Vowels and formants: the acoustics of the six sounds

Author: Raymond Hackland · Last reviewed: 18 October 2026 · Reading time: about 9 minutes

In short

• What this is: An introduction to how your voice gets its colour. Voice scientists describe it with the source-filter model: a sound source (your vocal folds, or rushing air) and a filter (the shape of your throat, mouth and lips). The filter’s peaks are called formants. We then look at what the six sounds of Liu Zi Jue are, acoustically.
• What the evidence says: The source-filter model and formants are textbook acoustics and phonetics. Two of the six sounds as we write them, SSSSS and SHHHH, are mainly hissing noise (fricatives). The other four, CHOOO, HAAAA, WHOOO and HEEE, are carried by a vowel shape. When the sounds are breathed rather than voiced, as the standard form ultimately aims for, the vowel shapes still filter the breath noise, much as in whispering.
• What it means for your practice: Changing the shape of your mouth genuinely changes the sound. That is the solid, physical side of the six sounds. The pairing of each sound with an organ is a separate, traditional layer of meaning, not an acoustic effect.

What science says

Source and filter. Label: Well studied. A common way to describe the voice is the source-filter model. A source produces a raw sound containing many frequencies, and the vocal tract (the airway from the vocal folds to the lips) then filters it, strengthening some frequencies and weakening others. You change the filter all the time with your tongue, jaw, lips and soft palate [1]. There are two main kinds of source. In voiced sounds, the vibrating vocal folds produce a buzz with a regular pitch and a series of harmonics. In whispering and in hissing consonants, the vocal folds don’t vibrate; instead, turbulent airflow produces broadband noise, a mixture of very many frequencies, which gives whispering its windy sound [1]. The model is a simplification: source and filter influence each other a little [1]. As a working picture, though, it explains a great deal.

Formants: the peaks that make a vowel. Label: Well studied. The filter has resonances, and in our article on what resonance is we described the vocal tract as a tube with resonances of its own. The resulting broad peaks in the spectrum of the sound are called formants, numbered from the bottom: F1, F2, F3 and so on. The term was defined in this sense by the speech scientist Gunnar Fant in 1960. Strictly, a resonance belongs to the tract and a formant to the sound, but their frequencies are usually close [2]. In languages such as English, listeners identify vowels largely by F1 and F2; F3 matters less, and the higher formants mostly affect the timbre of the voice [1].

What moves F1 and F2. Label: Well studied. For spoken vowels, the first resonance typically lies between about 200 Hz, with a small mouth opening, and 800 Hz; opening the mouth raises it. The second lies between about 800 and 2,000 Hz and depends mostly on where the tongue narrows the tract. Rounding or spreading the lips shifts the resonances too [1]. These ranges fit adult voices best, men’s in particular. Women and children have shorter vocal tracts, so their formants lie higher [1, 12], and “ee” has an especially high F2. In a large study of American English (45 men, 48 women and 46 children aged 10 to 12), the “ah” of “hod” had an average F1 of about 770 Hz in men, 940 Hz in women and 1,000 Hz in children, and the “ee” of “heed” an average F2 of about 2,320, 2,760 and 3,080 Hz [12]. So a wide-open “ah” has a high F1. “Ee” and “oo” both have a small opening and a low F1, and they sound so different mainly because the tongue narrows the tract at a different place (forward for “ee”, further back for “oo”, with rounded lips) [1]. Voice researchers often map vowels on a plane with F1 and F2 as axes, roughly corresponding to jaw height and tongue position [1].

Pitch and vowel are largely separate. Label: Well studied. The note you sing is set mainly by the source; the vowel is set by the filter. You can sing one pitch and move from “oo” to “ah” to “ee”, and the colour changes while the note stays the same [1]. Singers sometimes shape their vocal tract so that a resonance lines up with the note they are singing, which makes it louder for the same effort [1]. In short, the vowel and the pitch of a sound are two different things.

Hissing sounds: fricatives. Label: Well studied. In a fricative, such as s, sh or f, the tongue, teeth or lips narrow the airway so much that the air becomes turbulent and makes noise [1, 3]. The narrowing splits the tract into two parts, and in voiceless fricatives it is mainly the small space in front of the narrowing, towards the lips, that shapes the noise [3]. The shorter that front space, the higher the noise: English s has its main peak around 4 to 5 kHz, and sh typically around 2.5 to 3 kHz [3]. These sibilant sounds are loud because the air jet hits the teeth [3]. Mandarin Chinese has three hissing sounds of this kind (written s, sh and x in pinyin); their noise is highest for s, lowest for sh and in between for x [3].

Whispered vowels still have formants. Label: Well studied. Without vocal fold vibration, a whispered vowel has no pitch in the usual sense. The vocal tract still filters the breath noise, so the vowel remains recognisable, though listeners are less accurate than with voiced speech. Measured formants in whispered vowels tend to be somewhat higher than in voiced ones [4, 5].

What the six sounds are, acoustically. Label: Well studied (phonetics) · Tradition only (the traditional readings and mouth shapes). On our main site, the six sounds are written as English-friendly sustained sounds: SSSSS (lungs), CHOOO (kidneys), SHHHH (liver), HAAAA (heart), WHOOO (spleen) and HEEE (triple burner) [6]. The Chinese characters behind them are read in today’s Standard Chinese (pinyin) as si, chui, xu, he, hu and xi. Phonetically:

• SSSSS (si, 呬): a sustained s, a sibilant fricative with high-frequency noise [3]. In Standard Chinese, si is an s followed by a special “apical” vowel [7]. The standard form’s instructions keep the teeth together and say you should feel vibration and coolness at the front teeth [8].
• SHHHH (xu, 嘘): a sustained sh-like hiss. In Standard Chinese, xu begins with x, a hiss made with the front of the tongue near the palate, followed by the rounded vowel ü [7]. The standard form spreads the lips slightly and lets the air pass between the side teeth and out at the corners of the mouth [8].
• CHOOO (chui, 吹): an affricate, a brief closure released into a hiss (ch) [7], followed by a rounded “oo” vowel. In Standard Chinese the syllable continues into a glide towards “ei” [7], and the standard form describes it as the only sound with a moving mouth shape, in three steps: ch, u, i [8].
• HAAAA (he, 呵): breath noise followed by an open “ah” vowel with a high F1 [1]. In Standard Chinese, he has a different vowel, a close-mid back unrounded vowel, not “ah” [7]. Interestingly, the Chinese Health Qigong Association notes that one school, the Emei school, reads the character as ha [9].
• WHOOO (hu, 呼): breath noise into a rounded “oo” vowel [1, 7]. The standard form asks for rounded, slightly protruding lips, “as if blowing out a candle” [8].
• HEEE (xi, 嘻): breath noise into a front “ee” vowel. In Standard Chinese, xi begins with the same x hiss as xu [7]. The standard form keeps the teeth together, the molars lightly closed and the corners of the mouth drawn back [8].

So, acoustically, two sounds are mainly fricative noise and four are vowel shapes opened by a consonant. In Standard Chinese, the h of he and hu is not a light English h but a fricative made at the soft palate, further back in the mouth [7], and the x of xu and xi is a hiss [7]. Even in Chinese, then, all six syllables begin with friction noise, chui after a brief closure.

Breathed or voiced? Label: Tradition only (the instructions) · Well studied (the acoustics). Our article on the history of Liu Zi Jue explains that the earliest surviving description of the six sounds asks for an audible breath, while the 2003 standard form lets beginners voice the sounds and aims, in the end, at breathing out without sound [10, 11]. Acoustically, a breathed sound replaces the buzzing vocal folds with breath noise, while the mouth shapes still filter it. The standard form’s own instructions point to exactly this: hold the mouth shape, breathe out without voicing, and the breath itself sounds close to the syllable [8]. That is what acoustics would predict. In our practice, all six sounds are breathed out through the mouth, voiced through the breath rather than sung, and over time the exhalation becomes silent. That is how we teach them.

Where popular claims go beyond the evidence

Label for this section: Speculative.

• “Each sound vibrates its organ.” What changes from sound to sound is the shape of the vocal tract and therefore the filtering of the sound [1]. We know of no measurements showing that a particular vowel or hiss sets a particular organ into resonance; soft body tissue is heavily damped, as we explain in What is resonance?.
• “The sound has a healing frequency.” A vowel is not a single frequency. It is a pattern of formants, and the same vowel can be sung on any note [1]. A sibilant like s spreads its energy over thousands of hertz [3]. The frequencies our main site pairs with the sounds (396 to 963 Hz) are Solfeggio numbers [6]. We use them as musical tones, outside the usual 440 Hz concert tuning. They belong to the music, not to the acoustics of the sound itself, and we make no health claim for them; see our article on Solfeggio frequencies.
• “The original pronunciation is known.” The Chinese Health Qigong Association documents disagreement about the readings of he and si between schools, and reports that the standard form’s readings were settled after consulting specialists in Chinese phonology, who did not all agree on si, and a Qing-dynasty text on speech sounds [9].

How this relates to the Six Healing Sounds, Qigong and neigong

Label for this section: Well studied (acoustics) · Tradition only (organ and Five Phases pairings, qi).

The acoustics are the part you can hear and measure. Each of the six sounds is a different shape of your vocal tract, and so a different filter on your breath. When you voice a sound, harmonics from your vocal folds are shaped by formants; when you breathe it, noise is shaped instead. Moving from SSSSS to HAAAA is, physically, moving from a narrow hiss at the front teeth to an open vowel with a high first formant. That is why the sounds feel so different in the mouth and throat.

The pairings with organs and Five Phases are a different kind of knowledge. The Chinese Health Qigong Association grounds them partly in a traditional classification of speech sounds by where they are formed (throat, tongue, molars, front teeth and lips), each linked to one of the Five Phases [9, 11]. That classification is part of China’s long tradition of phonology, and it is worth knowing on its own terms. It is not the same as modern phonetics, though: in today’s Standard Chinese, he (classed as a tongue sound) and hu (a throat sound) begin with the same consonant [7, 9]. Read it as a traditional map of meaning, not as an acoustic measurement. In Qigong and neigong, the mouth shapes are also described as guiding the breath and qi; that is traditional language for an inner practice, not something acoustics can confirm or rule out.

Our spellings (SSSSS, CHOOO, SHHHH, HAAAA, WHOOO, HEEE) are an adaptation for English speakers and for sustained, music-assisted sounding. Some differ from today’s Standard Chinese readings, most clearly HAAAA for he. Resonance Heritage Qi is a personal, music-assisted practice inspired by Liu Zi Jue: connected to it, not a replacement for the traditional form or for study with a teacher of that tradition.

Try it (optional). Sit comfortably. On one easy note, hum softly, then open into “oo”, “ah” and “ee” without changing the note. Notice how the colour changes while the pitch stays put: those are your formants moving. Then breathe the same three shapes without voice and listen to the whispered versions. Keep it soft, never force the breath, and stop if you feel dizzy.

Sources

1. Wolfe J, Garnier M, Smith J. Voice acoustics: an introduction. Acoustics Group, University of New South Wales (UNSW), Sydney, accessed 18 October 2026. https://newt.phys.unsw.edu.au/jw/voice.html
2. Wolfe J. Formant: what is a formant? Acoustics Group, University of New South Wales (UNSW), Sydney, accessed 18 October 2026. https://newt.phys.unsw.edu.au/jw/formant.html
3. Jongman A, 2024. Phonetics of fricatives. Oxford Research Encyclopedia of Linguistics. Oxford University Press. https://doi.org/10.1093/acrefore/9780199384655.013.1086
4. Houle N, Levi SV, 2020. Acoustic differences between voiced and whispered speech in gender diverse speakers. Journal of the Acoustical Society of America 148(6):4002. https://doi.org/10.1121/10.0002952
5. Kallail KJ, Emanuel FW, 1984. Formant-frequency differences between isolated whispered and phonated vowel samples produced by adult female subjects. Journal of Speech and Hearing Research 27(2):245–251. https://doi.org/10.1044/jshr.2702.251
6. Resonance Heritage Qi. The six healing sounds (main website text), accessed 18 October 2026. https://resonanceheritageqi.com/
7. Lee W-S, Zee E, 2003. Standard Chinese (Beijing). Illustrations of the IPA. Journal of the International Phonetic Association 33(1):109–112. https://doi.org/10.1017/S0025100303001208
8. Zhang Mingliang, 2018. 健身气功·六字诀习练要领与技巧 (Health Qigong Liu Zi Jue: key points and techniques for practice). Health Qigong Management Center, General Administration of Sport of China, published 19 October 2018. https://www.sport.gov.cn/qgzx/n5407/c878295/content.html
9. Chinese Health Qigong Association, 2016. 健身气功·六字诀功法源流 (Health Qigong Liu Zi Jue: origins of the method). Health Qigong Management Center, General Administration of Sport of China, published 20 December 2016. https://www.sport.gov.cn/qgzx/n5407/c781291/content.html
10. Tao Hongjing (attributed). 養性延命錄 Yangxing yanming lu, juan 2, chapter 4 (服氣療病篇). Daoist Canon (Zhengtong Daozang) text, digital edition: Kanripo, KR5c0235. https://www.kanripo.org/text/KR5c0235/002
11. Zhang Mingliang, Ma Ling, 2016. 健身气功·六字诀养生撷萃 (Health Qigong Liu Zi Jue: essentials for nourishing life). Health Qigong Management Center, published 11 November 2016. https://www.sport.gov.cn/qgzx/n5407/c775164/content.html
12. Hillenbrand J, Getty LA, Clark MJ, Wheeler K, 1995. Acoustic characteristics of American English vowels. Journal of the Acoustical Society of America 97(5):3099–3111. https://doi.org/10.1121/1.411872

Source quality note: [1] and [2] are educational overviews by an active university voice-acoustics research group, with references to the primary literature. [3] is a peer-reviewed encyclopedia review of fricative research; the frequency values it gives are mainly for English and vary with speaker and vowel. [4] and [5] are laboratory studies of English speakers; [5] is from 1984 and small (20 speakers). [7] is the standard phonetic description of Standard Chinese. [12] is a large laboratory study of American English vowels (139 speakers) that repeated the classic Peterson and Barney study of 1952; formant values vary with dialect, speaker and era. [8], [9] and [11] are published by the Health Qigong Management Center; [8] is written by Zhang Mingliang, [9] by the Chinese Health Qigong Association and [11] by Zhang Mingliang and Ma Ling. They describe the standard form from inside the tradition. [10] is a historical primary text. [6] is our own website. None of these sources measured the acoustics of the six sounds themselves; the descriptions above apply general phonetics to them.