Rules for Modifying Vowels

clinical pedagogy vowels formants articulation voice-training
Last updated: 2025-01-19

Rules for Modifying Vowels

Understanding how specific articulatory maneuvers affect formant frequencies allows systematic and predictable vowel modification. These rules provide practical guidance for singers, actors, and voice clinicians.

Rule 1: Vocal Tract Length

All formant frequencies decrease uniformly as the length of the vocal tract increases.

Physical Basis

This rule agrees with intuition: larger objects resonate sound at lower frequencies. We use long organ pipes, long strings, large drums, and big loudspeakers to produce low notes.

Mechanisms for Changing Vocal Tract Length

Larynx Lowering:

  • Increases pharyngeal length

  • Approximately 10% length increase possible

  • Produces comparable percentage shift in formant frequencies

  • Darkens vowel quality

  • Used in “covered” singing technique

Larynx Raising:

  • Decreases pharyngeal length

  • Brightens vowel quality

  • Less effective than lowering (limited range)

  • Associated with “pressed” or “tight” voice quality

Lip Protrusion:

  • Extends vocal tract beyond teeth

  • Approximately 10% length increase possible

  • Similar acoustic effect to larynx lowering

  • More visible and easier to control than larynx height

Lip Retraction:

  • Effectively shortens vocal tract

  • Raises all formants

  • Associated with spreading, smiling configuration

Quantitative Effects

For a 17.5 cm vocal tract (average male):

10% lengthening (to 19.25 cm):

  • F₁: 500 Hz → 455 Hz (-45 Hz, -9%)

  • F₂: 1500 Hz → 1364 Hz (-136 Hz, -9%)

  • F₃: 2500 Hz → 2273 Hz (-227 Hz, -9%)

  • All formants reduced by approximately 10%

10% shortening (to 15.75 cm):

  • F₁: 500 Hz → 556 Hz (+56 Hz, +11%)

  • F₂: 1500 Hz → 1667 Hz (+167 Hz, +11%)

  • F₃: 2500 Hz → 2778 Hz (+278 Hz, +11%)

  • All formants increased by approximately 11%

Practical Exercise

Try this experiment:

  1. Phonate an [a] vowel at comfortable pitch

  2. Gradually lower your larynx (attempt to yawn)

  3. Note the darkening quality (move toward [ɔ])

  4. Now extend your lips forward during [a]

  5. Similar darkening should occur

Rule 2: Lip Rounding and Spreading

All formant frequencies decrease uniformly with lip rounding and increase with lip spreading.

Physical Basis

Lip rounding is similar to partially covering the mouth. In both cases, the effective tube length increases acoustically, which lowers all formant frequencies.

Lip Rounding Effects

Mechanism:

  • Creates small opening at lips

  • Builds up pressure behind lips

  • Acoustic zero-crossing extends beyond physical mouth

  • Increases effective acoustic length

Typical effects (5 mm protrusion with rounding):

  • All formants: -50 to -100 Hz per mm of extension

  • Total shift: -250 to -500 Hz for substantial rounding

Perceptual effects:

  • Darker, warmer tone quality

  • Reduced brightness

  • Associated with [u], [o], [ɔ] vowels

Lip Spreading Effects

Mechanism:

  • Creates larger opening

  • Reduces pressure buildup

  • Acoustic zero-crossing occurs inside physical mouth

  • Decreases effective acoustic length

Typical effects:

  • All formants: +50 to +100 Hz

  • Brighter, more forward tone quality

  • Associated with [i], [e], [æ] vowels

Covered vs. Open Singing

The contrast between “open” and “covered” singing by professionals involves lip configuration combined with larynx height adjustments:

Covered sound:

  • Lip rounding

  • Larynx lowering (or neutral)

  • Lower formants overall

  • Darker tone quality

Open sound:

  • Lips neutral to spread

  • Larynx higher (or neutral)

  • Higher formants overall

  • Brighter tone quality

Practical Exercise

  1. Phonate [a] with spread lips

  2. Gradually round lips while maintaining [a]

  3. Notice transition toward [ɔ] or [o]

  4. Alternate between spread and rounded

  5. Observe the consistent darkening with rounding

Rule 3: Jaw Lowering

F₁ can be raised substantially by lowering the jaw.

Physical Basis

Jaw lowering:

  • Increases mouth opening

  • Reduces constriction at mouth

  • Shortens effective acoustic length for F₁

  • Decreases acoustic impedance at lips

Application to Singing

This is particularly important in female singing at high pitches (Sundberg, 1977):

High soprano singing:

  • F₀ may exceed typical F₁ for vowel

  • Sopranos tend to align F₁ with F₀

  • Accomplished by jaw lowering

  • Maximizes output power

  • May sacrifice vowel identity for intensity

Example:

  • Soprano singing A5 (880 Hz)

  • Normal [i]: F₁ ≈ 300 Hz (poor alignment)

  • Modified [i]: F₁ raised to ~880 Hz (good alignment)

  • Accomplished by jaw lowering

  • Vowel sounds more like [e] or [ɛ]

Trade-offs

Benefits:

  • Increased acoustic output power

  • Better formant-harmonic alignment

  • Easier phonation at high pitches

Costs:

  • Reduced vowel intelligibility

  • Less precise articulation

  • May affect smoothness of tone quality

Voice teachers and singers must determine the optimal balance through trial and error based on:

  • Performance context (opera house vs. recital hall)

  • Repertoire requirements

  • Individual vocal characteristics

  • Desired aesthetic

Practical Exercise

  1. Sing or speak [i] at comfortable pitch

  2. Gradually lower jaw while maintaining [i]

  3. Notice F₁ rising (brightness increasing)

  4. Vowel moves toward [e] then [ɛ]

  5. Observe power increase with better resonance

Rule 4: Pharyngeal Constriction

A pharyngeal constriction raises F₁ and lowers F₂, creating spectral compacting.

Physical Basis

When the pharynx narrows (back of tongue raised):

  • Pharynx acts as smaller cavity

  • Mouth acts as larger cavity

  • F₁ pressure patterns show shortened acoustic length

  • F₂ pressure patterns show lengthened acoustic length

Acoustic Effects

For [a] vowel (pharyngeal constriction):

  • F₁: Raised from 500 Hz to ~700-900 Hz

  • F₂: Lowered from 1500 Hz to ~1000-1300 Hz

  • Net result: Compact spectrum (energy concentrated)

Perceptual quality:

  • Focused sound

  • Concentrated energy

  • Dark, back quality

  • May sound “throaty” if excessive

Applications

Positive uses:

  • Creating [a], [ɑ], [ɔ] vowels

  • Achieving darker tone quality

  • Balancing forward/back resonance

Caution:

  • Excessive constriction may cause tension

  • Can limit dynamic range

  • May create harsh or pressed quality

Rule 5: Oral Constriction

A mouth constriction lowers F₁ and raises F₂, creating spectral diffusion.

Physical Basis

When the mouth narrows (front of tongue raised):

  • Mouth acts as smaller cavity

  • Pharynx acts as larger cavity

  • F₁ pressure patterns show lengthened acoustic length

  • F₂ pressure patterns show shortened acoustic length

Acoustic Effects

For [i] vowel (oral constriction):

  • F₁: Lowered from 500 Hz to ~250-350 Hz

  • F₂: Raised from 1500 Hz to ~2200-2500 Hz

  • Net result: Diffuse spectrum (energy spread across frequencies)

Perceptual quality:

  • Bright, forward sound

  • Energy distributed high and low

  • Clear, penetrating quality

Applications

Positive uses:

  • Creating [i], [e], [ɛ] vowels

  • Achieving brighter tone quality

  • Increasing speech intelligibility

In singing:

  • May need modification at high pitches

  • Can create excessive brightness

  • Balance with pharyngeal adjustments

Constructing the F₁-F₂ Chart from Memory

Vowel chart construction

Figure 6.24: Constructing the F₁-F₂ vowel chart from a few simple rules.

These rules allow systematic construction of the vowel space:

  1. Draw square (0-1000 Hz F₁; 0-3000 Hz F₂)

  2. Place [ə] at center (500 Hz, 1500 Hz)

  3. [i] upper left: Oral constriction (F₁ ↓, F₂ ↑)

  4. [a] lower right: Pharyngeal constriction (F₁ ↑, F₂ ↓)

  5. [u] lower left: Lip rounding (both ↓)

  6. Connect corners: Form horseshoe

Practical Application Strategies

For Teachers

  1. Start with neutral: Establish [ə] as reference point

  2. Isolate variables: Change one parameter at a time

  3. Use visual feedback: Real-time spectral display helpful

  4. Develop awareness: Help students sense acoustic effects

  5. Apply systematically: Use rules consistently

For Clinicians

  1. Assess baseline: Measure formants in comfortable speech

  2. Identify problems: Determine if formants are appropriate

  3. Target modifications: Apply specific rules to shift formants

  4. Document changes: Measure before and after

  5. Establish new patterns: Stabilize improved configurations

For Performers

  1. Understand your instrument: Know your typical formant values

  2. Experiment systematically: Try each modification strategy

  3. Balance goals: Consider power, quality, intelligibility

  4. Develop consistency: Make modifications automatic

  5. Adapt to context: Modify based on venue and repertoire

Summary

Five main rules guide vowel modification: (1) Vocal tract lengthening lowers all formants uniformly; (2) Lip rounding lowers all formants uniformly while lip spreading raises them; (3) Jaw lowering primarily raises F₁; (4) Pharyngeal constriction raises F₁ and lowers F₂; (5) Oral constriction lowers F₁ and raises F₂. These rules provide systematic guidance for achieving specific acoustic goals through articulatory changes. Understanding and applying these rules allows voice professionals to make informed decisions about vocal technique and to predict acoustic outcomes of articulatory adjustments.

Key Takeaways

  • ✅ Vocal tract lengthening (larynx lowering, lip protrusion) lowers all formants uniformly

  • ✅ Lip rounding lowers all formants; lip spreading raises all formants

  • ✅ Jaw lowering primarily raises F₁, important for high-pitched singing

  • ✅ Pharyngeal constriction raises F₁ and lowers F₂ (spectral compacting)

  • ✅ Oral constriction lowers F₁ and raises F₂ (spectral diffusion)

  • ✅ These rules allow systematic vowel modification for specific acoustic goals

Further Reading

  1. Sundberg, J. (1977). The acoustics of the singing voice. Scientific American, 236, 82-91.

  2. Miller, D. G., & Schutte, H. K. (1990). Feedback from spectrum analysis applied to the singing voice. Journal of Voice, 4(4), 329-334.

  3. Titze, I. R. (2000). Principles of voice production (2nd ed.). National Center for Voice and Speech.

Connections across chapters

Practice all connections

Each prompt ties this topic to another chapter. Predict the outcome before you open the model answer.

Ch. 6 · Rules for Modifying Vowels (this page) Ch. 9 · Formant Tuning

When the pitch climbs above the first formant

A soprano sings [i] (F₁ ≈ 300 Hz) on a note with F₀ = 700 Hz. Predict the acoustic problem, then use the rules for modifying vowels to propose the adjustment singers actually make, the gain it gives, and the price paid in vowel identity.

Think it through, then show a model answer

With F₀ more than an octave above F₁, no harmonic can sit in the first formant, so the strongest part of the source gets no resonance help and the note is weak. The jaw rule says lowering the jaw raises F₁, so the soprano opens the jaw until F₁ climbs to about 700 Hz and meets the fundamental (F₁–H₁ tuning). That alignment yields on the order of 10–15 dB more intensity. Because the F₁ needed is that of an open vowel, all vowels converge toward [a] in the top of the range, which is why text is hard to understand on high soprano notes; the trade is accepted because the alternative is an inaudible note.

A good answer mentions

  • F₀ above F₁ leaves the strongest harmonic without resonance support
  • Jaw lowering raises F₁; tuning F₁ to F₀ gives roughly 10–15 dB
  • The cost is vowel convergence toward [a]

Ch. 6 · Rules for Modifying Vowels (this page) Ch. 7 · Vocal Tract Length

Same folds, longer tube

Two singers have the same membranous vocal fold length, but one has a vocal tract 15% longer. Predict their fundamental frequencies, their formants and their perceived voice colour, and explain what this means for classifying the longer-tract singer.

Think it through, then show a model answer

Fundamental frequency follows vocal fold length (F₀ ≈ 1700/Lₘ), so the two singers share the same natural pitch range. The formants follow the length rule instead: the longer tract has every formant about 15% lower, so its long-time average spectrum is shifted down and the voice sounds darker and "bigger". That singer is a hybrid: a range that says one category and a timbre that suggests a lower one. Such a voice can brighten by raising the larynx or spreading the lips, or the other can darken by lowering the larynx and rounding, but the 15% is not fully recoverable, so the Fach decision has to weigh range against colour.

A good answer mentions

  • F₀ range is set by fold length; timbre by tract length through the formants
  • A 15% longer tract lowers all formants by about 15% (darker LTAS)
  • Mismatched scaling gives a hybrid classification; adjustments are partial

Ch. 1 · Extrinsic Muscles of the Larynx Ch. 6 · Rules for Modifying Vowels (this page) Ch. 8 · Investigations with Electromyography

Lowering the larynx: darker vowels and a pitch side-effect

A singer engages the sternothyroid and other infrahyoid muscles to lower the larynx. Predict what happens to the formants and the perceived timbre, and explain why electromyography finds these muscles negatively correlated with F₀.

Think it through, then show a model answer

Lowering the larynx lengthens the vocal tract, and the length rule says all formants fall in proportion, so the vowel darkens: the "covered" quality of classical singing. The pitch side-effect comes from the anatomy: pulling the thyroid cartilage down and the larynx as a whole toward the sternum tends to bring thyroid and cricoid into a position that slackens the folds, and it opposes the elevating gesture that often accompanies high pitch, so activity in the sternohyoid and sternothyroid correlates negatively with F₀. A singer who wants a low larynx on a high note must therefore supply extra cricothyroid drive to keep the pitch.

A good answer mentions

  • Vocal tract lengthening lowers all formants uniformly (darker timbre)
  • Infrahyoid muscles depress the larynx and are negatively correlated with F₀ in EMG studies
  • Keeping a low larynx at high pitch requires compensating cricothyroid activity
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