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According to Wikipedia, bandpass filters were used before the digital era began. Can anyone explain to me how they appeared or were analyzed?

Spectrograms are usually created in one of two ways: approximated as a filterbank that results from a series of bandpass filters (this was the only way before the advent of modern digital signal processing), or calculated from the time signal using the FFT

I'm curious about the history of the science of phonetics and looking to learn more about how technological innovation shaped the development of the science. Thanks a lot.

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My first job as a graduate assistant in linguistics in 1965-66 was in Ilse Lehiste's acoustic phonetics lab at Ohio State, and for a time, I spent many hours running a Kay Sonagraph, making sonagrams (spectrograms) from taped informant sessions. I don't know how the variable bandpass filter was implemented, but you could see what was happening. The Sonagraph recorded a short sample of speech -- just a few seconds -- then replayed that over and over as the bandpass filter moved its band little by little.

A strip of special coated paper was strapped on a rotating drum, and an electrode burnt a line of varying darkness, depending on the output of the bandpass filter. The electrode moved upward on the drum (or downward, I can't remember), so you would be left at the end with a time vs. frequency analysis of the sample, with dark areas on the paper corresponding to the formants. At least for men's voices, you could also see the vertical striations for individual vocal cord vibrations, and count them to estimate the voice pitch.

At the time, Lehiste was investigating the acoustics of the overlong consonants of Estonian, and the rearticulations partway through the articulations of the consonants were clear on the Sonagrams as small bursts of energy (quite inaudible).

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I can't tell if you're asking about bandpass filtering or about linguistic applications for ideas in acoustics. For the latter, I suggest Martin Joos's monograph Acoustic Phonetics (1948, Language monographs). This paper gives an overview of the history of acoustic phonetics in the US, from which by reference-tracing you can generalize to outside the US (e.g. to the work of Helmholtz, or Chiba & Kajiyama). I think one can accurately say that when the sound spectrogram became publically available, it revolutionized research into speech, so while the underpinnings go back to before 1945, life starts in 1945 (or shortly after when a commercial model was actually available).

If you're interested in the technical details of the machine's workings, it is laid out (explained, sorta) here.

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