Hello Troubadourians! Last month I wrote, “I’ll figure out a way to explain resonance without your having to have a physics degree.” Let’s see if I’m successful. Every system has a resonant frequency, which is to say a frequency at which a system will oscillate at maximum amplitude with minimum external energy applied to the system. For musicians, the system is usually our instruments, more specifically, our acoustic instruments. Most often we experience this resonance in the form of feedback when we attempt to amplify our instruments. Guitars, by their very design and construction, are especially sensitive to resonance and generally have resonant frequencies that are major components of the musical keys we play in. For instance, Dreadnought-sized guitars often resonate in G, Bb, or Eb while Jumbo sized guitars often resonate in C or Bb. This body resonance is what makes a guitar sound full and is usually in the lower frequency range. Smaller guitars resonate at higher frequencies due to shallower as well as smaller bodies, which can make them slightly less susceptible to low-frequency resonance but more susceptible to high-frequency resonance in similar keys as their larger brethren.
So why do guitars resonate so easily? Well, as I stated earlier, as acoustic instruments they are specifically designed to resonate. Excellent for – and essential to – sounding good when we play them in our bedrooms, living rooms, and studios, but a potential for a howling mess when we try to amplify them and reproduce that good sound we’re accustomed to hearing. The top and back of a guitar work together to transform the vibrations of the strings into physical movement of air, which is what we are actually hearing when we listen to the guitar. This principle extends to all stringed instruments but differing design and construction makes a lot of difference in how susceptible each is to resonance. Because a guitar’s top and back are connected only at the edges (by the sides of the body), they resonate freely and easily. Violins – in fact all members of the viol family – have their tops and backs additionally connected by a sound post. The sound post transmits the sound effectively from the top to the back when the instruments are bowed but also effectively dampens the sound when they are only “plucked” instead. This makes them much less susceptible to resonance induced by external sources such as those associated with amplification.
Not all resonance is caused by amplification, though. There are several other external sources that can induce our guitars into resonance. Resonance can be induced by strong bass signals in the near environment, or be induced by wind when in outdoor environments, and it can be induced by low frequency non-musical sounds such as drums or machinery. I once played a gig on an airstrip as part of an air show. Periodically, they would fire-up the engine of the P-51 that was about 50 yards away from the stage. The plane was facing away from the stage and didn’t really drown out the music but the low frequency sound wave from the exhaust made my guitar resonate even when I stopped playing and damped the strings. Yikes!
Many people have, let’s say, an “interesting” concept of resonance and of what resonant frequencies are. I’ll give you an example. When I was in college, I needed to take an elective so I chose an “Introduction to Music” class. I figured it would be an easy A. Apparently, a lot of other people thought so too and I had to crash the class. The professor asked me what my major was and I told him, “engineering.” He asked me why would an engineering major want to take a music class and I explained that I was also a serious guitarist and that I wanted to better understand music, not just the guitar. I think he felt I had two strikes against me already in that I wasn’t a music major and I was a guitar player, but he decided to add me to the class anyway. The room was a small amphitheater with a small raised platform in front of which sat a 9-foot concert grand piano. A shiny, black beauty that was capable of some serious sounds. During one lecture, the professor stated, “The universe resonates in the key of F#.” To demonstrate this fact, he and his teaching assistant played a 20-finger F# chord on the piano. BAM! The walls and windows rattled like we were experiencing an earthquake. The class was impressed. Then I raised my hand… “Um, excuse me, professor,” I said. “That was really cool but I don’t think that actually proves that the universe resonates in F#.” “How so, Mr. … Loach is it…?” he responded. “Yes sir,” I said. “What you have shown is that this room resonates in F# but I don’t think that extends into the entire universe. Resonance requires that there be molecules vibrating at a specific frequency and a medium, such as air, to transmit those frequencies. Since the universe is essentially a vacuum, there is no medium in which to transmit the vibrations. Further, the temperature of space – of the universe – is absolute zero, which is defined as the state where there is no molecular movement. Therefore, the universe doesn’t resonate.” The class was silent for what seemed an eternity and all eyes were on me. The professor finally broke the silence. He let out a long, sad sigh and began to write in his notebook, and said – just loud enough for all of us to hear – “Note to self: never add engineering majors to ‘Intro to Music.’” We all laughed, including the professor, and I resolved to keep my mouth shut for the remainder of the semester.
So, while the universe might not be resonating in F#, your guitar might very well be doing just that. And sometimes it can seem like the entire universe is indeed resonating when all you wanted to do was play. More on how to deal with that next month.
Need to know? Just ask… Charlie (email@example.com)