How was the acoustic theory of speech production informed by electrical circuit theory?

Question

The acoustic theory of speech production as worked out by Gunnar Fant depends on a correspondence between the vocal tract and elementary electrical circuits. But the quote below perplexes me. In what way could electrical circuit theory be informative?

This was a truly pioneering era in speech research as an outgrowth from linguistics, electrical circuit theory, psychoacoustics and information theory. (source)

Wikipedia doesn't really shed much light on the topic, even if it mentions it there:

On a theoretical level, speech acoustics can be modeled in a way analogous to electrical circuits. Lord Rayleigh was among the first to recognize that the new electric theory could be used in acoustics, but it was not until 1941 that the circuit model was effectively used, in a book by Chiba and Kajiyama called "The Vowel: Its Nature and Structure".

In this book, I can find some diagrams like the one below which purports to be a "four terminal network representation of the production of a any non-nasal sound", but I can't make heads or tails of it.

I'd be grateful for any information that could shed light on how these two disciplines connect. Thanks a lot.

Answer 1

Mechanical and electrical components are analogous. Ideal resistors are mathematically identical to ideal dashpots, ideal inductors are identical to ideal springs, and ideal capacitors are identical to ideal masses (see for example, Analogous Electrical and Mechanical Systems).

This means that mechanical filters, like those that shape speech in the vocal system, can be described in terms of equivalent analog electrical filters (see wikipedia's article on Mechanical Filters), where the mathematical principles are much better-developed.

In your specific example, from what I can tell, the "current" is the acoustic energy, the "resistance" is the viscosity of the air, the "inductance" is the elasticity of the air, the "current source" is the vocal cords, and the "impedance" is energy absorption by the vocal tract. You can use then use the equations for these electrical components to describe the mechanical behavior of the sound.

Answer 2

Electrical circuits are well-enough understood that you can talk about them in terms of equations. If speech is sufficiently like circuitry, speech can be described as an equation, and quantification is the holy grail of science.