Introduction to Harmonics and Formants

Matthew Reeve

When a sustained sound is produced the vocal folds are vibrating hundreds of times per second. The number of times per second the vocal folds vibrate is referred to as the fundamental frequency. At the same time as producing the fundamental frequency a range of other higher frequencies called overtones or harmonics are also produced. These arise from the complex wave effects that occur when the folds vibrate. This movement can be viewed on stroboscopic images of vocal fold vibration and is often referred to as the mucosal wave.

What are harmonics?

Harmonics can be defined as frequencies (or more simply vibrations) that are integral multiples of the fundamental, that occur simultaneously with the fundamental vibration and which contribute to the resonant quality or timbre of the sound. Thus when you pluck a string on a guitar it not only vibrates across the whole of its length, it also vibrates at half its length, a quarter, a third and so on.

Every musical instrument favours certain clusters of harmonics according to its shape. The shape and properties of the instrument will cause the harmonics clusters to be boosted or dampened. This creates the difference between the same note played on different instruments. The voice is like an instrument in this way. By changing and modifying the shape of the vocal tract, the shape and acoustic properties of the tube are altered. This causes different harmonics to be boosted or dampened.

This is demonstrated when the larynx is raised or lowered whilst maintaining the same pitch – the resonance changes are clearly audible. The sounds could be described as being darker or brighter. Acoustically, in the lower laryngeal position, lower frequency harmonics were boosted, whilst in the higher position higher frequency harmonics were boosted.

What are formants?

Formants are the clusters of harmonics created in the vocal tract. As sound travels through the vocal tract it is filtered by the resonance properties of the tract. The harmonics that are close to the resonance frequencies of the vocal tract get stronger and others will be damped. A series of peaks and valleys are formed in the sound spectrum. The peaks are given numbers that relate to their position in the sound spectrum and the relative positioning of the peaks is determined by the dimensions of the vocal tract. For example, moving the tongue forward in the mouth is known to raise the second formant, lowering the jaw raises the first formant and so on. Formant readings are also used to identify vowels and the relationship between laryngeal height and the tongue.

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