You'll search Google News in vain for stories about most technical terms in phonetics — no recent coverage of lenition, for example — but "vocal fry" has been prominent in the popular press for several years. Despite all the coverage, many people seem to be unclear about what it is and where it comes from — so today I thought I'd spend a few minutes on the phenomenon from a phonetician's perspective.
The media focus began back in 2011, in response to Lesley Wolk, Nassima B. Abdelli-Beruh, and Dianne Slavin, "Habitual Use of Vocal Fry in Young Adult Female Speakers", Journal of Voice. Wolk et al. used both perceptual and acoustic measures of "vocal fry", and found that
(1) vocal fry was used in sentence reading by more than two-thirds of this population of  female college students, (2) vocal fry rarely occurred in sustained vowels, (3) vocal fry occurred most often at the end of utterances, and (4) statistically significant differences were found for several acoustic measures between vocal fry and normal register.
This was widely interpreted as signaling an "epidemic" of "pathological" voice quality among young American women, though in fact the study claimed no such thing, and offered no evidence that the situation would have been different 30 or 50 or 100 years earlier. (Nor, for that matter, any evidence that the phenomenon is more common among young women than among older women, or among women than among men.) So in "Vocal fry: 'creeping in' or 'still here'?", 12/12/2011, I took at look at the "TIMIT Acoustic-Phonetic Continuous Speech Corpus", recorded in 1986, and found vocal fry in the very first female-speaker sentence that I looked at — from a woman born in 1957 — and in many others, though I didn't try to quantify the prevalence in that collection.
But Wolk et al. unfortunately did not publish the recordings on which their paper was based, so it's hard to know whether my criterion for "vocal fry" was the same as theirs.
Vocal fry made it into the news again, due to Rindy C. Anderson et al., "Vocal Fry May Undermine the Success of Young Women in the Labor Market", PLOSOne 5/28/2014. Anderson et al. asked male and female speakers to produce sentences both with and without vocal fry, and then asked listeners which version they would be more likely to hire. The result was that the non-fry sentences were overwhelmingly preferred, with the anti-fry preference being slightly stronger in the case of female speakers.
Anderson et al. did publish the audio from their experiment, for which they (and the journal editors) are to be commended. This allows us to confirm what Christian DiCanio suggested we ought to expect ("Vocal fry probably doesn't harm your career prospects", 6/7/2014), namely that the vocal-fry version of the sentences are rather exaggerated and fake-sounding. Here are the first and second female-speaker examples:
But maybe real-life contemporary-woman vocal fry is like that? Let's take a look at some examples from Kim Kardashian, widely regarded as the beau ideal of the modern female fryer. Her interview on The Early Show certainly provides plenty of examples, e.g.
On the other hand, the show's host exhibits pretty similar patterns:
And for that matter, so does Hillary Clinton in her recent Fresh Air interview:
And just to forestall any suspicions that HRC is pandering to the young female vote, or is a secret member of the Kardashian family, or whatever, here's George W. Bush in a 2010 interview with BIll O'Reilly:
In all of the real examples — from TIMIT, from Kim Kardashian and her interviewer, from Hillary Clinton and George W. Bush — the region of period-doubling and/or erratic glottal pulses occupies about 200-500 milliseconds at the end of a intonational phrase that falls to the bottom of the speaker's range. In the (fake and fake-sounding) examples from the Anderson et al. study, the comparable period is often more than a second long, e.g.:
So why does this happen?
The real question, how is it ever possible for it not to happen?
The mathematical framework for such phenomena was discovered by Mitchell J. Feigenbaum in the late 1970s. As he explains in Universal behavior in nonlinear systems", Physica D 1983,
[S]ome very simple schemes to produce erratic numbers behave identically to some of the erratic aspects of natural phenomena. More specifically, there is now cogent evidence that the problem of how a fluid changes over from smooth to turbulent flow can be solved through its relation to the simple scheme described in this article. Other natural problems that can be treated in the same way are the behavior of a population from generation to generation and the noisiness of a large variety of mechanical, electrical, and chemical oscillators. Also, there is now evidence that various Hamiltonian systems-those subscribing to classical mechanics, such as the solar system-can come under this discipline.
The feature common to these phenomena is that, as some external parameter (temperature, for example) is varied, the behavior of the system changes from simple to erratic. More precisely, for some range of parameter values, the system exhibits an orderly periodic behavior; that is, the system's behavior reproduces itself every period of time T. Beyond this range, the behavior fails to reproduce itself after T seconds; it almost does so, but in fact it requires two intervals of T to repeat itself. That is, the period has doubled to 2T. This new periodicity remains over some range of parameter values until another critical parameter value is reached after which the behavior almost reproduces itself after 2T, but in fact, it now requires 4T for reproduction. This process of successive period doubling recurs continually (with the range of parameter values for which the period is 2nT becoming successively smaller as n increases) until, at a certain value of the parameter, it has doubled ad infinitum, so that the behavior is no longer periodic.
In fact, some oscillatory behavior starts out in the chaotic regime. In particular, if you release pressurized gas through an elastic passage which is almost but not quite able to seal off the flow, you will get a cycle in which
(1) the pressure forces the passage open,
(2) gas begins to flow through the passage,
(3) bernoulli forces pull the passage closed again, returning us to step (1).
But there's a wrinkle, well known to anyone who has tried to learn the oboe or the trumpet — unless the gas pressure and the elastic passage are very carefully regulated, the result will be something like a Bronx cheer, with irregular oscillation from the start:
The mammalian larynx probably evolved to close off the airway, partly to protect the lungs from food, drink, and vomit, and partly to allow pressurizing of the lungs to make the trunk a more rigid platform for the arms and legs. The role of the larynx in vocalization is presumably a secondary adaptation — but in any case, it takes a delicate balance in genetic design, in phenotypic development, and in learned behavioral control to produce a nearly-periodic pitched sound rather than a croak or a bark or a burp or a squeak. Not all of our mammalian relatives can manage this feat. And we ourselves don't manage it all the time.
So it's not at all surprising that as the system relaxes, it tends to exhibit period-doubling and even transition to chaos. The surprising thing is that this doesn't happen more often.
For an entrancing example of a simple system that exhibits deterministic chaos, consider the double pendulum: