The Physics of Loudmouths: Why Some Voices Carry

A human mouth. CREDIT: National Institutes of Health View full size image

You know that guy with the voice heard 'round the world? The one who — no matter how far away he is — sounds as if he is shrieking directly into your ear? He seems to show up at every party, restaurant and (worst of all) office.

Voices that "carry" contain a pitch of sound that strongly resonates with both the human vocal tract and the human ear, said acoustics expert John Smith, a biophysicist at the University of New South Wales in New Zealand. This piercing pitch, dubbed the "speaker's formant," has a frequency around 3,000 hertz, or 3,000 beats per second: about the same frequency as that of fingernails scraping a chalkboard. (To hear such a voice, click here, audio-visual courtesy of the National Center for Voice and Speech.)

By comparison, most human speech falls in the gentler 80- to 250-hertz (Hz) range.

How do loudmouthed people generate that much faster and much more earsplitting frequency? Understanding their method requires a short lesson on how speech works.

We generate sound by rapidly vibrating two small flaps of mucous membrane called vocal folds in our voice boxes, Smith said. The back-and-forth motions of these folds interrupt the flow of air from our lungs to create "puffs" of sound. If our vocal folds wiggle back and forth 100 times each second, they produce puffs with a frequency of 100 beats per second (Hz). However, additional motions of the vocal folds, such as collisions with each other, can generate additional frequencies that are multiples of that fundamental frequency: "harmonics" at 200 Hz, 300 Hz, 400 Hz and so on. [Why Are Beeps So Annoying?]

All these frequencies travel together through the vocal tract — the tubelike cavity leading from the voice box up through the throat and mouth to the outside world. Depending on its shape, this tract resonates at certain frequencies, meaning it vibrates in time with them. In the same way that an organ pipe increases the amplitude of the sound waves that travel through it, the resonance of the vocal tract amplifies those resonant frequencies, making them louder.

And that's the trick: Whether done subconsciously or on purpose, loud talkers have learned to harness the natural resonances of their vocal tracts to pump up the volume of their voices. First, they manipulate their vocal folds to generate a harmonic frequency up around 3,000 Hz. "This could involve using a stronger flow of air from the lungs and controlling the folds so they undergo a motion that results in a sound that is intrinsically richer in harmonics," Smith told Life's Little Mysteries. Most people mainly produce harmonics at lower frequencies. [7 Biggest Mysteries of the Human Body]

Second, they alter the shape of their vocal tract, often narrowing or restricting the tract just above the vocal folds, so that it will resonate at that high frequency, making the high frequency louder. "This increase in sound level occurs in a frequency range where the ear is more sensitive and where background noise is usually reduced," Smith said. "Coupled with any increased harmonic content from the vocal folds, it can produce a large increase in subjective loudness."

Recent and ongoing research by Smith and his colleagues shows that singers make similar adjustments to their vocal tracts in order to project their voices. The scientists have managed to characterize how this is done at different pitches. In a report published in January in the Journal of the Acoustic Society of America, they showed that even trumpet players seem to control their own vocal resonances in order to play very high notes.

The difference is, singers and musicians are paid to project their sounds. Loudmouths do it voluntarily.

This story was provided by Life's Little Mysteries, a sister site to LiveScience. Follow Natalie Wolchover on Twitter @nattyover or Life's Little Mysteries @llmysteries. We're also on Facebook & Google+.