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Auditory phonetics is a branch of phonetics concerned with the hearing of speech sounds and with speech perception . The range of sounds that are exploited phonetically by the world's languages represents only a portion of what humans are capable of producing vocally. Moreover, among attested phonetic segments there is enormous variation in the frequency of occurrence across languages: Most segments are relatively rare, while a few occur almost universally. A major task of phonetic theory is to explain these patterns of selection. Traditionally, many phoneticians have believed that two principles-articulatory economy and perceptual distinctiveness-play a role in shaping sound patterns and segment inventories. However, these principles have not often been formulated with sufficient precision to have genuine explanatory content. The focus of this presentation is on the role of auditory factors in structuring vowel systems. First, attempts to predict vowel inventories on the basis of a principle of auditory dispersion (i.e., sufficient auditory contrast) are reviewed. Second, a corollary of the dispersion principle, the auditory enhancement hypothesis, that provides a general account of certain widespread patterns of phonetic covariation in the production of vowels is explored. Finally, how the notion of sufficient contrast may explain some puzzling acoustic differences between male and female tokens is considered.
If articulatory phonetics studies the way in which speech sounds are produced, auditory phonetics focuses on the perception of sounds or the way in which sounds are heard and interpreted. Remembering our conventional division of linguistic communication into several stages of a process unfolding between two parties, the sender of the message and its addressee, we may say that while articulatory phonetics is mainly concerned with the speaker, auditory phonetics deals with the other important participant in verbal communication, the listener.
It is again, obviously, a field of linguistic study which has to rely heavily on biology and more specifically on anatomy and physiology. We should say from the very beginning, however, that the mechanism and physiology ofsound perception is a much hazier field that the corresponding processes related to the uttering of the respective sounds. This is so because speech production is a process that takes place roughly along the respiratory tract which is, comparatively, much easier to observe and study than the brain where most processes linked to speech perception and analysis occur.
Our presentation so far has already revealed a fundamental characteristic of acoustic phonetics which essentially differentiates it from both articulatory and acoustic phonetics: its lack of unity. We are in fact dealing with two distinct operations which, however, are closely interrelated and influence each other: on the one hand we can talk about audition proper, that is the perception of sounds by our auditory apparatus and the transforming of the information into a neural sign and its sending to the brain and, on
the other hand, we can talk about the analysis of this information by the brain which eventually leads to the decoding of the message, the understanding of the verbal message.
When discussing the auditory system we can consequently talk about its peripheral and its central part, respectively. We shall have a closer look at both these processes and try to show why they are both clearly distinct and at the same time they are closely related.
Before the sounds we perceive are processed and interpreted by the brain, the first anatomical organ they encounter is the ear. The ear has a complex structure and its basic auditory6 functions include the perception of auditory stimuli, their analysis and their transmission further on to the brain. We can identify three components: the outer, the middle and the inner year. The outer ear is mainly represented by the auricle or the pinna and the auditory meatus or the outer ear canal. The auricle is the only visible part of the ear, constituting its outermost part, the segment of the organ projecting outside the skull. It does not play an essential role in audition, which is proved by the fact that the removing of the pinna does not substantially damage our auditory capacity.
The auricle rather plays a protective role for the rest of the ear and it also helps us localize sounds. The meatus, or the outer ear canal is a tubular structure playing a double role: it, too, protects the next segments of the ear - particularly the middle ear - and it also functions as a resonator for the sound waves that enter our auditory system. The middle ear is a cavity within the skull including a number of little anatomical structures that have an important role in audition. One of them is the eardrum. This is a diaphragm or membrane to which sound waves are directed from outside and which vibrates, acting as both a filter and a transmitter of the incoming sounds. The middle ear also contains a few tiny bones:
the mallet, the anvil and the stirrup. The pressure of the air entering our auditory system is converted by the vibration of the membrane (the eardrum) and the elaborate movement of the little bones that act as some sort of lever system into mechanical movement which is further conveyed to the oval window, a structure placed at the interface of the middle and inner ear. As pointed out above, the middle ear plays an important protection role.
The muscles associated with the three little bones mentioned above contract in a reflex movement when sounds having a too high intensity reach the ear. Thus the impact of the too loud sounds is reduced and the mechanism diminishes the force with which the movement is transmitted to the structures of the inner ear. It is in the middle ear too, that a narrow duct or tube opens. Known as the Eustachian tube it connects the middle ear to the pharynx. Its main role is to act as an outlet permitting the air to circulate between the pharynx and the ear, thus helping preserve the required amount of air pressure inside the
middle ear. The next segment is the inner ear, the main element of which is the cochlea, a cavity filled with liquid. The inner ear also includes the vestibule of the ear and the semicircular canals.
The vestibule represents the central part of the labyrinth of the ear and it gives access to the cochlea. The cochlea is a coil-like organ, looking like the shell of a snail. At each of the two ends of the cochlea there is an oval window, while the organ itself contains a liquid. Inside the cochlea there are two membranes: the vestibular membrane and the basilar membrane. It is the latter that plays a central role in the act of audition. Also essential in the process of hearing is the so-called organ of Corti, inside the
cochlea, a structure that is the real auditory receptor. Simplifying a lot, we can describe the physiology of audition inside the inner ear as follows: the mechanical movement of the little bony structures of the middle ear (the mallet, the anvil and the stirrup) is transmitted through the oval window to the liquid inside the snail-like structure of the cochlea; this causes the basilar membrane to vibrate: the membrane is stiffer at one end than at the other, which makes it vibrate differently, depending on the pitch of the sounds that are received. Thus, low-frequency (grave) sounds will make vibrate the membrane at the less stiff (upper) end, while highfrequency (acute) sounds will cause the lower and stiffer end of the membrane to vibrate.
The cells of the organ of Corti, a highly sensitive structure because it includes many ciliate cells that detect the slightest vibrating movement, convert these vibrations into neural signals that are transmitted via the auditory nerves to the central receptor and controller of the entire process, the brain.
The way in which the human brain processes auditory information and, in general, the mental processes linked to speech perception and production are still largely unknown. What is clear, however, regarding the perception of sounds by man's auditory system, is that the human ear can only hear sounds having certain amplitudes and frequencies. If the amplitudes and frequencies of the respective sound waves are lower than the range perceptible by the ear, they are simply not heard.
If, on the contrary, they are higher, the sensation they give is one of pain, the pressure exerted on the eardrums being too great. These aspects are going to be discussed below when the physical properties of sounds are analyzed. As to the psychological processes involved by the interpretation of the sounds we hear, our knowledge is even more limited. It is obvious that hearing proper goes hand in hand with the understanding of the sounds we perceive in the sense of organizing them according to patterns already existing in our mind and distributing them into the famous acoustic images that Saussure spoke of. It is at this level that audition proper intermingles with psychological processes because our brain decodes, interprets, classifies and arranges the respective sounds according to the linguistic (phonological) patterns already existing in our mind.
It is intuitively obvious that if we listen to someone speaking an unknown language it will be very difficult for us not only to understand what they say (this is out of the question given the premise we started from) but we will have great, often insurmountable difficulties in identifying the actual sounds the person produced. The immediate, reflex reaction of our brain will be to assimilate the respective sounds to the ones whose mental images already exists in our brain, according to a very common cognitive reaction of humans that always have the tendency to relate, compare and contrast new information to already known information.
There is a growing consensus that developmental dyslexia is associated with a phonological-core deficit. One symptom of this phonological deficit is a subtle speech-perception deficit. The auditory basis of this deficit is still hotly debated. If people with dyslexia, however, do not have an auditory deficit and perceive the underlying acoustic dimensions of speech as well as people who read normally, then why do they exhibit a categorical-perception deficit? A potential answer to this conundrum lies in the possibility that people with dyslexia do not adequately handle the context-dependent variation that speech signals typically contain. A mathematical model simulating such a sensitivity deficit mimics the speech-perception deficits attributed to dyslexia. To assess the nature of the dyslexic problem, the authors examined whether children with dyslexia handle context dependencies in speech differently than do normal-reading individuals. Contrary to the initial hypothesis, children with dyslexia did not show less context sensitivity in speech perception than did normal-reading individuals at auditory, phonetic, and phonological levels of processing, nor did they reveal any categorization deficit. Instead, intrinsic properties of online phonological processes, not phonological representations per se, may be impaired in dyslexia.
An auditory illusion is an illusion of hearing, the aural equivalent of an optical illusion: the listener hears either sounds which are not present in the stimulus, or "impossible" sounds. In short, auditory illusions highlight areas where the human ear and brain, as organic, makeshift tools, differ from perfect audio receptors (for better or for worse).