Redoubt is a special case, however, because its volcanic "scream" reaches such a crescendo that it can actually be audible to human ears.

"At the very highest frequency, it would just be barely perceptible as a low-bass hum," explained Alicia Hotovec-Ellis, a geophysicist at the University of Washington.

The frequency of Redoubt's harmonic tremor is so high, in fact, that it can't easily be explained by many of the accepted theories.

"The pitch that Redoubt got up to was so much higher than any other volcano that we've recorded, and that's kind of pushed the limits of all the models that were out there," Hotovec-Ellis said in an interview Monday.

But in two new studies, published in the Journal of Volcanology and