A mathematician is not necessarily the first person that you would associate with speech science. But as Samuli Siltanen demonstrates in this short film, researchers in his field can bring fresh approaches to long-standing challenges relating to the human voice.

Siltanen, who works at the University of Helsinki, explains how vowel sounds are generated in the vocal tract. Never one to shy away from giving a demonstration, Siltanen reveals his own vocal folds in action by visiting a clinic where a laryngoscope attached to a camera is inserted into his throat.

Vowel sounds are produced thanks to two independent processes. First is what Siltanen refers to as the "excitation signal", which is generated as we push air through our lungs into the vocal folds, causing them to flap. The sound waves then travel up towards the mouth through the vocal tract, and it is this stage where speech generates its "colour" or "texture".

An inverse approach

Siltanen demonstrates how an electronic larynx can be used to provide the excitation signal in cases where people have had their vocal folds damaged or removed, perhaps following surgery for cancer. His particular expertise is an area of mathematics called "inverse problems", so he is interested in using available data to work out the missing information in speech generation. What this means in practise is that he is building a clearer picture of the physiology and processes relating to human speech based on available measurements.

Why is this useful? Siltanen explains that it can be the case that a young girl or woman who has lost her voice has to use a computer synthesis device that speaks with a grown man's voice. The reason is related to so-called glottal inverse filtering, which means it is much more difficult to recreate a realistic female or a child's voice because of the higher fundamental frequencies involved compared with a mature male voice.

Siltanen will be explaining the challenges of speech synthesis, and how mathematics can help, in a feature he is writing that will appear in a future issue of Physics World.