“Oh. OH! Wow, Uncle Sy, this changes everything!”

“Which ‘this,’ Teena?”

“The resonator thing. Our music teacher, Ms Searcy, has been going on about us singing too much in our heads. I thought she’s been saying we’re thinking about the music too much and should just let the singing happen.”

“Sounds very Zen.”

“I’m too young to know Zen stuff. But now I think maybe she’s saying we’re using those head resonators you just told me about — singing with our sinuses and nasal cavities instead of somewhere else. She’s never been clear on what we can do about that.”

“You’re probably right. No music teacher since Harold Hill would try to get away with ‘think the music.’ I’m sure Ms Searcy’s complaint is about head tones as opposed to chest tones. If so, she’s got a good excuse for being unclear.”

“How can I have chest tones when you said vibrations come from my voice box and that’s above my chest?”

“Sound wave energy is about molecules colliding against any neighboring molecules. Up, down left, right — none of those matter. When air from your lungs makes your vocal cords buzz against each other, much of that buzzing goes up through your throat and head resonators. However, some of the buzz energy travels into your chest cavity. That’s your biggest resonator and it’s where your lowest tones come from.”

“I guess Ms Searcy wants us to send even more buzzing there, but how do we do that?”

“That’s a hard question. People have been interested in it since they started teaching singing and oratory to other people. We learned one part of the answer in medieval times when we began studying anatomy up close and personal. For instance, your voice box, which I really ought to call your larynx now we’re getting into detail, relates to about a dozen different muscles.”

“What’re the other parts?”

“The easiest part was kinesiology — figuring out what action each muscle supports. The hardest part was teaching a student how to feel and control the right muscles to make their voice do exactly what they want.”

“How can that be hard? I can flex my arm and leg muscles any time I want.”

“Can you make your larynx move up and down in your throat?”

“Easy.”

“That motion depends on a chain of muscles running between the back of your tongue and the top of your chest. One way to send buzzes downward is to activate the muscles that pull the larynx in that direction. Shorter distance makes for more efficient buzz transmission. It can also help to pull your head back a little. That tenses those pulled‑down tissues and improves transmission even more.”

<slightly deeper voice> “Like this?”

“That’s the idea. Learning to do that without having to pay attention to it is part of vocal training. Now, can you spread your toes out?”

“That’s weird. I can on my right foot but not my left.”

“Very common. Each foot has little tiny muscles, called intrinsic muscles, buried deep inside. Some people can control the whole set, some not so much. Your larynx has two pairs of intrinsic muscles that govern how your vocal cords work together. Some voice teachers claim the intrinsic muscles inside the larynx are the key to proper voice technique. Unfortunately, you can’t see them or get a feel for controlling them other then ‘keep trying things until you get it right and remember what you did.’ That’s Ms Searcy’s strategy. It’d be much harder with helium.”

“That’s right! Our voices with the balloons were all head tones. How come?”

“The speed of sound.”

“That’s a sideways answer, Uncle Sy.”

“Okay, this is more direct. We’ve said resonance was about waves whose wavelength just fit across a cavity. Picture two waves, the same number of waves per second, but the wave in helium travels about 3½ times faster than the one in air. The helium wave stretches about 3½ times farther between peaks. Whatever peaks per second your vocal cords make, in helium your chest cavity is 3½ times too small to resonate. Your head cavities, though, can resonate to overtones of those frequencies.”

“Squeaky overtones.”

~ Rich Olcott

<chirp chirp> “Moire here.”

“Hi, Uncle Sy.”

“Hi, Teena, How was the birthday party?”

“Pretty fun. We had balloons but Mom wouldn’t let us fly them into the sky.”

“Because animals might eat them when they come down, I suppose.”

“Yeah, that’s what she said. What we did was we untied the knotty part so we could breathe in the helium and sing squeaky Happy Birthdays. It didn’t make much difference to my voice but Brian’s came out weird ’cause his voice is breaking anyway. It was almost a yodel.”

“I thought you didn’t like Brian any more.”

“That was last month, Uncle Sy. He ‘poligized on accounta he’s still learning social skills. We had fun playing with the sounds.”

“Sure you did. Do you remember the slide whistle I gave you once?”

“That was a long time ago. It was fun until Brian bent the slider and it didn’t work any more.”

“No surprise. How’d you make it give a high note?”

“I’d push the slider all the way in. Slider-out made a low note, but Brian could make a high note even there if he blew really hard.”

“Well, he would. It all has to do with resonant cavities.”

“I don’t have any cavities! Mom makes sure I brush and floss every day. And what’s resonant?”

“You don’t have dental cavities but there are other kinds. ‘Cavity‘ is another word for ‘hole‘ and some of them are important. You breathe through two nasal cavities that join up to be your posterior nasal cavity and that connects to sinus cavities in your skull and down to your voice box and lungs. Your mouth cavity resonates with all those cavities and vibrations from your voice box to make your speaking sounds.”

“That’s twice you used that ‘resonate‘ word I don’t know.”

“Break it down. ‘–son–‘ means ‘sound,’ like ‘sonic.’ Then—”

“‘Re–‘ means ‘again‘ like ‘rebuild‘ so resonate is sounding again like an echo.”

“Right. Except a resonant cavity is picky about what sounds it works with. Here, I’m sending you a video. Did you get it?”

“Yeah, I see a couple of wiggly lines. Wait, the blue line stays the same size but the orange line gets littler until it’s all gone and then the picture goes yellow and starts over.”

“What happens over on the right‑hand side?”

“The blue line bounces back from zero but the orange line waves all over the place. Is that how sound works?”

“Sort of. The air molecules in a sound wave don’t go up‑and‑down, they go to‑and‑fro. The wiggly lines are a graph of where the energy is. Where the molecules bunch together they bang into each other more often than in the in‑between places. In open air the energy pushes along even though the molecules stay pretty much where they are. In a resonant cavity, sounds are trapped like the blue line if they have just the right wavelength. A cavity’s longest trappable wavelength is its lowest note, called its fundamental. The energy in the fundamental is sustained as long as new energy’s coming in.”

“What happens to the orange line?”

“It doesn’t get a chance to build up. The energy in those waves spreads out until the wave just isn’t any more.”

“Aww, poor wavey. Wait, what about when Brian blows extra‑hard and gets that high note?”

“He gives the commotion inside the whistle enough energy to excite a second trapped wave with twice the number of crowded places. That’s called an overtone of the fundamental. Sometimes you want to do that, sometimes you don’t.”

“Brian always did on the whistle.”

“Well, he would. The resonant cavity thing also explains why Brian’s voice breaks and how talking works. Your windpipe is a resonating cavity. Brian’s windpipe has grown just large enough that sometimes it resonates in his new fundamental and sometimes switches to some other fundamental or overtone. Talking depends on tuning the resonances inside your mouth cavity. Try saying ‘ooo‑eee‘ while holding your lips steady in ‘ooo‘ position. On ‘eee‘ your tongue rises up to squeeze out the low‑pitched long waves in ‘ooo‘, right?”

“oooeeeoooeeeooo“

• Thanks to Xander, whose question inspired this story arc.

~ Rich Olcott