Vocal cords are able to produce a wide range of sound frequencies because of the larynx's ability to stretch vocal cords and the cords' molecular composition - according to a new paper published in PLOS Computational Biology. Scientists, led by Ingo Titze at the University of Utah, show how these two characteristics of various species' larynxes can closely predict the range of frequencies each species can produce. The results reveal the evolutionary roots of how and why voice arose.
While most people know the structures in our throats that produce speech as "vocal cords," the term is not universally used among voice researchers. Some have preferred "vocal folds" since the mid-1970s, when studies of vocal anatomy showed a folding of the vocal ligament (the cord) during vibration. This research shows that the phrase "vocal cord" may be more apt, due to consideration of the cord's string-like properties.
At birth, vocal cords are composed of a uniform, gel-like material. As the vocal cords mature, fibers develop within the gel, eventually forming a multilayered, laminated string. Imagine a set of guitar strings glued close together with gelatin; when one string is plucked, the entire gel-fiber set shakes along with it. The muscles in the larynx further modulate the sound the cords produce, lengthening and shortening the cords to change the pitch.
Titze and his colleagues, Tobias Riede of Midwestern University in Arizona and Ted Mau of the University of Texas Southwestern Medical Center, compiled measurements of larynx characteristics for 16 species, including humans and animals ranging from mice to elephants. As expected, larger animals had larger larynxes, and body size correlated well with the average frequency an animal could produce.
But body size could not predict an animal's range of possible frequencies. "So, one asks, what's going on inside the larynx that allows this quite different outcome for pitch range across species, where the mean pitch is so well-correlated with size?" Titze says.
The team found that two factors were much better at predicting range: A factor measuring the amount of length change possible in the vocal cord, or how far it could stretch, and a factor measuring the stiffness of the cord due to the fiber structures within.
Titze says that creating a manmade instrument with the same properties as a vocal cord might prove technically daunting. The first step, he says, would be to fashion a laminated string, with the layers cross-linked together and supported by fluid. "But then we'd have to figure out how to pull it, elongate it, and how to distribute the tension to one layer versus another layer versus another layer," he says. "Nature has figured this out, how to literally play the dominant layer for a given pitch."
The results may help surgeons repair damaged vocal cords. Because both cord stretchiness and stiffness factor into range, doctors may have more options to design treatments to restore much of a patient's range. The findings also have implications for vocal training, and suggest that singers can increase their ranges by either stretching their vocal cords or by engaging in exercises that affect fiber spacing and cord stiffness - again, more options to achieve the same goal.
Despite the complexities of the vocal cord structure, Titze says he was surprised at how well the model of a simple vibrating string explained the cord's range. "Most people would laugh at using a simple vibrating string model for something as complicated as a 3-D, nonhomogeneous tissue structure," he says. "But the string model does an incredibly good job of explaining this range of frequencies."