Airstream mechanism
Encyclopedia
In phonetics
, the airstream mechanism is the method by which airflow is created in the vocal tract
. Along with phonation
, it is one of two mandatory aspects of sound production; without these, there can be no speech sound.
The organ generating the airstream is called the initiatior; for this reason the production of airflow is called initiation. There are three initiators used in spoken human languages:
. These changes in pressure are often said to involve outward and inward airflow, and are therefore termed egressive and ingressive mechanisms; however, ingressive mechanisms often only reduce outward airflow.
Of these six possible airstream mechanisms, four are found in words around the world:
These mechanisms may be combined into airstream contours
, such as clicks which release into ejectives.
The Khoisan languages
have pulmonic, ejective, and click consonants, the Chadic languages
have pulmonic, implosive, and ejective consonants, and the Nguni languages utilize all four, pulmonic, click, implosive, and ejective, in normal vocabulary.
In interjection
s, the other two mechanisms may be employed. For example, in countries as diverse as Canada, Sweden, Turkey, and Togo, a pulmonic ingressive ("gasped" or "inhaled") vowel is used for back-channeling or to express agreement, and in France a lingual egressive (a "spurt") is used to express dismissal. The only language where such sounds are known to be contrastive in normal vocabulary is the ritual language Damin
; however, that language appears to have been intentionally designed to be different from normal speech.
s (actually the diaphragm and ribs) is called pulmonic initiation. The vast majority of sounds used in human languages are pulmonic egressives. In most languages, including all the languages of Europe (excluding the Caucasus
), all phoneme
s are pulmonic egressives.
The only attested use of a phonemic pulmonic ingressive is a lateral fricative in Damin
, a ritual language formerly used by speakers of Lardil
in Australia
. This can be written with the extended version of the International Phonetic Alphabet
as [ɬ↓]. !Xóõ
has ingression as a phonetic detail in one series of its clicks, which are ingressive voiceless nasals with delayed aspiration, [↓ŋ̊ʘʰ ↓ŋ̊ǀʰ ↓ŋ̊ǁʰ ↓ŋ̊!ʰ ↓ŋ̊ǂʰ]. Peter Ladefoged
considers these to be among the most difficult sounds in the world. Other languages, for example in Taiwan
, have been claimed to have pulmonic ingressives, but these claims have either proven to be spurious or to be occasional phonetic detail.
In interjection
s, but not in normal words, pulmonic ingressive vowels such as [ə↓] frequently occur in Scandinavian languages, e.g. Swedish
, and contrary to the common view that pulmonic ingressive speech is restricted to northern Europe, speech technologist Robert Eklund has recently shown that it occurs on most continents, in a wide variety of languages of different typological origins, e.g. and Ewe
. In Ewe, [ə↓] is used for back-channeling, to indicate that one is listening (like ah or I see in English). See ingressive sound for details.
. This is known as glottalic initiation.
For egressive glottalic initiation, one lowers the glottis (as if to sing a low note), closes it as for a glottal stop
, and then raises it, building up pressure in the oral cavity and upper trachea
. Glottalic egressives are called ejectives. Since the glottis must be fully closed to form glottalic egressives, it is impossible to pronounce voiced
ejectives.
For ingressive glottalic initiation, the sequence of actions performed in glottalic pressure initiation is reversed: one raises the glottis (as if to sing a high note), closes it, and then lowers it to create suction in the upper trachea and oral cavity. Glottalic ingressives are called implosive
s, although they may involve zero airflow rather than actual inflow. Because it is not necessary to fully close the glottis, implosives may be voiced; indeed, voiceless implosives are exceedingly rare.
It is usual for implosives to be voiced. Instead of keeping the glottis tightly closed, it is tensed but left slightly open to allow a thin stream of air through. Unlike pulmonic voiced sounds, in which a stream of air passes through a usually-fixed glottis, in voiced implosives a mobile glottis passes over a nearly motionless air column to cause vibration of the vocal cords. More open phonation
s than modal voice, such as breathy voice, are not conducive to glottalic sounds because in these the glottis is held relatively open, allowing air to readily flow through and preventing a significant pressure difference from building up behind the articulator.
Because the oral cavity is so much smaller than the lungs, vowel
s and approximants cannot be pronounced with glottalic initiation. So-called glottalized
vowels and other sonorants use the more common pulmonic egressive airstream mechanism.
, and are almost universally ingressive. The word lingual is derived from Latin lingua, which means tongue.
To produce a lingual ingressive airstream, first close the vocal tract at two places: at the back of the tongue, as in a velar
or uvular
stop, and simultaneously with the front of the tongue or the lips, as in a coronal
or bilabial stop. These holds may be voiced or nasalized. Then lower the body of the tongue to rarefy the air above it. The closure at the front of the tongue is opened first, as the click "release"; then the closure at the back is released for the pulmonic or glottalic click "accompaniment" or "efflux". This may be aspirated
, affricated, or even ejective. Even when not ejective, it is not uncommon for the glottis to be closed as well, for a triply articulated consonant, and this third closure is released last to produce a glottalized click. Clicks are found in very few languages, notably the Khoisan languages
of southern Africa and some nearby tongues such as Zulu
. They are more often found in extra-linguistic contexts, such as the "tsk tsk" sound many Westerners use to express regret or pity (a dental click
), or the clucking noise used by many equestrian
s to urge on their horses (a lateral click).
Lingual egressive initiation is performed by reversing the sequence of a lingual ingressive: the front and back of the tongue (or lips and back of the tongue) seal off the vocal cavity, and the cheeks and middle of the tongue move inward and upward to increase oral pressure. The only attested use of a lingual egressive is a bilabial nasal egressive click in Damin
. Transcribing this also requires the use of the Extended IPA, [ŋʘ↑].
Since the air pocket used to initiate lingual consonants is so small, it is not thought to be possible to produce lingual fricatives, vowels, or other sounds which require continuous airflow.
Clicks may be voiced, but they are more easily nasalized
. This may be because the vocal cavity behind the rearmost closure, behind which the air passing through the glottis for voicing must be contained, is so small, clicks cannot be voiced for long. Allowing the airstream to pass through the nose enables a longer production.
Nasal clicks involve a combination of lingual and pulmonic mechanisms. The velum is lowered so as to direct pulmonic airflow through the nasal cavity during the lingual initiation. This nasal airflow may itself be egressive or ingressive, independently of the lingual initiation of the click. Nasal clicks may be voiced, but are very commonly unvoiced and even aspirated, which is rare for purely pulmonic nasals.
Phonetics
Phonetics is a branch of linguistics that comprises the study of the sounds of human speech, or—in the case of sign languages—the equivalent aspects of sign. It is concerned with the physical properties of speech sounds or signs : their physiological production, acoustic properties, auditory...
, the airstream mechanism is the method by which airflow is created in the vocal tract
Vocal tract
The vocal tract is the cavity in human beings and in animals where sound that is produced at the sound source is filtered....
. Along with phonation
Phonation
Phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, phonation is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the definition used among those who study laryngeal anatomy and physiology...
, it is one of two mandatory aspects of sound production; without these, there can be no speech sound.
The organ generating the airstream is called the initiatior; for this reason the production of airflow is called initiation. There are three initiators used in spoken human languages:
- the diaphragmThoracic diaphragmIn the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...
together with the ribs and lungs (pulmonic mechanisms), - the glottisGlottisThe glottis is defined as the combination of the vocal folds and the space in between the folds .-Function:...
(glottalic mechanisms), and - the tongueTongueThe tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...
(lingual or "velaric" mechanisms).
Types of airstream mechanism
Each of the three initiators − diaphragm, glottis or tongue − may act by either increasing pressure in the airstream or by reducing it with suctionSuction
Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...
. These changes in pressure are often said to involve outward and inward airflow, and are therefore termed egressive and ingressive mechanisms; however, ingressive mechanisms often only reduce outward airflow.
Of these six possible airstream mechanisms, four are found in words around the world:
- pulmonic egressive, where the air is pushed out of the lungs by the ribs and diaphragm. All human languages employ such sounds (such as vowels), and nearly three out of four use them exclusively.
- glottalic egressive, where the air column is pushed upward by the glottisGlottisThe glottis is defined as the combination of the vocal folds and the space in between the folds .-Function:...
. Such consonants are called ejectivesEjective consonantIn phonetics, ejective consonants are voiceless consonants that are pronounced with simultaneous closure of the glottis. In the phonology of a particular language, ejectives may contrast with aspirated or tenuis consonants...
. Ejective and ejective-like consonants occur in 16% of the languages. - glottalic ingressive, where the air column is rarefied as the glottis moves downward. Such consonants are called implosivesImplosive consonantImplosive consonants are stops with a mixed glottalic ingressive and pulmonic egressive airstream mechanism. That is, the airstream is controlled by moving the glottis downward in addition to expelling air from the lungs. Therefore, unlike the purely glottalic ejective consonants, implosives can...
. Implosive and implosive-like consonants occur in 13% of the world's languages. - lingual ingressive, AKA velaric ingressive, where the air in the mouth is rarefied by a downward movement of the tongue. These are the clickClick consonantClicks are speech sounds found as consonants in many languages of southern Africa, and in three languages of East Africa. Examples of these sounds familiar to English speakers are the tsk! tsk! or tut-tut used to express disapproval or pity, the tchick! used to spur on a horse, and the...
s. Clicks are regular sounds in ordinary words in fewer than 2% of the world's languages, all but one in Africa.
These mechanisms may be combined into airstream contours
Contour (linguistics)
In phonetics, contour describes speech sounds which behave as single segments, but which make an internal transition from one quality, place, or manner to another. These sounds may be tones, vowels, or consonants....
, such as clicks which release into ejectives.
The Khoisan languages
Khoisan languages
The Khoisan languages are the click languages of Africa which do not belong to other language families. They include languages indigenous to southern and eastern Africa, though some, such as the Khoi languages, appear to have moved to their current locations not long before the Bantu expansion...
have pulmonic, ejective, and click consonants, the Chadic languages
Chadic languages
The Chadic languages constitute a language family of perhaps 200 languages spoken across northern Nigeria, Niger, Chad, Central African Republic and Cameroon, belonging to the Afroasiatic phylum...
have pulmonic, implosive, and ejective consonants, and the Nguni languages utilize all four, pulmonic, click, implosive, and ejective, in normal vocabulary.
In interjection
Interjection
In grammar, an interjection or exclamation is a word used to express an emotion or sentiment on the part of the speaker . Filled pauses such as uh, er, um are also considered interjections...
s, the other two mechanisms may be employed. For example, in countries as diverse as Canada, Sweden, Turkey, and Togo, a pulmonic ingressive ("gasped" or "inhaled") vowel is used for back-channeling or to express agreement, and in France a lingual egressive (a "spurt") is used to express dismissal. The only language where such sounds are known to be contrastive in normal vocabulary is the ritual language Damin
Damin
Damin was a ceremonial language register used by the advanced initiated men of the Lardil and the Yangkaal tribes in Aboriginal Australia. Both inhabit islands in the Gulf of Carpentaria, the Lardil on Mornington Island, the largest island of the Wesley Group, and the Yangkaal and Forsyth Islands...
; however, that language appears to have been intentionally designed to be different from normal speech.
Pulmonic initiation
Initiation by means of the lungLung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s (actually the diaphragm and ribs) is called pulmonic initiation. The vast majority of sounds used in human languages are pulmonic egressives. In most languages, including all the languages of Europe (excluding the Caucasus
Languages of the Caucasus
The languages of the Caucasus are a large and extremely varied array of languages spoken by more than ten million people in and around the Caucasus Mountains, which lie between the Black Sea and the Caspian Sea....
), all phoneme
Phoneme
In a language or dialect, a phoneme is the smallest segmental unit of sound employed to form meaningful contrasts between utterances....
s are pulmonic egressives.
The only attested use of a phonemic pulmonic ingressive is a lateral fricative in Damin
Damin
Damin was a ceremonial language register used by the advanced initiated men of the Lardil and the Yangkaal tribes in Aboriginal Australia. Both inhabit islands in the Gulf of Carpentaria, the Lardil on Mornington Island, the largest island of the Wesley Group, and the Yangkaal and Forsyth Islands...
, a ritual language formerly used by speakers of Lardil
Lardil language
Lardil is a moribund language spoken on Mornington Island , in the Wellesley Islands of Queensland in northern Australia. Lardil is unusual among Australian languages in that it features a ceremonial register, called Damin...
in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. This can be written with the extended version of the International Phonetic Alphabet
International Phonetic Alphabet
The International Phonetic Alphabet "The acronym 'IPA' strictly refers [...] to the 'International Phonetic Association'. But it is now such a common practice to use the acronym also to refer to the alphabet itself that resistance seems pedantic...
as [ɬ↓]. !Xóõ
!Xóõ language
Taa, also known as !Xoon or ǃXóõ, is a Khoisan language known for its large number of phonemes. As of 2002, it was spoken by about 4,200 people worldwide. These are mainly in Botswana , but some are in Namibia. The people call themselves ǃXoon or ‘N|ohan Taa, also known as !Xoon or ǃXóõ, is a...
has ingression as a phonetic detail in one series of its clicks, which are ingressive voiceless nasals with delayed aspiration, [↓ŋ̊ʘʰ ↓ŋ̊ǀʰ ↓ŋ̊ǁʰ ↓ŋ̊!ʰ ↓ŋ̊ǂʰ]. Peter Ladefoged
Peter Ladefoged
Peter Nielsen Ladefoged was an English-American linguist and phonetician who traveled the world to document the distinct sounds of endangered languages and pioneered ways to collect and study data . He was active at the universities of Edinburgh, Scotland and Ibadan, Nigeria 1953–61...
considers these to be among the most difficult sounds in the world. Other languages, for example in Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
, have been claimed to have pulmonic ingressives, but these claims have either proven to be spurious or to be occasional phonetic detail.
In interjection
Interjection
In grammar, an interjection or exclamation is a word used to express an emotion or sentiment on the part of the speaker . Filled pauses such as uh, er, um are also considered interjections...
s, but not in normal words, pulmonic ingressive vowels such as [ə↓] frequently occur in Scandinavian languages, e.g. Swedish
Swedish language
Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...
, and contrary to the common view that pulmonic ingressive speech is restricted to northern Europe, speech technologist Robert Eklund has recently shown that it occurs on most continents, in a wide variety of languages of different typological origins, e.g. and Ewe
Ewe language
Ewe is a Niger–Congo language spoken in Ghana, Togo and Benin by approximately six million people. Ewe is part of a cluster of related languages commonly called Gbe, spoken in southeastern Ghana, Togo, and parts of Benin. Other Gbe languages include Fon, Gen, Phla Phera, and Aja...
. In Ewe, [ə↓] is used for back-channeling, to indicate that one is listening (like ah or I see in English). See ingressive sound for details.
Glottalic initiation
It is possible to initiate airflow in the upper vocal tract by means of the vocal cords or glottisGlottis
The glottis is defined as the combination of the vocal folds and the space in between the folds .-Function:...
. This is known as glottalic initiation.
For egressive glottalic initiation, one lowers the glottis (as if to sing a low note), closes it as for a glottal stop
Glottal stop
The glottal stop, or more fully, the voiceless glottal plosive, is a type of consonantal sound used in many spoken languages. In English, the feature is represented, for example, by the hyphen in uh-oh! and by the apostrophe or [[ʻokina]] in Hawaii among those using a preservative pronunciation of...
, and then raises it, building up pressure in the oral cavity and upper trachea
Vertebrate trachea
In tetrapod anatomy the trachea, or windpipe, is a tube that connects the pharynx or larynx to the lungs, allowing the passage of air. It is lined with pseudostratified ciliated columnar epithelium cells with goblet cells that produce mucus...
. Glottalic egressives are called ejectives. Since the glottis must be fully closed to form glottalic egressives, it is impossible to pronounce voiced
Phonation
Phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, phonation is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the definition used among those who study laryngeal anatomy and physiology...
ejectives.
For ingressive glottalic initiation, the sequence of actions performed in glottalic pressure initiation is reversed: one raises the glottis (as if to sing a high note), closes it, and then lowers it to create suction in the upper trachea and oral cavity. Glottalic ingressives are called implosive
Implosive consonant
Implosive consonants are stops with a mixed glottalic ingressive and pulmonic egressive airstream mechanism. That is, the airstream is controlled by moving the glottis downward in addition to expelling air from the lungs. Therefore, unlike the purely glottalic ejective consonants, implosives can...
s, although they may involve zero airflow rather than actual inflow. Because it is not necessary to fully close the glottis, implosives may be voiced; indeed, voiceless implosives are exceedingly rare.
It is usual for implosives to be voiced. Instead of keeping the glottis tightly closed, it is tensed but left slightly open to allow a thin stream of air through. Unlike pulmonic voiced sounds, in which a stream of air passes through a usually-fixed glottis, in voiced implosives a mobile glottis passes over a nearly motionless air column to cause vibration of the vocal cords. More open phonation
Phonation
Phonation has slightly different meanings depending on the subfield of phonetics. Among some phoneticians, phonation is the process by which the vocal folds produce certain sounds through quasi-periodic vibration. This is the definition used among those who study laryngeal anatomy and physiology...
s than modal voice, such as breathy voice, are not conducive to glottalic sounds because in these the glottis is held relatively open, allowing air to readily flow through and preventing a significant pressure difference from building up behind the articulator.
Because the oral cavity is so much smaller than the lungs, vowel
Vowel
In phonetics, a vowel is a sound in spoken language, such as English ah! or oh! , pronounced with an open vocal tract so that there is no build-up of air pressure at any point above the glottis. This contrasts with consonants, such as English sh! , where there is a constriction or closure at some...
s and approximants cannot be pronounced with glottalic initiation. So-called glottalized
Glottalization
Glottalization is the complete or partial closure of the glottis during the articulation of another sound. Glottalization of vowels and other sonorants is most often realized as creaky voice...
vowels and other sonorants use the more common pulmonic egressive airstream mechanism.
Lingual (velaric) initiation
The third form of initiation in human language is lingual or velaric initiation, where a sound is produced by a closure at two places of articulation, and the airstream is formed by movement of the body of the tongue. Lingual stops are more commonly known as clicksClick consonant
Clicks are speech sounds found as consonants in many languages of southern Africa, and in three languages of East Africa. Examples of these sounds familiar to English speakers are the tsk! tsk! or tut-tut used to express disapproval or pity, the tchick! used to spur on a horse, and the...
, and are almost universally ingressive. The word lingual is derived from Latin lingua, which means tongue.
To produce a lingual ingressive airstream, first close the vocal tract at two places: at the back of the tongue, as in a velar
Velar consonant
Velars are consonants articulated with the back part of the tongue against the soft palate, the back part of the roof of the mouth, known also as the velum)....
or uvular
Uvular consonant
Uvulars are consonants articulated with the back of the tongue against or near the uvula, that is, further back in the mouth than velar consonants. Uvulars may be plosives, fricatives, nasal stops, trills, or approximants, though the IPA does not provide a separate symbol for the approximant, and...
stop, and simultaneously with the front of the tongue or the lips, as in a coronal
Coronal consonant
Coronal consonants are consonants articulated with the flexible front part of the tongue. Only the coronal consonants can be divided into apical , laminal , domed , or subapical , as well as a few rarer orientations, because only the front of the tongue has such...
or bilabial stop. These holds may be voiced or nasalized. Then lower the body of the tongue to rarefy the air above it. The closure at the front of the tongue is opened first, as the click "release"; then the closure at the back is released for the pulmonic or glottalic click "accompaniment" or "efflux". This may be aspirated
Aspiration (phonetics)
In phonetics, aspiration is the strong burst of air that accompanies either the release or, in the case of preaspiration, the closure of some obstruents. To feel or see the difference between aspirated and unaspirated sounds, one can put a hand or a lit candle in front of one's mouth, and say pin ...
, affricated, or even ejective. Even when not ejective, it is not uncommon for the glottis to be closed as well, for a triply articulated consonant, and this third closure is released last to produce a glottalized click. Clicks are found in very few languages, notably the Khoisan languages
Khoisan languages
The Khoisan languages are the click languages of Africa which do not belong to other language families. They include languages indigenous to southern and eastern Africa, though some, such as the Khoi languages, appear to have moved to their current locations not long before the Bantu expansion...
of southern Africa and some nearby tongues such as Zulu
Zulu language
Zulu is the language of the Zulu people with about 10 million speakers, the vast majority of whom live in South Africa. Zulu is the most widely spoken home language in South Africa as well as being understood by over 50% of the population...
. They are more often found in extra-linguistic contexts, such as the "tsk tsk" sound many Westerners use to express regret or pity (a dental click
Dental click
Dental clicks are a family of click consonants found, as constituents of words, only in Africa and in the Damin ritual jargon of Australia. The tut-tut! or tsk! tsk! sound used to express disapproval or pity is a dental click, although it isn't a speech sound in that context.The symbol in the...
), or the clucking noise used by many equestrian
Equestrianism
Equestrianism more often known as riding, horseback riding or horse riding refers to the skill of riding, driving, or vaulting with horses...
s to urge on their horses (a lateral click).
Lingual egressive initiation is performed by reversing the sequence of a lingual ingressive: the front and back of the tongue (or lips and back of the tongue) seal off the vocal cavity, and the cheeks and middle of the tongue move inward and upward to increase oral pressure. The only attested use of a lingual egressive is a bilabial nasal egressive click in Damin
Damin
Damin was a ceremonial language register used by the advanced initiated men of the Lardil and the Yangkaal tribes in Aboriginal Australia. Both inhabit islands in the Gulf of Carpentaria, the Lardil on Mornington Island, the largest island of the Wesley Group, and the Yangkaal and Forsyth Islands...
. Transcribing this also requires the use of the Extended IPA, [ŋʘ↑].
Since the air pocket used to initiate lingual consonants is so small, it is not thought to be possible to produce lingual fricatives, vowels, or other sounds which require continuous airflow.
Clicks may be voiced, but they are more easily nasalized
Nasal consonant
A nasal consonant is a type of consonant produced with a lowered velum in the mouth, allowing air to escape freely through the nose. Examples of nasal consonants in English are and , in words such as nose and mouth.- Definition :...
. This may be because the vocal cavity behind the rearmost closure, behind which the air passing through the glottis for voicing must be contained, is so small, clicks cannot be voiced for long. Allowing the airstream to pass through the nose enables a longer production.
Nasal clicks involve a combination of lingual and pulmonic mechanisms. The velum is lowered so as to direct pulmonic airflow through the nasal cavity during the lingual initiation. This nasal airflow may itself be egressive or ingressive, independently of the lingual initiation of the click. Nasal clicks may be voiced, but are very commonly unvoiced and even aspirated, which is rare for purely pulmonic nasals.
External links
- Eating the Wind: a satirical, but illustrative example of sound symbolismSound symbolismSound symbolism or phonosemantics is a branch of linguistics and refers to the idea that vocal sounds have meaning. In particular, sound symbolism is the idea that phonemes carry meaning in and of themselves.-Origin:...
and iconicity of airstream mechanisms.
- http://journals.cambridge.org/action/displayIssue?jid=IPA&volumeId=38&seriesId=0&issueId=03: Robert Eklund (2008). Pulmonic ingressive phonation: Diachronic and synchronic characteristics, distribution and function in animal and human sound production and in human speech. Journal of the International Phonetic Association, vol. 38, no. 3, pp. 235–324.
- http://ingressivespeech.info: Robert Eklund's website devoted to ingressive speech. Maps, sound files, and spectrograms.