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:

Phonate an [a] vowel at comfortable pitch
Gradually lower your larynx (attempt to yawn)
Note the darkening quality (move toward [ɔ])
Now extend your lips forward during [a]
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

Phonate [a] with spread lips
Gradually round lips while maintaining [a]
Notice transition toward [ɔ] or [o]
Alternate between spread and rounded
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

Sing or speak [i] at comfortable pitch
Gradually lower jaw while maintaining [i]
Notice F₁ rising (brightness increasing)
Vowel moves toward [e] then [ɛ]
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:

Draw square (0-1000 Hz F₁; 0-3000 Hz F₂)
Place [ə] at center (500 Hz, 1500 Hz)
[i] upper left: Oral constriction (F₁ ↓, F₂ ↑)
[a] lower right: Pharyngeal constriction (F₁ ↑, F₂ ↓)
[u] lower left: Lip rounding (both ↓)
Connect corners: Form horseshoe

Practical Application Strategies

For Teachers

Start with neutral: Establish [ə] as reference point
Isolate variables: Change one parameter at a time
Use visual feedback: Real-time spectral display helpful
Develop awareness: Help students sense acoustic effects
Apply systematically: Use rules consistently

For Clinicians

Assess baseline: Measure formants in comfortable speech
Identify problems: Determine if formants are appropriate
Target modifications: Apply specific rules to shift formants
Document changes: Measure before and after
Establish new patterns: Stabilize improved configurations

For Performers

Understand your instrument: Know your typical formant values
Experiment systematically: Try each modification strategy
Balance goals: Consider power, quality, intelligibility
Develop consistency: Make modifications automatic
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

Rules for Modifying Vowels

Rules for Modifying Vowels

Rule 1: Vocal Tract Length

Physical Basis

Mechanisms for Changing Vocal Tract Length

Quantitative Effects

Practical Exercise

Rule 2: Lip Rounding and Spreading

Physical Basis

Lip Rounding Effects

Lip Spreading Effects

Covered vs. Open Singing

Practical Exercise

Rule 3: Jaw Lowering

Physical Basis

Application to Singing

Trade-offs

Practical Exercise

Rule 4: Pharyngeal Constriction

Physical Basis

Acoustic Effects

Applications

Rule 5: Oral Constriction

Physical Basis

Acoustic Effects

Applications

Constructing the F₁-F₂ Chart from Memory

Practical Application Strategies

For Teachers

For Clinicians

For Performers

Summary

Key Takeaways

Related Topics

Further Reading