The double-resonance theory
The single-resonance theory was the standard for a couple of hundred years until the end of the 19th century, expressing the belief that a typical vowel tone arose in an anterior (mouth) cavity whose dimensions determined the pitch of the resonance. Alexander Melville Bell’s vowel model was based on this theory in his Visible Speech (1867). But Helmholtz had already discovered and reported a second, lower, resonance in front vowels in (1863, Die Lehre von den Tonempfindungen, Braunschweig). Alexander Graham Bell (1879, Vowel Theories, American Journal of Otology 1) reported two resonances for all vowels, assigning the lower resonance to a posterior (throat) cavity and the higher resonance to an anterior (mouth) cavity. Finally, Lloyd (1890-92, Speech sounds, their nature and causation, Phonetische Studien 3-5) found a third resonance and assigned it to a cavity between the lips.
Sir Richard Paget (1930, Human Speech, London, p 41) was told by Jones in the 1920s that the lower resonance was not generally recognized except for the front vowels. If Jones’ assessment were correct, it would imply that a majority of phoneticians around 1920 would still not have ventured beyond Helmholtz, and would have rejected A. G. Bell’s two resonances, and Lloyd’s third resonance. That was apparently the solution of the day at the beginning of the 20th century, two resonances for front vowels only, each resonance uniquely in its own cavity, doubting A G Bell’s two resonances in back vowels, and overlooking Lloyd’s contribution in silence. The double-resonance theory, a central theme of Paget (1930), placed F1 in the pharynx and F2 in the mouth for all vowels. It was a useful approach for a few more decades, seemingly underpinning a more ambitious spectral explanation for the Bell model, supported by electronic models. It was finally superseded by the work of Chiba & Kajiyama and Fant who demonstrated any number of formants simultaneously, each with its own standing wave throughout the entire length of the vocal tract, the frequency of every formant being modified by local narrowing anywhere in the vocal tract. So it finally turned out that F2 is not tuned uniquely in the mouth, nor F1 uniquely in the pharynx, demonstrated again by Fant (1980, The relation between area functions and the acoustic signal, Phonetica 37:55-86) who showed that F2 of [i] had substantially more reactive energy in the pharynx but little in the palatal passage (i.e. that F2 of [i] is more sensitive to tongue movement posteriorly in the pharynx than anteriorly in the palatal passage, contradicting the expectation of the double resonance theory).