Researchers at York University and their team have uncoupled the mystery of how Tuvan throat singers produce what sounds like two different pitches at once – a low rumble and a high whistle-like tone.
Fascinated with how this form of throat singing, Khoomei, creates this dual tone, researchers studied members of the Tuvan performing group Huun Huur Tu to see first-hand how they do it.
“They can produce two different pitches, which goes against the typical way we think about how speech sounds are produced,” says lead researcher Associate Professor Christopher Bergevin of the Faculty of Science. “It was a bit of a mystery how they did it and it’s something researchers have wondered about for the last two decades.”
Bergevin worked with Associate Professor and linguist Chandan Narayan of the Faculty of Liberal Arts and Professional Studies and the team, including Professor Brad Story of the University of Arizona, an expert on the acoustics of singing, and Assistant Professor Natasha Mhatre of Western University. They found that the Tuvan singers were able to uniquely constrict their vocal tract in two key spots simultaneously – once at the front of their mouth using their tongue and another at the back of their throat. This had the effect of creating the dual sounds.
“We adjust our pitch, we change our loudness or amplitude, and we extend the vowels. These are all things that we do in normal speaking,” says Narayan. “What is interesting about this type of throat singing is that it does something different. It’s a highly unusual sound that you don’t hear in other forms of singing.”
To figure out the mechanisms involved, researchers recorded the singers in a sound booth and shot a series of images of one the Tuvan performers singing while in an MRI scanner at York. Those images were sent to Mhatre, who helped reconstruct the vocal tract shape, as well as Story, who modelled and simulated the singing.
Birds and some frogs can produce two distinct tones, but it’s unknown in humans except in throat singers from Tuva and Mongolia.
“The question becomes, why are there two pitches heard when Tuvan singers sing? They don’t have two sets of vocal cords,” says Narayan.
In humans, vocal folds make sound by vibrating creating a buzzing noise. How fast or slow the vocal cords vibrate determines whether a high- or low-pitched sound is produced. The faster they vibrate, the higher the pitch of the voice. But they also produce a series of harmonics or “overtones.” The mouth and tongue shape theses overtones, creating resonances at certain frequencies called formants. Vowels in human speech are determined by the first three formants – F1, F2 and F3.
Each formant is usually distinct, but Tuvan singers can merge multiple formants to create one exceedingly sharpened formant.
“The Tuvans are able to make this sound through such precise control of their vocal track that they can kind of tease these things out and create simultaneously sounds. One of the things that’s so remarkable about it is that it doesn’t sound like any human could do this, to have that degree of motor control,” says Bergevin.
The paper, “Overtone focusing in biphonic Tuvan2throat singing,” was published in full today in the journal eLife.