Skinny On

The Ocean in a Seashell

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By Hannah Holmes

    "Why is it that you hear a sound when you put a glass or seashell to your ear?"
    -- Greg Rudy
Oh, the allure of the inobvious answer! Someone once told me that when you put a shell to your ear, you're hearing the sound of blood coursing through your own skull. It was such a surprising notion that I assumed it had to be true.

It's not.

"Do you have a coffee cup on your desk?" asked Dr. Charles Berlin, an audiologist talking to me from the Kresge Hearing Research Laboratory in New Orleans.

"Yeah."

"Put it up against your ear. Wait -- it's an empty cup, right?"

(I'm working on a theory: Scientists think English majors have a dusty tangle of old typewriter ribbons where the brain is supposed to be.)

"OK. Press it against your head, then rock it. Hear the change?" (To try this at home, alternately seal the glass against your head then break the seal.)

"Around you is this constant barrage of noise that you tend to ignore," he continues. "When you put something near your ear, it strengthens a certain group of frequencies. You're just enhancing environmental noise with that cup."

Even when the cup is sealed against your head, some sound waves are beating through the ceramic and bouncing around inside the cavity, then registering on your ear drum.


The dimensions of the cup -- or shell, or organ pipe, or mandible -- determines which sound waves will be amplified and which will be squelched. Wait ... mandible?
.
Dr. Berlin tells me to put down the cup and press the phone to my ear. I hear a musical, "tloonk!" like a wooden stick on a wooden bowl. It's actually his wooden finger against his wooden throat. He plays me a series of notes. It's not a song, thank heavens, just a fleshy, tubular sort of scale.

"Just by changing the shape of the resonating cavity, I'm changing the pitch," he explains. Certain wavelengths prefer a wide-open mouth; others thrive in a cramped space. You can do a cheap imitation of this trick by snapping a finger against your cheek as you form smaller and larger Os with your lips.

That's the basic answer: There's all this noise flying around all the time. Forcing these sound waves to bounce off something before they enter your ear changes the mix of frequencies that are resonated, or strengthened. So the sound waves suddenly sound different. And sometimes, they sound like waves on a beach.


But there's another dynamic at work in the seashell or the coffee cup.
.
This second type of resonance is called "Helmholtz," after the guy who described mathematically what happens when you blow across the mouth of a bottle. Helmholtz's resonance is caused by the vibration of a "spring" of air inside the bottle, which quakes under the assault of your breath. (It's nothing personal, I'm sure.) Both the Helmholtz and "coffee cup" kinds of resonance contribute to the noise you hear in a seashell.

Helmholtz is described to me by an Illinois acoustician who makes high-performance hearing aids and ear plugs. This guy, Dr. Mead Killion, talks to me on the speaker phone in his car as he drives. He's talking a blue streak, thinking a red streak, and the white noise punctuated by toll-booth exchanges and cell-phone static make him impossible to follow. But I do prick up my ears when he tells me that people used to break the tip off big seashells and use them as hearing aids.

"As the sound goes through the shell, it's forced into a smaller and smaller space," he explains, every other word lost to a fire engine or an underpass. "There's less motion, and higher pressure." When the sound waves are finally freed at the end of the shell, they rap much harder on the eardrum.

Straining to hear Dr. Killion, I roll a cone of paper, put my phone in the big end and stick the little end in my ear. I hear nothing. And when I pull it out, it is limp with coffee, and bits of old typewriter ribbon cling to the end.


Vocabulary

intensity, n.: "You know that old thing, 'If a tree falls in the forest and no one hears it, did it make a sound?'" Berlin asks me. "Well, the answer is pretty simple: There is intensity, but there is no loudness." Physicists think of sound waves in terms of their intensity, or forcefulness. Psychologists describe them in terms of how people perceive them, or their loudness. No people in the forest, no loudness.


Check out more of "The Skinny On ..." stories:

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  • Why Asparagus Makes Your Pee Stink
  • Why We Can't Tell What Time It Is
  • Why We Fear Nuclear Power, Not Peanut Butter
  • Tongue Rolling
  • Itty Bitty Life Forms
  • Sewing Up Baseballs
  • Strange Sneezing Situations
  • The Evil Eye
  • Why Ice Cubes Shrink in the Freezer
  • Why Toilet Bowl Water Twirls Clockwise
  • Why Teflon Sticks to the Pan
  • Lunacy and the Full Moon
  • Sunscreen Testing
  • Where Fruit Flies Come From
  • Smelly Sports Clothing
  • Why Beans Give You Gas
  • Why You Never See Baby Pigeons
  • Latin Names for Living Things
  • The Color of Snow
  • Pizza and Thirst
  • Why There's No Channel 1 on a TV
  • Falling-Asleep Twitches
  • Deaf People's Inner Voice
  • Truck Tailgating


    		•
    To really hear the sea, Hannah Holmes just walks to the shoreline from her home in Portland, Maine. A fresh "Skinny On ... " arrives here every other Friday, adding to her extensive work for Discovery Online. Hannah's writing also appears on the pages of Escape, Outside, Sierra, Backpacker, Eco Traveler and Women's Sports and Fitness. Send her a note at skinny@online.discovery.com.


    Learn more about unexpected connections to everyday things with the "Connections III" CD-ROM game, which you can buy online in Discovery Shopping.


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