1. Investigating physical properties of speech sounds
Acoustic Phonetics
Investigating physical properties of speech sounds
From chapter 7, Rogers (2000)

2. Speech Sound Representation Reconsidered
Articulatory phonetic approach:
Describing sounds depending on how they are produced
Problems of this approach
Representation is only in terms of symbols  Sounds are not like that in reality
It’s not reflected that some sounds are more confusing each other when perceived while others are not
Eg) i/e vs. a/k s/f vs. s/m
So we need another way of describing speech sounds
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3. Acoustic representation of speech sounds
Representing sounds as they are
Visual other than symbolic representation
Depending more upon perception than production or articulation
Physical properties are analyzed
Similarities and differences of sounds are disclosed
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4. Acoustic definition of sound
Variation in air pressure
Movements of air particles
An audible disturbance of a medium produced by a source
The source: any object that vibrates
Eg) musical instruments, human vocal cords, microphone
The medium: any elastic object that carries vibration
Eg) air, water
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5. Advantages of acoustic representation
Real/physical mechanism of speech communication is represented
No convention, no confusion, no controversy
Gradual change of sounds are shown
Example) How loud a sound is
Small variations are shown
Helpful for understanding how computers synthesize speech and how speech recognition works
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6. What to represent? Three aspects sounds that can differ Pitch Loudness
Quality
(Length)
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7. How to represent acoustically?
Sound is air particle movements
The best and agreed way of expressing air particle movements:
Waveform
Another necessary way of representing sound:
Spectrum
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8. Waveforms

9. Waveform properties Simple harmonic movement + Time elapse  Waveform
Individual particles move only backward and forward
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10. Air particle movement No force Initial force Time Elasticity Inertia
Displacement
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11. Simple Waveform
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12. Speech sound properties shown in waveforms
Differentiation of sounds
Sounds are different, which is crucial in human speech as a communication method
Ways in which sounds can differ
Perceptually: Pitch, Loudness, Quality
Acoustically: Frequency, Amplitude, Phase
Waveform shows differences in
Acoustic correlate of Loudness  Amplitude
Acoustic correlate of Pitch  Frequency
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13. Amplitude representing Loudness
(b)
파란색보다 빨간색 파형의 진폭이 2배로 크다  빨간색 소리가 더 큼
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14. Amplitude (cntd.) Air pressure fluctuation
The extent of the maximum variation in air pressure from normal during a sound
Unit: Bel, Decibel(dB; 1/10 of Bel), Bark
dB: Common logarithm of power ratios
Twice amplitude is not heard as twice loud
Loud sound: particles move farther and more rapidly
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15. Frequency representing Pitch
(a)
(b)
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16. Frequency (cntd.) The rate at which sound source vibrates
Sound sources: tuning forks, vocal cords, etc
Units: Hz, cps (cycle per second)
Depending upon
Length of the pendulum
Length of tuning fork prongs
F(requency) = 1/T(period)
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17. Frequency (cntd.) Standard A frequency: 440 Hz
Octave: a note which is exactly twice the frequency of another note
Eg) A(440Hz), A’(880Hz), A’’(1760Hz)
Audible Frequency
Human: 20Hz(or16Hz) – 20KHz
Bats: 20KHz – 100KHz
Fastest telephone vibration: 35KHz
Most of the human speech sound frequency: below 8KHz
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18. Frequency (cntd.) Pitch and frequency are not in linear relationship
Only in the low frequency, fairly linear
Hz difference sounds greater than Hz difference
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19. Phase difference
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20. Phase (cntd.) Phase differences cause different waveforms But
Human ears do not perceive phase differences
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21. Waveform is not sufficient..
Two sounds with the same pitch and loudness can still differ
Example) Violin A vs. Piano A
Example) [i] vs. [a]
Another way of representation needed
Spectrum
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22. More about waveform first..
To know about spectrum and its representation of quality, we need to know more about waveform
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23. Types of Waveforms: Pure tones vs. Complex waves
Most sounds, including human speech, sources produce complex vibrations
Pure tone: single harmonic motion (SHM), with only one frequency
Complex wave: more than one harmonic motion, multiple frequency
Pure tone + pure tone of the same frequency and phase  another pure tone
Pure tone + pure tones of different frequency  a complex tone
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24. Pure tone (Simple Wave, simple harmonic motion, Sinusoid, Sine wave)
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25. Complex wave
100 Hz Hz Hz
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26. [a] production by a female speaker
Complex wave
[a] production by a female speaker
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27. Types of Waveform: Repetitive vs. non-repetitive wave
Strictly Repetitive (periodic):
sine wave, ideal sounds
Virtually Repetitive (periodic):
vowels, sonorants
Non-repetitive (aperiodic):
obstruents
white noise (most complex)
click
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28. Periodic vs non-periodic wave
Aperiodic [s]
Periodic wave [a]
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29. Limitation of Waveform Representation
Sound can be heard in 3 different way
Loudness, Pitch, Quality
Quality can’t be represented directly in waveforms
A new way of representation needed
Spectrum
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30. Spectrum

31. Background Knowledge on Spectrum
Sound waves can be either simple or complex
Simple: sinusoid
Complex: Combined simple waves with different frequency
Sound quality can be determined by the way such simple waves combine into a complex wave
If a complex wave can be split into each simple wave we can see the secret
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32. Waveform and Spectrum (100Hz + 200Hz + 300Hz )4 2
100
200
300 Hz
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33. An Example of Spectrum
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34. Formants shown in spectrum
Frequency component(s) with boosted energy
Formant frequency: Its frequency
Reason for formant shaping: Filtering function in vocal tract
Decisive aspect of sound quality
For vowels three formants (F1, F2, F3) are especially important for their distinction
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35. An Example of Formant : Vowel [«]
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36. An Example of Formant: Vowel [e]1 2 3 4 5 6 50
300
550
800
1050
1300
1550
1800
2050
2300
2550
2800
3050
3300
3550
3800
4050 Hz
Amplitude F1 F2 F3
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37. Disadvantages of Spectrum Representation
Less intuitive
X-axis denotes frequency level
No time varying representation
Hard to see interaction with Waveforms
Thus, a new way of representation needed
 Spectrogram
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38. Spectrogram & its reading

39. What is spectrogram? Begin to be used since 1940s
Another representation of frequency domain analysis
The most popular way of representing spectral information
3 dimensional representation
X-axis: Time
Y-axis: Frequency
Darkness (or color): Energy
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40. Waveform & Spectrogram aligned
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41. Spectrogram example (color resolution of word “compute”)
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42. Spectrogram example (grayscale of word “compute”)
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43. Types of spectrogram Wideband spectrogram Narrowband spectrogram
better time resolution
Narrowband spectrogram
better frequency resolution
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44. Wideband vs. Narrowband spectrograms of the question "Is Pat sad, or mad?" The 5th, 10th and 15th harmonics have been marked by white squares in two of the vowels
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45. Advantages & Disadvantages
Time alignment
Disadvantages
Less reliable than waveform
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46. Vowel Spectrogram
Formant frequencies are critical cues for vowel distinction
F1: Height
high vowels: low F1
F2: Backness
back vowels: low F2
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47. Examples of formant frequencies of English monophthongsà F3
2900
2550
2490
2640
2380
2300
2500
2390 F2
2250
1900
1770
1660
1100
1030
870
1500
1190 F1
280
400
550
690
710
450
310
900
640
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48. From http://hctv.humnet.ucla.edu/departments/linguistics
"heed, hid, head, had, hod, hawed, hood, who'd" (a male speaker, American English)
From
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49. Consonant Spectrogram
General
Acoustic structure more complicated than vowels
Adjacent sounds (especially vowels) convey important information  locus
High frequency characteristics
 especially for fricatives and affricates
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50. What is LOCUS
Information of formant transition from vowels into obstruents or from obstruents into vowels
The target frequency that each formant transition is heading toward as an obstruction is made, or the frequency the transition comes as the obstruction is released
The characteristic of the consonantal place and manner  roughly the same in different vowel contexts
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51. Stops General Fairly distinct locus for each place Burst
Silence during the closure (only at syllable onset position)
Virtually no difference during the closure
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52. Stops (cntd.) Voicing distinction
voiced: vertical striations for voiced sounds, less abrupt burst, frequently weakened to be like fricatives or approximants
voiceless: generally abrupt burst at higher frequency area
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53. Stops (cntd.) Place distinction bilabial alveolar velar
relatively low F2, F3 locus  rising into and falling out of vowel
weak and spread vertical lines
alveolar
F2 locus about 1800 Hz
Strong vertical lines
velar
Velar pinch: vowels F2, F3 merging
often double burst
long formant transitions
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54. Stops (cntd.) Manner distinction Silence duration, VOT, Following V F0
Aspirated [pH]
short
long
high
Tense [p’]
Lax [p]
mid
low
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55. Examples -- “a bab, a dad, a gag”
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56. Place dependent loci
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57. Fricatives
General
Random noise pattern especially in high frequency regions
Place distinction
Labiodental [f, v]: rising locus into the following vowel
Dental [T, D]: major energy above 6000Hz
Alveolar [s, z]: major energy above 4000Hz
Alveopalatal [s&, zà]: major energy above 2000Hz
Glottal [h]: the trace of formant frequencies of neighbouring vowels
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58. Fricatives (cntd.) Weak vs. strong Strong [s, z, s&, zà]: darker bands
Weak [f, v, T, D]: spread and fainter
Voiced [v, D ]: often so weak and confused with nasals or approximants
Cues to tell [T] from [f]: higher formants of [T] fall into adjacent vowels
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59. Example – “fie, thigh, sigh, shy”
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60. Example – “ever, weather, fizzer, pleasure”
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61. Nasals General Place distinction
Formants similar to vowels but fainter
Very low F1 (about 250Hz), F2 (about 2500Hz), and F3 (about 3250Hz)
Place distinction
bilabial [m]: downward F2, F3 locus
alveolar [n]: less amount of F2 transition
velar [N ]: velar pinch
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62. Examples -- “a Pam, a tan, a kang”
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63. Liquids & Approximants
General
Formants similar to vowels but fainter (especially at high frequency regions)
Approximately F1(250Hz), F2(1200Hz), F3(2400Hz)
Slow formant movements
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64. Liquids & Approximants (cntd.)
Phone specific properties
Labial glide [w]:
very low F1, F2 ( Hz|) and gets too close to each
relatively low F3
rapid falloff of spectral amplitude (formant movements)
Palatal glide [y]:
extremely low F1
extremely high F2, F3
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65. Liquids & Approximants (cntd.)
Phone specific properties (cntd.)
Flap [R]: soft burst, short duration
Retroflex [r]:
F3 dipping down close to F2
General lowering of F3, F4
Lateral [l]:
Low F1, F2 (approx. F1 250Hz, F2 1200Hz)
usually substantial energy in the high F region
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66. Example – “led, red, wed, yell”
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67. Final remarks on spectrogram
Spectrogram is not the only cue for acoustic distinction of speech sounds.
When there is a mismatch between waveform & spectrogram, the waveform is more reliable in general.
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68. References & Links
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