Acoustic phonetics is a subfield of phonetics which deals with acoustic aspects of speech sounds. It investigates properties such as the mean squared amplitude of a waveform, its duration, its fundamental frequency or other properties of its frequency spectrum, and to abstract linguistic concepts like phrases and utterances.
The study of acoustic phonetics was greatly enhanced in the late 19th century by the invention of the Edison phonograph. The phonograph allowed the speech signal to be recorded and then later processed and analyzed. By replaying the same speech signal from the phonograph several times, filtering it each time with a different filter, a spectrogram or graphic image of the speech recording could be created.
A series of papers by Ludimar Hermann investigated the spectral properties of vowels and consonants using the Edison phonograph, and it was in these papers that the term formant was first introduced. Hermann also played back vowel recordings made with the Edison phonograph at different speeds in an effort to distinguish between varying theories of vowel production.
Further advances in acoustic phonetics were made possible by the development of the telephone industry. During World War II, work at the Bell Telephone Laboratories greatly facilitated the systematic study of the spectral properties of periodic and aperiodic speech sounds, vocal tract resonances and vowel formants, voice quality and prosody.
On a theoretical level, acoustic phonetics advanced greatly when it became clear that speech acoustics could be modeled in a way analogous to electrical circuits. During the early 1900s, 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 1952, the book Preliminaries to Speech Analysis was published, tying acoustic phonetics and phonological theory together. This was followed in 1960 by Gunnar Fant’s Acoustic Theory of Speech Production, which has remained the major theoretical foundation for speech acoustic research in both the academy and industry.