2. A vowel sound contains a number of different
pitches simultaneously.
There is the pitch at which it is actually spoken, and
there are the various overtone pitches that give it its
distinctive quality.
We distinguish one vowel from another by the differences in
these overtones, the overtones are called formants.
Source/Filter
Theory
3. The lowest three formants distinguish vowels from one
another.
First formant = the lowest formant and can be symbolized
as F1.
The second formant = can be symbolized as F2, goes down
in pitch in the series of vowels [ i, l, ɛ, ɶ, ɑ, ɔ, ʊ, u ] as can
be heard more easily when these vowels are whispered.
The third formant = can be symbolized as F3, adds to
quality distinctions.
Source/Filter
Theory
4. How do these
formants arise?
They are echoes in the vocal tract. Sound travels from a noise-making source (in voiced
sounds, this is the vocal fold vibration) to the lips. Then, at the lips, most of the sound energy
radiates away from the lips for a listener to hear, while some of the sound energy reflects
back into the vocal tract—it echoes.
5. The vocal folds are a source of sound energy.
The vocal tract, due to the interaction of the reflected sound
waves in it, is a frequency filter altering the timbre of the
vocal fold sound.
In phonetics, the timbre of a vowel is called the vowel
quality
Source/Filter
Theory
7. The length of the resonating portion of the vocal tract also differs
substantially for different speech sounds.
In vowels, the whole vocal tract, from glottis to lips, serves as the
acoustic filter for the noise generated by the vibrating
vocal folds.
In fricatives, the resonating portion of the vocal tract is shorter.
in [s], the portion of the vocal tract that serves as the acoustic filter is
from the alveolar ridge to the lips. Thus, the lowest formant in [s] will
have a much higher frequency than the F1 found in vowels.
The only fricative that does not have higher resonant frequencies than
those found in vowels is the glottal fricative [h]. In [h], the whole vocal
tract, from glottis to lips, is involved.
Source/Filter
Theory
8. In most voiced sounds, three formants are produced every time the
vocal folds vibrate.
The vocal folds may vibrate faster or slower, giving the sound a higher
or lower pitch, but the formant frequencies will remain the same as long
as there are no changes in the shape of the vocal tract.
The general theory of formants was stated by the great German
scientist Hermann Helmholtz about one hundred fifty years ago.
A vowel is the rapid repetition of its peculiar two or three notes
(corresponding to its formants).
All voiced sounds are distinguishable from one another by their formant
frequencies.
Tube Models
9. A bird in the hand is worth two
in the bush
1. We can hear just the variations in the first formant, which sounds like a muffled version of the
sentence.
2. The variations of these overtone pitches convey much of the vowel quality.
3. This formant adds to the overall quality of the sound, but in this sentence, it does not play a very
significant role.
4. The three formants are added together. With this, the sentence becomes highly intelligible.
5. A slight improvement in quality occurs by adding some additional, fixed, formants.
6. Enables us to hear the sounds of the bursts of noise and the turbulence of the fricatives by themselves.
7. We can hear the entire sentence in a monotone.
8. The fundamental pitch can be heard.
11. For each formant, there are locations in the vocal tract where
constriction will cause the formant frequency to rise, and locations
where constriction will cause the frequency to fall.
The perturbation theory says that if there is a constriction at a velocity
maximum (V) in a resonant wave, then the frequency of that resonance
will decrease, and if there is a constriction at a point of maximum
pressure (P), then the frequency of the resonance will increase.
Constriction near the glottis is closer to a pressure maximum (P) than to
a velocity maximum (V) and constriction near the lips is closer to a
velocity maximum.
Perturbation
Theory
13. Acoustic
Analysis
A spectrogram of the words heed, hid, head, had, hod, hawed, hood, who’d as
spoken by a male speaker of American English. The locations of the first three
formants are shown by arrows.
14. Computer programs that are used to make
spectrograms:
1. One of the best is WaveSurfer from the Centre for Speech Technology (CTT) at KTH in Stockholm,
Sweden.
2. Also very widely used is Praat, which is a product of the University of Amsterdam.
16. We can see some
of the
relationships
between
traditional
articulatory
descriptions and
formants A formant chart showing the frequency of the first formant on the ordinate (the
vertical axis) plotted against the second formant on the abscissa (the horizontal
axis) for eight American English vowels. The scales are marked in Hz, arranged at
Bark scale intervals.
17. Acoustics of Consonants
The acoustic structure of consonants is usually more complicated than that of vowels.
20. Acoustic
Analysis
A spectrogram of fie, thigh, sigh, shy. The frequency scale goes up
to 8000 Hz in this figure. The arrows mark the onsets of the second
formant transitions. Only the first word is shown in full. The second part
of the diphthong has been deleted for the other words.
