1.1. Acoustic phonetics
Phonetics is the science that aims to describe speech, phonology is the science that aims to describe language. Phonetics studies the sounds of human language from all angles, whereas phonology is only interested in these same sounds in terms of the role they have in the functioning of a language. Consequently, the objects described by phonetics are a priori independent of their function in the linguistic system, whether they are articulatory, acoustic or perceptive phonetics. While articulatory phonetics is very old (see the well-known scene from Le Bourgeois Gentilhomme, in which Monsieur Jourdain is having the details of the articulation of consonants and vowels explained to him very specifically, which every speaker realizes without being aware of the mechanisms involved), acoustic phonetics could only develop with the appearance of the first speech recording instruments, and the realization of instruments based on mathematical tools, to describe their physical properties.
During the 20th Century, recording techniques on vinyl disc, and then on magnetic tape, made it possible to preserve sound and analyze it, even in the absence of the speakers. Thanks to the development of electronics and the invention of the spectrograph, it was possible to quickly carry out harmonic analysis, sometimes painstakingly done by hand. Later, the emergence of personal computers in the 1980s, with faster and faster processors and large memory capacities, led to the development of computerized acoustic analysis tools that were made available to everyone, to the point that phonologists who were reluctant to investigate phonetics eventually used them. Acoustic phonetics aims to describe speech from a physical point of view, by explaining the characteristics that are likely to account for its use in the linguistic system. It also aims to describe the links between speech sounds and the phonatory mechanism, thus bridging the gap with traditional articulatory phonetics. Lastly, in the prosodic field, it is an essential tool for data acquisition, which is difficult to obtain reliably by auditory investigation alone.
1.2. Sound waves
Speech is a great human invention, because it allows us to communicate through sound, without necessarily having visual contact between the actors of communication. In acoustic phonetics, the object of which is the sound of speech, the term “sound” implies any perception by the ear (or both ears) of pressure variations in an environment in which these ears are immersed, in other words, in the air, but also occasionally under water for scuba divers. Pressure variations are created by a sound source, constituted by any material element that is in contact with the environment and manages to locally modify the pressure. In a vacuum, there can be no pressure or pressure variation. Sound does not propagate in a vacuum, therefore, a vacuum is an excellent sound insulator.
Pressure variation propagates a priori in all directions around the source, at a speed which depends on the nature of the environment, its temperature, average pressure, etc. In air that is 15°C, the propagation speed is 340 m/s (1,224 km/h) at sea level, while in seawater it is 1,500 m/s (5,400 km/h). Table 1.1 gives some values of propagation velocities in different environments. We can see that in steel, the speed of sound is one of the highest (5,200 m/s or 18,720 km/h), which explains why, in the movies from our childhood, outlaws of the American West could put an ear to a train rail to estimate its approach before attacking it, without too much danger. The possibility of sound being perceived by humans depends on its frequency and intensity. If the frequency is too low, less than about 20 Hz (in other words, 20 cycles of vibration per second), the sound will not be perceived (this is called infrasound). If it is too high (above 16,000 Hz, however this value depends on the age of the ear), the sound will not be perceived either (it is then called ultrasound). Many mammals such as dogs, bats and dolphins do not have the same frequency perception ranges as humans, and can hear ultrasound up to 100,000 Hz. This value also depends on age. Recently, the use of high-frequency sound generators, that are very unpleasant for tee...
