to the music (Sarangi, Wikipedia).
If the frequency must be exact to create resonance, does this mean that while speaking I used precisely the correct note to excite my guitar’s strings?
The sympathetic strings in a sarangi, or its relatives, do not solely resonate at the fundamental frequency of those plucked, but also at octave steps from the main strings. Resonance can even be heard, though less prominently, at intervals like fifths or thirds.
Experimenting
Normally, when a guitar string is plucked, the resonance is occurring mainly within the guitar body, but I wonder what would happen if I were to pluck the string of one guitar as it sits facing another. I intend to measure the effectiveness of the resonance by watching the chromatic clip-on tuner on each guitar select the note that is being played. These tuners are ideal because they reference the vibrations within each respective guitar without taking in sound from the surrounding environment.
https://youtu.be/Srx9rbr_kR8 (Video recording tuners on each guitar after low E is plucked on only one)
After testing for the low E string as well as A, I find that the readings on the respective tuners do in fact match.
Due to the specificity of guitar strings, of which there are only six unlike the sarangi's 37... resonance works most effectively only at tones E B D and A. My findings on this did not list an effective resonance at G or the high E string. I tested this by plucking the high E on one guitar and observing the tuner on the other for the same response as with the lower strings. Of course this format of study can not be 100% accurate as the tuner will indicate the note it senses without showing varying strength of the resonance.
Voice
The resonant range of a guitar aligns somewhat well with a typical vocal range, perhaps primarily on the slightly lower side.
When I sing an ‘A’ into the body of my guitar, I check the tuner and it also registers an A. Does this mean that only the A string is activated?
https://youtu.be/YIYLpcjmaOQ (Slow-mo video of strings while A is being activated)
While I attempted to determine if every string vibrated by taking a slow-motion video, my camera was not slow enough, nor of adequate quality. Therefore despite the terrifying sound of my slow-mo-ed voice, no string movement is clearly seen. Since this experiment did not adequately answer my question I have to estimate an answer.
Guitar players often mute string they do not want to be heard when playing other notes, for example a fretted note that resonates with A will cause the A string to vibrate, as we may glean from the discussion up to this point. I can assume then that while the A string most definitely vibrates when I sing A into the body of my guitar, the other strings do not vibrate to an amount that is of any consequence, unless maybe they were being fretted to mimic the A frequency.
References
https://study.com/academy/lesson/acoustic-resonance-definition-calculation.html
https://en.wikipedia.org/wiki/Sympathetic_string#Sympathetic_string_resonance_in_music_instruments