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The Physics of ... Singing

How a simple cough made us musical.

By Curtis Rist
Jan 19, 1999 12:00 AMMay 17, 2019 8:28 PM


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Singing came naturally to the opera mezzo-soprano Marilyn Horne, who made her public debut at a town picnic at age four. Her granddaughter Daisy, now just ten months old, appears to be even more precocious. "When I picked her up the other day, I swear I heard her begin to hum," says Horne. "To test it out, I came in on harmony--and she held her pitch. My daughter heard us and nearly passed out." 

Even after decades of operatic training, Horne is mystified about what makes it possible for her—let alone her granddaughter—to lift her voice in song. “I mean, something’s going on down there,” Horne says, motioning to her chest. “But I really don’t give it a thought, not while I’m singing.” When Horne raises the rafters with a powerhouse aria and Daisy hums a faint melody, they are each relying on some unusual anatomic aspects of the throat and mouth that allow humans to transform life-giving breath into dynamic sound waves.

The sound of music begins with the vocal cords, two skin-covered flaps of muscle that open and close across the passage to the lungs and oscillate with a wave motion as air rushes out during exhalation. The flaps first appeared some 300 million years ago in the sea creatures that climbed out of the water onto land and breathed with lungs rather than through gills. By quickly closing and opening, the two folds of muscle served as natural valves, blocking everything but air from entering the respiratory system and allowing the ejection of phlegm from the lungs. Otherwise the creatures would have choked to death. In humans and most of the higher animals, closing off air-filled lungs with this valve mechanism also stabilizes the body during muscular exertion, including evacuation of the bowels.

Vocal noisemaking came as something of an evolutionary afterthought. Eventually, various animals equipped with lungs, including humans, discovered they could sound warnings or attract mates by blowing air out and vibrating the vocal cords. “It’s a perfect illustration of what is known as Darwinian preadaptation,” says Tecumseh Fitch, a biologist at Harvard.

One set of human vocal cords looks much like another, except that men’s are slightly bigger than women’s. “You probably couldn’t tell the difference between Pavarotti and Mick Jagger by looking at the vocal cords alone,” says Scott Kessler, a Manhattan otolaryngologist. For that matter, human vocal cords closely resemble those of other species. “If somebody did a weird transplant where a singer got a dog’s vocal cords, they could probably do a good job of singing,” says Fitch.

In making sound, the vocal cords operate like the open end of a blown-up balloon, which buzzes rapidly if pulled tight so air can escape more slowly. Ingo Titze, a physicist at the University of Iowa and director of the National Center for Voice and Speech, says that air rushing out of the lungs creates a wave motion in the cords. They slam into one another, only to blow open again, buffeted by the steady airflow from the lungs. “This vibration releases an energy that the human ear and the brain interpret as sound,” Titze says.

The sound box in which exhaled air becomes musical notes is the larynx—the tip of which forms the Adam’s apple—a cartilaginous shell that contains and protects the vocal cords. In humans, the position of the larynx in the throat is relatively low, allowing the tongue greater freedom to move and thus shape sounds. In dogs, by contrast, the larynx is almost even with the mouth, severely limiting their ability to control how the bark sounds. Humans can alter their vocal tone by raising and lowering the larynx. In addition, trained singers relax and contract muscles within the larynx in order to produce the resonant waver of vibrato.

The rest of the human vocal instrument consists of the throat, tongue, mouth, lips and, of course, the respiratory system. Although the pitch depends on the number of times per second the vocal cords collide—ranging from as few as 55 times per second for a low A sung by a bass, to 1,047 times per second for a soprano’s high C—the refinement and articulation of the musical notes take place in the singer’s throat and mouth.

In contrast to a trombonist, who pushes the slide out or pushes it in, making air take a longer or shorter route to get to the bell, a singer must modulate the passage of breath with a series of mostly unconscious physical movements. The range of sounds produced is astounding considering the limited length of the air passage. “The human throat is remarkably short—just six or seven inches from the larynx to the lips,” says Titze. Finishing touches come from the lips and tongue, which shape vowels ranging from a mellow U to a sunny E.

Like other instruments, vocal cords can be mistreated and damaged. Some singers stretch their voices trying to hit high ranges they weren’t born to hit. Over time this can produce vocal nodes, calluslike knobs that grow on the vocal cords and prevent them from closing completely during their oscillation. This condition leads to a raspy, hoarse voice, even in an eight-year-old songstress struggling to belt out “Tomorrow” in Annie. Blisters, calluses, and even hemorrhages affect vocal cords, all impeding a singer’s sound. So the best singers learn to use their whole bodies to safely summon a precisely controlled flow of air and to modulate their larynx without straining themselves. “My doctor once called me a laryngeal athlete,” says Horne. “And it’s true.”

Singers—like other athletes—even find themselves resorting to drugs to repair the damage brought on by physical stress. “For many of these people, ‘the show must go on,’ ” says Kessler, who tends to some of the most famous throats in the world and sometimes provides steroids to alleviate hoarseness or other ailments. Singers suffer strain from dry heat in the winter and secondhand cigarette smoke, both of which dry out and inflame the mucosal lining of the vocal cords. They tax their voices because of grueling schedules, such as the eight-show-a-week regimen of Broadway performers. Or like Bill Clinton, they occasionally are reduced to using gravelly whispers because of a condition called acid reflux, often resulting from overeating late in the evening, and then burping up stomach acids during the night. “As singers, we’re really at the mercy of our bodies,” says Horne, tossing up her hands. “With all that can go wrong with your vocal cords, it’s a wonder anyone sings at all.”

Singing may come naturally to all humans, but vocal artistry, of course, is rare and elusive. “It all comes down to that little thing we all call talent,” says Horne. “You can’t get it, you can’t buy it, and you can’t coax it. As the song says, ‘you’ve either got it—or you ain’t.’ ”

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