Vowels, Vowel Formants and Vowel Modification

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Vowels

The term ‘vowel’ is commonly used to mean both vowel sounds and the written symbols that represent them.

In phonetics, a vowel (from the Latin word ‘vocalis’, meaning ‘uttering voice’ or ‘speaking’) is a sound in spoken language that is characterized by an open configuration of the vocal tract, in contrast to consonants, which are characterized by a constriction or closure at one or more points along the vocal tract. Vowels are understood to be syllabic, meaning that they usually form the peak or nucleus (the central part) of a syllable - whereas consonants form the onset (any consonant or sequence of consonants preceding the nucleus) and coda (any consonant or sequence of consonants following the nucleus).

Vowels are extremely important to singing. They almost always carry the greatest energy in the speech signal because, during vowel phonation, the vocal tract is most open. Vowels are also stable segments of speech during which the articulators do not move, allowing the resonance frequencies of the vocal tract to remain more stable (minus the characteristic waxing and waning of the frequencies due to the rapid and periodic opening and closing of the vocal folds during phonation). Because of these characteristics, vowels are probably the easiest speech category to recognize in a spectrogram - an electronic device that mesures peaks in the harmonic spectrum of the voice during singing. A singer needs to learn to sing vowels while not allowing consonants, which resonate and ‘project’ more poorly than vowels do, to get in the way.

The different ways in which the vibrations and tension in the larynx affect the quality of a vowel are called phonation. The simplest phonation is called voicing. Voice or voicing is a term used in phonetics and phonology to characterize speech sounds, with sounds described as either voiced or voiceless (or unvoiced). A voiced sound is one in which the vocal folds, which are cartilages inside the larynx, vibrate during the articulation of the vowel. At the articulatory level, a voiceless sound is one in which the folds do not vibrate in order to produce the sound. (Voicing is the difference between the pairs of sounds that are associated with the English letters ‘s’ and ‘z’, with the 'z' sound being voiced.) In all languages, without exception, most vowels are voiced sounds. (Most languages, in fact, have only voiced vowels.) In whispered speech, vowels are devoiced.

In both singing and speech, optimal vowel phonemes are voiced, and are tense and therefore particularly distinct. Optimal consonants, on the other hand, are voiceless and lax.

(Oral) vowels are formed with no major obstacles in the vocal tract so that there is no build-up of air pressure at any point above the glottis (the supra-glottal spaces). The air stream, once out of the glottis, passes through the speech organs and is not cut off or constricted by the supra-glottal resonators, nor by the articulators themselves, which means that they are ‘open’ sounds. The resonators, then, cause only resonance, reinforcing certain frequency ranges.

This isn’t the case with consonants and nasal sounds, however. With consonants, there is a constriction or closure at some point along the vocal tract. For consonants, there are also antiresonances in the vocal tract at one or more frequencies due to oral constrictions. An antiresonance is the opposite of a resonance, such that the impedance is relatively high rather than low. Consequently, consonants attenuate or eliminate formants at or near these frequencies, so that they appear weakened or are missing altogether when looking at spectrograms. For these reasons, singers need to sing on open vowels, and not sustain consonant sounds, if they wish to maximize the resonance and carrying power of their voices.

In addition, for nasal consonants and nasal vowels, the vocal tract divides into a nasal branch and an oral branch, and interference between these branches produces more antiresonances. Furthermore, nasal consonants and nasal vowels can exhibit additional formants, called nasal formants, arising from resonance within the nasal branch. Consequently, nasal vowels may show one or more additional formants due to nasal resonance, while one or more oral formants may be weakened or missing due to nasal antiresonance. For these reasons, singers should avoid singing with excessive nasality in their tones, even when singing in a foreign language that utilizes nasal vowels, if they hope to achieve a more fully resonant sound.

There are three possible resonators involved in the articulation of a vowel: the oral cavity, the labial cavity, and the nasal cavity. The number of resonators involved distinguish between the type of vowels created. With oral vowels, there is no nasal resonance, as the soft palate is raised and air does not enter the nasal cavity. With nasal vowels, the nasal resonator is activated by lowering the soft palate and allowing air to pass through the nose and mouth simultaneously. In rounded vowels, a third resonator – the labial resonator – is activated from oral vowels when the lips are pushed forward and rounded. If, on the other hand, the lips are spread sideways or pressed against the teeth, as in the case of unrounded vowels, no labial resonator is formed.

A vowel's timbre (or colour or quality) depends on the number of active resonators (among the oral, labial and nasal cavities), the shape of the oral cavity and the size of the oral cavity. The articulators, primarily the tongue and lips, shape the vocal tract into these different resonating cavities, giving a characteristic acoustic quality to each vowel.

The shape of the oral cavity is determined by the general position of the tongue in the mouth. Based on positioning of the tongue, the vowels are divided into three classes: front, back and central vowels. In front vowels, the tongue body is held in the pre-palatal region. In back vowels, the tongue body is held in the post-palatal or velar region. In central vowels, the tongue body is in the medio-palatal region.

The size of the oral cavity is the last factor considered in the articulatory classification of a vowel. The size of the oral cavity depends directly upon the degree of aperture (opening) of the mouth ; that is, upon the distance between the hard palate (roof of the mouth) and the tongue's highest point. Arbitrarily, four degrees of aperture are distinguished, from the most closed (first degree) to the most open (fourth degree).

I will describe these features in greater detail in the section on Cardinal Vowels.

IPA Symbols For Vowels

Before continuing with this discussion on singing vowels, I would like to take some time to explain the symbols that I use for the spoken vowel sounds, which are those used internationally to represent the same sounds in language.

The International Phonetic Alphabet (IPA) is a system of phonetic notation, originally developed in 1886 by a group of French and British language teachers that later became known as the International Phonetic Association, that is used as a standardized representation of the sounds of spoken language. In 1888, the alphabet was revised so as to be uniform across languages, thus providing the base for all future revisions.

The IPA is used by foreign language students and teachers, linguists, speech pathologists and therapists, singers, actors, lexicographers, and translators. IPA symbols are used in phonetic transcriptions in modern dictionaries. More relevant to singers, IPA also has widespread use among classical singers for preparation, especially among English-speaking singers who rarely sing in their native language. In fact, opera librettos are authoritatively transcribed in IPA.

The Association created the IPA so that the sound values of most vowels and consonants taken from the Latin alphabet would correspond to ‘international usage’. For this reason, most symbols are either Latin or Greek letters, or modifications thereof. The sound values of modified Latin letters can often be derived from those of the original letters. However, there are symbols that are neither. Despite its preference for letters that harmonize with the Latin alphabet, the International Phonetic Association has occasionally admitted symbols that do not have this property. Beyond the letters themselves, there are a variety of secondary symbols which aid in transcription. Apart from the fact that certain kinds of modification to the shape of a letter generally correspond to certain kinds of modification to the sound represented, there is no way to deduce the sound represented by a symbol from the shape of the symbol.

Since its creation, the IPA has undergone a number of revisions. Apart from the addition and removal of symbols, changes to the IPA have consisted largely in renaming symbols and categories, and modifying typefaces. As of 2008, there are 107 distinct letters (representing consonants and vowels) and 56 prosody marks and diacritics - glyphs or marks that may appear above or below a letter, or in some other position such as within the letter or between two letters, whose main function is to change the sound value of the letters to which they are added, although some are used to further specify these sounds and still others to indicate such qualities as length, tone, stress, and intonation.

The IPA is designed to represent only those qualities of speech that are distinctive in spoken language: phonemes, intonation, and the separation of words and syllables. The general principle of the IPA is to provide one symbol for each distinctive sound (or speech segment). This means that it does not use letter combinations to represent single sounds, or single letters to represent multiple sounds (the way that the letter <x> represents both [ks] or [gz] in English). There are no letters that have context-dependent sound values, (as the letter ‘c’ does in English and other European languages), and finally, the IPA does not usually have separate letters for two sounds if no known language makes a distinction between them (a property known as ‘selectiveness’).

Neither broad nor narrow transcription using the IPA provides an absolute description; rather, they provide relative descriptions of phonetic sounds. This is especially true with respect to the IPA vowels: there exists no hard and fast mapping between IPA symbols and formant frequency ranges, and, in fact, one set of formant frequencies may correspond to two different IPA symbols, depending on the phonology of the language in question. (I will be explaining more about how every vowel has its own distinct formant frequency in the section entitled Vowel Formants.)

The most difficult thing about learning the IPA and learning to transcribe English phonetically is breaking the habit of associating sounds with spellings. English speakers must learn the sound values for certain symbols whose IPA values are different from their most typical English spelling values. An IPA symbol always has the same sound value, regardless of who is speaking; it does not have more than one pronunciation. Therefore, the same vowel sound should be represented by the same symbol, regardless of a word's conventional spelling. This means that if two words are homonyms – they sound the same but have different meanings, as in ‘heard’ and ‘herd’ or ‘cite’, ‘site’ and ‘sight’ - they should have exactly the same transcription. Also, if two words rhyme, they should have the same vowel symbol (and the same symbols for any following consonants).

This is different from English spelling. In spelling, some of these sounds, especially the vowels, vary from one region of the country to another. (This is a major pitfall for the vowels. If some of the words listed in the chart below don't seem quite right to you, this may be why.)

The IPA symbols for vowels are, therefore, international standards for vowel sounds, (each having specific and unique articulatory features and auditory criteria), not necessarily for the vowels themselves. The symbol [a], therefore, does not stand for the vowel in the English word ‘father’, (as I might vainly attempt to tell my students). Instead, it stands for the vowel sound that is the furthest back and the lowest possible vowel in the vowel space - the vowel with the highest f1 and the closest f2 to f1 frequencies. Likewise, the symbol [o] refers to a vowel made with the tongue body in a relatively exact place, and which will therefore have the formants at certain frequencies. Any deviations from this place - any subtle change in the vocal posture that produces a slightly different sound - would also be observed on a spectrogram, through changes in the formants.

In the following sections of this article, I have endeavoured to give an example of an English word containing the vowel sounds whenever I have written about them, although, due to differences in regional accents and dialects of English, these words, when pronounced by different speakers, may produce vowel sounds that are merely similar or approximate. The sounds made by different speakers for each written vowel may differ considerably between languages or dialects within the same language. (Acoustically speaking, speech vowels are not necessarily ‘pure’, as they may vary considerably depending upon regional accents, producing forms of vowels that are less than uniform among all speakers. Spoken values also vary according to languages and dialects.)

Teachers should not take for granted that their own pronunciation of a specific vowel in the context of a word is also the typical pronunciation across all dialects and regional accents. As a Canadian who is teaching vocal students within the northeastern United States, I have noticed that the words that I tend to use to demonstrate the particular vowel sounds that I would like my students to use do not necessarily produce the desired results. For example, Canadian English just happens to have the vowel in ‘father’ very close to the cardinal position for [a], which means that when I say the word ‘father’, I am producing the sound that matches the sound for that particular symbol on the IPA vowel chart. In the northern U.S. pronunciation of the same word, however, the sound is more front and is not in the lowest, most far back part of the vowel space. It is not a question of whose accent is most deserving of falling into the desired category. It is a matter of accent, and accent alone.

To counter this confusion about correct ‘pure’ vowel sounds, a teacher may need to abandon the idea of giving examples of words containing the desired vowel sound, and instead demonstrate the ideal vowel sound in isolation (without any other context). Having no words to use in reference may be more challenging for a singer, but a few reminders and corrections during vocal exercises will eventually help the singer to memorize and correctly imitate these sounds.

Learning to truly hear the sounds in a word is another challenge for singers. This comes with practice. It becomes especially important for singers who need to be able to analyze the words of text in order to find the vowel core or the primary vowel sound of diphthongs and triphthongs.

To avoid becoming too entangled in the details of phontec transcriptions, I’d like to turn our attention to the more practical IPA symbols used for the sounds that vocalists sing the most – those pertaining to vowel sounds. Since most of my readers and students speak and sing primarily in English, I’d like to focus more specifically (and practically) on English vowel sounds.

Below is a chart of the common vowel sounds found in English, their standard IPA symbols, and some words to help guide my readers to a correct pronunciation of the sound. Since the goal of IPA is that interpretation should not depend on the reader's dialect, not all of these words will be relevant to the dialects of every native speaker. Also, some sounds made in certain dialects may not be represented in this table. To clear up any confusion caused by the differences between regional accents and dialects, I have included a link to another website that enables a singer to click onto an IPA symbol for a vowel and hear an audio clip of the correct, standard, internationally accepted pronunciation of the sound.

Cardinal Vowels

Cardinal vowels are a set of reference vowels used by phoneticians in describing the sounds of languages. In the early 20th century, phonetician Daniel Jones developed the cardinal vowel system to describe vowels in terms of their common features: height (vertical dimension), backness (horizontal dimension) and roundedness (lip position). These three parameters are indicated in the schematic IPA (International Phonetic Alphabet) vowel diagram below. There are, however, still more possible features of vowel quality, such as the velum position (which, if lowered, contributes to nasality), type of vocal fold vibration (phonation), and tongue root position.

Vowel Height

Vowel height refers to the vertical position of the tongue relative to either the roof of the mouth or the aperture of the jaw. In high vowels, such as [i] and [u], the tongue is positioned high in the mouth, whereas in low vowels, such as [a], the tongue is positioned low in the mouth. The IPA prefers the terms close vowel and open vowel, respectively, which describes the jaw as being relatively closed or open. However, vowel height is an acoustic rather than articulatory quality, and is defined today not in terms of tongue height, or jaw openness, but according to the relative frequency of the first formant (f1). The higher the f1 value, the lower (more open) the vowel; height is thus inversely correlated to f1.

The International Phonetic Alphabet identifies seven different vowel heights: close vowel (high vowel), near-close vowel, close-mid vowel, mid vowel, open-mid vowel, near-open vowel, and open vowel (low vowel). The parameter of vowel height appears to be the primary feature of vowels cross-linguistically in that all languages use height contrastively. No other parameter, such as front-back or rounded-unrounded (see below), is used in all languages. Some languages have vertical vowel systems in which, at least at a phonemic level, only height is used to distinguish vowels; the vowel height is used as the sole distinguishing feature.

Although English contrasts all six heights in its vowels, these are interdependent with differences in backness, and many are parts of diphthongs - unitary vowels (contour vowels) that change quality during their pronunciation, or ‘glide’, with a smooth movement of the tongue from one articulation to another, as in the English words ‘eye’, ‘boy’, and ‘cow’. Diphthongs often form when separate vowels are run together in rapid speech. The English language also has some triphthongs, as in certain pronunciations of the word ‘flower’. These contrast with monophthongs (‘pure’ vowels), where the tongue is held still, as in the English word ‘hat’. However, there are also unitary diphthongs, as in the English examples above, which are heard by listeners as single vowel sounds (phonemes).

Vowel Backness

The International Phonetic Alphabet identifies five different degrees of vowel backness: front vowel, near-front vowel, central vowel, near-back vowel and back vowel. Vowels are defined as either front or back not according to actual articulation (e.g., the position of the tongue), but according to the relative frequency of the second formant (f2). The higher the f2 value, the fronter the vowel. The lower the f2 value, the more back the vowel. Although English has vowels at all five degrees of backness, there is no known language that distinguishes all five without additional differences in height or rounding.

Vowel Roundedness

Roundedness refers to whether the lips are rounded or not. In most languages, roundedness is a reinforcing feature of mid to high back vowels, and is not distinctive. Usually the higher a back vowel is, the more intense the rounding. However, some languages treat roundedness and backness separately. Nonetheless, there is usually some phonetic correlation between rounding and backness: front rounded vowels tend to be less front than front unrounded vowels, and back unrounded vowels tend to be less back than back rounded vowels. That is, the placement of unrounded vowels to the left of rounded vowels on the Cardinal vowel chart is reflective of their typical position.

Rounding is generally realized by a complex relationship between f2 and f3 that tends to reinforce vowel backness. One effect of this is that back vowels are most commonly rounded while front vowels are most commonly unrounded. Another effect is that rounded vowels tend to plot to the right of unrounded vowels in vowel charts. That is, there is a reason for plotting vowel pairs the way they are.

Labialisation is a secondary articulatory feature of sounds in some languages. Labialised sounds involve the lips while the remainder of the oral cavity produces another sound. The term is normally used to refer to consonants. When vowels involve the lips, they are usually called rounded. The most common labialised consonants are labialized velars - a consonant with an approximant-like secondary articulation. (Labial-velar refers to a consonant made at two places of articulation, one at the lips and the other at the soft palate.)

Labialisation may also refer to a type of assimilation process. Assimilation is a common phonological process by which the phonetics of a speech segment become more like that of another segment in a word (or at a word boundary).

A related process is co-articulation, where one segment influences another to produce an allophonic variation, such as vowels acquiring the feature nasal before nasal consonants when the velum opens prematurely or ‘b’ becoming labialised (lips pursed in preparation for the vowel) as in "boot".

In mid to high rounded back vowels the lips are generally protruded (‘pursed’) outward, a phenomenon known as exolabial rounding because the insides of the lips are visible. In mid to high rounded front vowels, however, the lips are generally ‘compressed’, with the margins of the lips pulled in and drawn towards each other. This latter phenomenon is known as endolabial rounding. (In English, [u] is exolabial.) In many phonetic treatments, both are considered types of rounding, but some phoneticians do not believe that these are subsets of a single phenomenon of rounding, and prefer instead the three independent terms rounded (exolabial), compressed (endolabial), and spread (unrounded).

Vowel Tenseness

Tenseness is used to describe the opposition of tense vowels, as in ‘leap’ and ‘suit’ versus lax vowels as in ‘lip’, ‘soot’. In the English language, ‘tense and lax’ are often used interchangeably with ‘long and short’, respectively. In most Germanic languages, (including English), lax vowels can only occur in closed syllables. Therefore, they are also known as ‘checked vowels’ (those vowels that usually must be followed by a consonant in a stressed syllable), whereas the tense vowels are called ‘free vowels’ since they can occur in any kind of syllable - they may stand in a stressed open syllable with no following consonant.

Below, I have copied a standard chart for the cardinal vowels. Again, cardinal vowels are vowels that are formed with the articulators in specific (extreme) positions. Since many new students of voice haven’t had any exposure to the IPA symbols that are used for each vowel sound, I have included a link (below) to audio recordings for each sound. This link will take the reader to this same vowel chart posted on another website, where each individual symbol can then be clicked onto in order to hear the recording of the correct pronunciation. (Click onto the diagram below to be taken to the other website.)

The vowel symbols in the IPA chart represent where in the human mouth their corresponding sounds are formed. You can imagine the chart as being superimposed on the mouth of the human that is facing to the left of this page.

Across the top of the chart, are the labels ‘front’, ‘central’, and ‘back’. Front is the left most label because it corresponds with the front of a left-facing human model’s mouth.

The vertical axis of the chart is mapped by vowel height. Vowels pronounced with the tongue lowered are at the bottom, and vowels pronounced with the tongue raised are at the top. For example, [ɑ], (as in ‘palm’), is at the bottom because the tongue is lowered in this position. However, [i], (as in ‘meet’), is at the top because the sound is made with the tongue raised close to the roof of the mouth.

Vowel backness determines the horizontal axis of the chart. Vowels with the tongue moved towards the front of the mouth, (such as [ε], the vowel in ‘met’), are to the left in the chart, while those in which it is moved to the back, (such as [Λ], the vowel in ‘but’) are placed to the right in the chart.

In places where vowels are paired, the right represents a rounded vowel (in which the lips are rounded) while the left is its unrounded counterpart.

Three of the cardinal vowels, [i], [ɑ] and [u] have articulatory definitions that place them at the ‘corners’ of the vowel diagrams, with [i] and [a] having the most extreme tongue body positions, high front and low back, respectively. Other vowels are placed on the vowel chart using these three cardinal vowels as ‘landmarks’, with the rounded vowels always listed to the right of the corresponding unrounded vowel. (Besides being consistent, this also reflects their relative positions on formant graphs. Lip rounding will lower f2, and lower second formants are toward the right end of the standard vowel chart.)

The other vowels are 'auditorily equidistant' – meaning that there is equal spacing between the height levels that can be determined articulatorily (by making the tongue body move in four equal steps from high to low) or acoustically (by dividing the f1 dimension into four levels from lowest to highest) - between these three 'corner vowels', at four degrees of aperture or 'height': close (high tongue position), close-mid, open-mid, and open (low tongue position).

The diagrams above show the heights for the eight primary cardinal vowels. These degrees of aperture plus the front-back distinction define eight reference points on a mixture of articulatory and auditory criteria. Notice that five of the vowels are pronounced with spread lips and are consequently unrounded, while three of the back vowels are rounded vowels. Vowels like these are common in the world's languages.

The secondary cardinal vowels are obtained by using the opposite lip-rounding on each primary cardinal vowel. For example, if the feature ‘rounded’ for all the eight vowels is modified, the first five pronounced with rounded lips and the last three with spread lips, the secondary cardinal vowel chart is obtained. Thus, in this newly established set, all front vowels are round. (Since English does not have any front rounded vowels, this secondary cardinal vowel chart is not really relevant for the study of the vocalic system of English.)

The primary and secondary cardinal vowels are often referred to by a number, as well as their symbols.

Below is a chart depicting the vowels of the IPA that may be used as an alternative to the above cardinal vowel chart. These vowels are mapped mainly according to the position of the tongue, but also the degree of backness and the roundedness of the lips.

Articulatory Features (Definitions) of the ‘Five Pure Italian Vowels’

The five pure Italian vowels – so named because their pronunciation follows that of the Italian pronunciation and because they are ‘pure’ monophthongs – generally refer to [e], as in ‘name’, [i], as in ‘feet’, [a], as in ‘father’, [o], as in ‘bone’, [u], as in ‘boot’. Again, as stated in earlier sections of this article, the way in which I pronounce these particular vowel sounds falls in line with the IPA criteria for the sounds. The Spanish pronunciations of [ e, i, a, o, u ] are also close to their IPA values. However, not everyone may pronounce these vowel sounds in precisely the same way, so it is best to listen to these audio clips of the individual vowel sounds to know how to correctly pronounce them. These are the vowels that vocal instructors generally use, either individually or in various combinations, in training their vocal students to balance their singing tone. Learning to pronounce these sounds correctly allows for more consistency in vocal training, as teachers and students will be on the same ‘acoustic’ page, and because their formant frequencies are easily tracked.

Each of these vowels has its own unique articulation, giving it its own unique characteristics and qualities. The articulatory features that distinguish different vowel sounds are said to determine the vowel's quality, (by establishing their individual formant frequencies).

The features of the vowel [e] - a close-mid front unrounded vowel - include a vowel height that is close-mid, which means that the tongue is positioned halfway between a close vowel and a mid vowel. Its vowel backness is front, which means the tongue is positioned as far forward as possible in the mouth without creating a constriction that would be classified as a consonant. Its vowel roundedness is unrounded, which means that the lips are not rounded.

In the close front unrounded vowel [i], its vowel height is close, which means that the tongue is positioned as close as possible to the roof of the mouth without creating a constriction that would be classified as a consonant. Its vowel backness is front, which means that the tongue is positioned as far forward as possible in the mouth without creating a constriction that would be classified as a consonant. Its vowel roundedness is unrounded, which means that the lips are spread.

In an open back unrounded vowel, such as [a], the vowel height is open, which means that the tongue is positioned as far as possible from the roof of the mouth. The vowel backness is back, which means that the tongue is positioned as far back as possible in the mouth without creating a constriction that would be classified as a consonant. The vowel roundedness is unrounded, which means that the lips are not rounded.

[o] is a close-mid back rounded vowel, which means that it has a vowel height that is close-mid, which means that the tongue is positioned halfway between close vowel and a mid vowel. Its vowel backness is back, which means that the tongue is positioned as far back as possible in the mouth without creating a constriction that would be classified as a consonant. Its roundedness is endolabial, which means that the lips are rounded and protrude, with the inner surfaces exposed.

The vowel height of the close back rounded vowel [u] is close, which means that the tongue is positioned as close as possible to the roof of the mouth without creating a constriction that would be classified as a consonant. Its vowel backness is back, which means that the tongue is positioned as far back as possible in the mouth without creating a constriction that would be classified as a consonant. Its roundedness is endolabial, which means that the lips are rounded and protrude, with the inner surfaces exposed. This sound can be approximated by adopting the posture to whistle a very low note, or blow out a candle.

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