Characteristics of vowels, such as those of the International Phonetic Alphabet, are typically plotted on a flat chart by their first and second formant frequencies or by qualities such as front, central, back, close, mid and open along the axes. The two dimensional charts with articulatory features as axes typically show the vowels in a diagram with vowels having one of the articulatory features in common being joined by a straight line, usually such that the space of vowels is bound by a quadrilateral or a roughly triangular shape. Sometimes the distinctions between the vowels are displayed in a three dimensional plot with the acoustic properties of the first, second, and third formants as axes or by articulatory features with the three axes as frontness, height, and roundness. While the plots in two dimensions by the first two formant frequencies are often compared to those in which height and frontness are the two axes, comparison of articulatory and acoustic plots in three dimensions are not so usual as a method of teaching phonetics.
On the Base Dozen Forum there is presented a portrayal of vowels in a three dimensional plot by articulatory features of high versus low, back versus front, and rounded versus unrounded as the three axes. The space of vowels is let be a cube with extreme vowels at its corners. For example, the most high and rounded back vowel is one of the corners, and the lowest and furthest back unrounded vowel is another corner. Another corner is allocated to the most high and front unrounded vowel. These are three of the standard cardinal vowels proposed by the phonetician Daniel Jones. In the three dimensional space that is here proposed, these three cardinal vowels form the vertices of an equilateral triangle in the cube. To the other corners are assigned vowels according to the resulting articulatory dimensions of the axes aligned with the edges of the cube. Apart from vowels at the corners of the cube, other vowels of intermediate qualities are plotted between the corners, either along the edges or on the faces of the cube. At the centre of the cube is a schwa.
To compare this cube of vowels by their articulatory features to their acoustic properties on a formant chart, the cube is transformed by rotational orientation and scaling before it is projected onto the planar formant chart. The result is that the positions of the projected vowels agree extraordinarily well to their positions by their measured formant frequencies. Using the outlines of the edges of the projected cube, it is possible to predict the effects of changes in articulation on the acoustic qualities of the vowel. This should make this proposal more useful for understanding what a vowel will sound like in relation to known vowels than an ordinary chart in which unrounded and rounded vowels having the same features in other respects are merely placed beside each other in pairs without indicating how their formants differ, although it is expected that rounding applied to front vowels or unrounding applied to back vowels will centre them acoustically.
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Ideas & Observations 