CHAPTER TWO
RESONANCE
CHAPTER OVERVIEW
I. RESONANCE
II. ACOUSTIC RESONANCE
 Tube Resonance and Standing Waves
 Acoustic Resonators as Filters
 Bandwidth
 Cutoff Frequencies
 Resonance Curves
 Parameters of a Filter
 Types of Filters
III. VOCAL TRACT RESONANCE
 Characteristics of the Vocal Tract Resonator
 Source-Filter Theory of Vowel Production
 Formant Frequencies Related to Oral and Pharyngeal Volumes
 Vowel Formant Frequencies
 F1/F2 Plots
KEY CONCEPTS
ACOUSTIC RESONANCE
APPLIED FREQUENCY
BANDWIDTH
BROADLY TUNED
CUTOFF FREQUENCY
FORMANTS
F1/F2 PLOTS
HALF-WAVE RESONATOR
NATURAL FREQUENCY
QUARTER-WAVE RESONATOR
RESONANCE CURVE
RESONANT FREQUENCY
SHARPLY TUNED
SOURCE-FILTER THEORY
VOWEL FORMANT FREQUENCIES
EMPHASES AND INSTRUCTIONAL ACTIVITIES
1. Introduce the difference between free and forced vibration. Explain that forced vibration is the
basis of resonance. You can easily demonstrate forced vibration by placing a radio on a light table.
Turn up the volume to a moderate level and have students feel the table. Increase the volume while
the students continue to feel the table. Have the students think about the table’s vibration in terms
of natural frequency, free vibration, forced vibration and driving frequency.
2. Discuss acoustic resonance and tube resonance.
3. Explain the filtering properties of acoustic resonators. You can use the common experience of
holding a seashell to the ear and hearing “the sound of the sea” as an example of acoustic resonance.
There is always background noise present even in a very quiet room. This noise is composed of low
intensity sound waves with a broad range of frequencies. The background noise creates vibrations
within the shell. As with any air-filled container, the seashell has a natural frequency or frequencies
at which it will vibrate with greatest amplitude. Some of the frequencies in the room (applied
frequencies) are likely to be close to the natural frequencies of the shell, creating constructive
interference and causing those frequencies to become audible when the shell is held close to the ear.
4. Review resonance curves, bandwidth, center frequency, cutoff frequency and different types of
filters. Have students draw resonance curves of broadly tuned and sharply tuned resonators.
5. Introduce vocal tract resonance, listing the characteristics of the vocal tract in its role as an
acoustic resonator.
6. Describe the source-filter theory. Relate the theory to formants and the formant frequencies that
are influenced by oral and pharyngeal cavity volumes.
7. Explain vowel formant frequencies and how manipulation of vocal tract length, location of
constriction and degree of constriction influence the resonant frequencies of the vocal tract.
8. Discuss F1/F2 plots, noting the relationship between the articulation of the corner vowels and
their acoustic correlates. Prepare a handout of several examples of F1/F2 plots and have students
interpret them. Discuss how these graphs may be used clinically to show differences in vowel
articulation before and after treatment.
9. Download the free acoustic analysis program, Praat (www.praat.org). Ask students to record
different vowels and compare the vowel formants. Point out the inter-individual differences between
students’ productions of the same vowel, and discuss the reasons for these differences.