I always have the hardest time starting up these series, because I spend a lot of time trying to figure out where to start. I always know what the ending conclusion should be, but what's the beginning? What's basic without being too basic? So I'm going to start out where I think it should start out, but if I'm not being basic enough, please feel free to post any questions you may have.
Everybody always talks about resonance in the singing world. Resonance, resonance, resonance. Let's face it, as opera singers, we're pretty obsessed about it. And why wouldn't we be? It is, after all, the key to how opera singing works. It is exactly how we are able to sing over an orchestra for hours at a time without hurting our voices. The only issue I have with all this resonance talk is that it is painfully obvious that (some) singers have absolutely no clue what resonance really is. It often gets talked about as a subjective thing that changes from person to person. This is understandable given that so much of the sensation of singing is subjective, and therefore, how we teach singing is subjective. It only makes sense that singers would start to think everything about singing is subjective somehow. However, when we take something from the hard sciences, like resonance, and think of it as something that acts differently from person to person, as if it doesn't follow the laws of nature, we kinda sound like fools. The other issue with all this resonance-as-subjective talk is that it makes what could be very clear pedagogy very fuzzy and confusing. So, in order to fully understand what resonance is and how it can help us sing better, let's start with how a single sound wave works.
There are a lot of things in nature that function like waves: Light, sound, the water in your bathtub...(okay, fine, ocean water too), but what exactly does that mean for sound to have a wave-like pattern of behavior? Well, here's the definition of a wave from physics: "a disturbance (an oscillation) that travels through space and time, accompanied by a transfer of energy...often with no permanent displacement of the particles of the medium (Wikipedia)." Sounds pretty fancy, am I right? But it does make a lot of sense. If you drop a rock straight down into a body of still water, the rock disturbs the water's stillness causing a rippling of waves that travel out to the edges of that body of water. Energy was transferred from the rock to the water which then traveled out to the edges of the body of water. The water itself, though, will return to being still, i.e. it doesn't just keep traveling away from the rock until there's no water left, so there wasn't a permanent displacement of the particles of that water (you know, H2O).
So what's the "medium" for sound waves? Air particles! All the lovely little air particles that make up our atmosphere is the medium for all the sound waves we hear, and the ones we don't hear too (i.e. ultrasound, infrasound, etc). For our purposes, we'll think of a sound wave as beginning with air particles at rest. An external force then comes along and sets those particles in motion (like when the electric slide is played at a wedding...sorry, couldn't resist.) Anyways, let's imagine those particles are all lined up nicely next to one another. The particles in row A, the ones closest to the external force, then get "pushed" up towards the particles in row B. This is where we say the row A particles are "compressed" against row B, which then gets pushed up against row C, and so on. (Anyone who's ever seen elementary-school kids line up for recess knows what I'm talking about here.) So while each row is going into it's period of compression with the particles in front of it, the rows that have already been compressed then go into a period of rarefaction. This would be when row A, after compressing with row B, swings back towards it's resting position. But instead of landing at rest, row A actually over-shoots its resting position and ends up being spaced out farther from the row B particles. If we want to get even more specific here, the property of inertia for those particles causes row A to compress with row B, then the property of elasticity over-takes row A's inertia, sending the particles back towards resting. However, the property of inertia for that row of particles then over-takes elasticity and causes row A to over-shoot it's resting position. But, don't fear, cause elasticity will over-take inertia and send row A back to towards resting. This process will repeat itself until row A is again completely at rest. Sounds complicated, but if you've ever set a pendulum into motion and watched until it came to rest again, you've seen this same action at work. (*Edit to add: This pattern of motion is called simple harmonic motion and is actually what pretty much everything in nature can be reduced to.)
This pattern of compression and rarefaction makes up what we call the sound wave. This is why sound waves are sometimes called compression waves, but more commonly, they are called longitudinal waves. (If you clicked on the link I had above on "compressed," you probably saw that coming.) I encourage you to go ahead over to the link for longitudinal waves, because there are some very nice animations over there for you to see these waves in action.
One last thing before I sign off: This pattern of compression and rarefaction is often graphically represented as a waveform. Typically, waveforms are set on a typical Cartesian coordinate system (the graphs with x and y from math class), with the y, or vertical, axis representing the amount of displacement, which also happens to be the amplitude of the sound wave, and the horizontal axis representing the amount of time the wave has traveled. We'll go over this all a bit more later, but I wanted to introduce it here for you just to get you more familiar with the terminology I'll be using.
Raphel, L. J., Borden, G. J., Harris, K. S. (2007). Speech science primer: Physiology, acoustics, perception of speech (5th ed.). Philadelphia, PA: Lippincott Williams & Williams.
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