Category:PHYS341-2017/Physics Behind Overtone Singing

The Physics Behind Overtone Singing

Overtone Singing is a vocal technique used by singers which creates the perception of singing more than one pitch simultaneously. The technique has been used across many cultures and continents, from Asia to Europe to Africa. Often times, listeners will hear a single 'drone' frequency, while also hearing a 'melody' above it due to movement in the overtones of the fundamental frequency. Despite being an ancient technique, overtone singing makes use of physics that were only understood much later.

Overtones

Musical sounds are often generated from a source, such as a string or block, that emits a series of frequencies when vibrating. The lowest frequency is referred to in music as the fundamental frequency while the following frequencies are referred to as the overtones. While the brain usually perceives the fundamental frequency alone, sound waves according to the overtone frequencies are also emitted at varying levels of power. The formants of a tone are emphasised differently depending on the shape of the vocal tract, which in fact is that which allows the human ear to perceive differences in vowels, consonants, tonal inflections, and so on. In essence, the manipulation of the vocal tract is a natural human ability which allows us to hear variations in speech and tone.^[1].

Mongolian throat singer: throat singing is a variation of overtone singing where additional muscle pressure is applied in the throat to create a unique timbre

The human brain typically perceives the fundamental frequency and does not naturally distinguish it from its overtones during speech and conventional singing, however changes in the emphasis of different overtones do create a perceivable change in tone and articulation. In music, this tonal quality is often referred to as the timbre, and allows human brains to distinguish between the sounds of different instruments, voices, and other mediums. For vocalists, different syllables and timbres are created through the movement and adjustment of the vocal system, including the vocal folds, jaw movement, lip shape, etc.^[2].

How overtone singing is achieved

Overtone singing gives the perception through vocal technique of one of the overtones of a sung fundamental frequency being distinctively heard as well by the human ear. Vocalists who use this technique are also able to give the perception of the movement of the higher frequency to other overtone frequencies, while simultaneously maintaining the fundamental frequency. In many cultures, such as Tuvan culture, overtone singers will create a drone, or a long and constant fundamental lower pitch, while the higher overtones move in a melody above it.^[3].

Diagram of the different components of the vocal tract used to create different timbre

Adept overtone singers achieve the desired effect through altering the shape of the vocal tract, in the same way that most people do so to create syllables in their speech. While the sounds produced by conventional singers are typically perceived as single notes, overtone singers have the ability to emphasise certain overtones of their notes while damping others^[4]. Singers will change the shape of their vocal tract in order to align the frequency of a formant with that of a harmonic, therefore causing the overtone to resonate more strongly and create a perceivable change. A large number of components in the vocal tract is used to do overtone singing, with manipulations in the larynx and vocal folds, jaw movement to manipulate the volume of the mouth, changing lip shape, as well as the position and thickness of the tongue.^[5]. Just as human speech makes use of changes in the vocal tract to create different vowel and consonant sounds as well as tonal inflections, overtone singing also uses shape changes in the vocal tract in order to highlight the different partials of a tone in order to create its effect.

Scientific phenomenon in overtone singing

Diagram showing how the partials can be stronger than the fundamental itself in overtone singing

Overtone singers are able to manipulate the sound of their voice so that the desirable partials in their sound are emitted at comparable, and sometimes much greater loudness than the fundamental frequency. Furthermore, they are able to emit the other undesired partials quietly, so that the fundamental frequency and the overtone are highlighted.

Despite the perception of two frequencies appearing at once during overtone singing, research has shown that in fact two partials that are of close proximity both resonate at high levels on top of the fundamental frequency. Studies have suggested that the distinct clarity and strength of the sung overtones are a result of Helmholtz resonance within the vocal tract, as biologically the vocal tract is not well equipped to produce such high intensity formants due to damping mechanisms, and thus another cause must be the result.^[6] Helmholtz resonance is the resonance of air in a cavity, creating musical pitches within.

The overtone singer must be highly trained in the manipulation of the vocal tract, as slight adjustments can cause significant changes to the produced sound. In order to produce the unique effect, overtone singers must be aware of a large variety of factors in order to optimise both the tone and amplitude of their sound. For instance, minimising the size of the mouth may be beneficial for the conservation of energy while singing. However, increasing the size of the mouth will also increase the radiation efficiency of the sound. The shape of the tongue also affects the articulation of the sound of the formants^[7]. Such factors indicate that the overtone singer must be extremely adept in order to recreate the ancient sound.

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