When most people praise the quality of a duet, they expect to be congratulating two people. But, as a video that’s gone viral this week has proved, that assumption need not always hold true. In just over eight days German singer Anna-Maria Hefele has racked up 4 million views on YouTube by demonstrating her phenomenal ability to sing two separate melodies at the same time.

Hefele’s performance is an example of polyphonic overtone singing, a technique which allows her to produce two distinct notes in perfect harmony.The lower of the two is generated by the vibrations of vocal folds in the larynx: the same process as occurs in everyday speech. This sound wave is said to have a fundamental frequency, as it has the longest wavelength that will fit inside the resonant cavity formed by the speaker’s mouth and throat.

Produced alongside this low note are numerous overtones or harmonics, waves with higher frequencies that can also fit into the body’s cavity a whole number of times. By moving the tongue, lips and soft palate, the shape of the cavity can then be adjusted so as to isolate individual overtones at will. Ordinarily, this process occurs in tandem with the vibration of the larynx to produce a single identifiable sound.

When these two processes are separated, however, and allowed to work in isolation, the body becomes capable of producing a high-pitched overtone at the same time as the low-pitched fundamental. With sufficient training a singer can even alternate between notes, allowing accomplished performers like Anna-Maria Hefele to sing entire melodies in the higher register while the resonant frequency continues to sound underneath.

According to Ingo Titze, Director of the National Centre for Voice and Speech and Professor of Speech and Voice at the University of Iowa, polyphonic singing of this type is theoretically accessible to us all. But choosing the right fundamentals and harmonics can be a tricky business, as only notes whose frequencies have the right mathematical relationship can be sung at the same time. Take the A above middle C, for example, which is defined as having a frequency of 440Hz. If this were to serve as a singer’s fundamental note, then all harmonics would need to have frequencies corresponding to multiples of 440Hz – such as 880Hz (the A an octave higher) or 1320Hz (the E above that).

It is this awareness of the precise location of different harmonics that explains Hefele’s remakable ability (demonstrated from 3:25 onwards) to make the two notes move in seemingly opposite directions. Surely if the fundamental frequency decreases, the frequency of the harmonic should decrease with it? “The fundamental and harmonics always move in the same direction”, confirms Professor Titze, “but what changes is whether the singer is jumping up or down the harmonics available to her.” He compares the situation to someone climbing up a ladder being carried downwards: they may appear to be rising as the ladder falls, but are still only moving up rungs of a falling ladder.

“The brain has to do a lot of work,” he stresses. “The clever bit is working out ahead of time which resonances in the vocal tract you can use and then finding the right harmonics to match those changing fundamentals.”

Although Hefele’s video has taken the world by storm this week, the skills she displays are nothing new. Polyphonic singing (often misleadingly called throat singing) has existed in many cultures for centuries, and the techniques shown in the above video are well-known amongst practitioners of the art. “She is clearly a fantastic instrument player,” concludes Professor Titze, “but she has not invented the instrument.”