in articulation to retracted tongue root but is acoustically distinct. A stronger degree of pharyngealisation occurs in the Northeast Caucasian languages and the Khoisan languages. They might be called epiglottalized since the primary constriction is at the tip of the epiglottis. The greatest degree of pharyngealisation is found in the strident vowels of the Khoisan languages, where the larynx is raised, and the pharynx constricted, so that either the epiglottis or the arytenoid cartilages vibrate instead of the vocal cords. Note that the terms pharyngealized, epiglottalized, strident, and sphincteric are sometimes used interchangeably. Rhotic vowels Rhotic vowels are the "R-colored vowels" of American English and a few other languages. Reduced vowels Some languages, such as English and Russian, have what are called 'reduced', 'weak' or 'obscure' vowels in some unstressed positions. These do not correspond one-to-one with the vowel sounds that occur in stressed position (so-called 'full' vowels), and they tend to be mid-centralized in comparison, as well as having reduced rounding or spreading. The IPA has long provided two letters for obscure vowels, mid and lower , neither of which are defined for rounding. Dialects of English may have up to four phonemic reduced vowels: , , and higher unrounded and rounded . (The non-IPA letters and may be used for the latter to avoid confusion with the clearly defined values of IPA letters like and , which are also seen, since the IPA only provides for two reduced vowels.) Acoustics The acoustics of vowels are fairly well understood. The different vowel qualities are realized in acoustic analyses of vowels by the relative values of the formants, acoustic resonances of the vocal tract which show up as dark bands on a spectrogram. The vocal tract acts as a resonant cavity, and the position of the jaw, lips, and tongue affect the parameters of the resonant cavity, resulting in different formant values. The acoustics of vowels can be visualized using spectrograms, which display the acoustic energy at each frequency, and how this changes with time. The first formant, abbreviated "F1", corresponds to vowel openness (vowel height). Open vowels have high F1 frequencies, while close vowels have low F1 frequencies, as can be seen in the accompanying spectrogram: The and have similar low first formants, whereas has a higher formant. The second formant, F2, corresponds to vowel frontness. Back vowels have low F2 frequencies, while front vowels have high F2 frequencies.