1. Syllables and Stress
October 21, 2015

2. What is Stress? Examples of stress in English: (V) vs. (N)
Phonetically, stress is hard to define
I.e., it is hard to measure.
It seems to depend on an interaction of three quantifiable variables:
Pitch
Duration
Loudness
And also: quality

3. Loudness How do we measure how loud a sound is?
Recall: one parameter of a sinewave is its amplitude.
Peak amplitude (for sound) is the highest sound pressure reached during a particular wave cycle.
peak-to-peak amplitude

4. Amplitude/Loudness Examples
The higher the peak amplitude of a sinusoidal sound, the louder the sound seems to be.

5. RMS amplitude
Peak-to-peak amplitude is sufficient for characterizing the loudness of sinewaves, but speech sounds are more complex.
Another method of measuring loudness:
root-mean-square (RMS) amplitude
To calculate RMS amplitude:
Square the pressure value of the waveform at each point (sample) in the sound file
Average all the squared values
Take the square root of the average
Write the steps on the board.

6. RMS example
A small sampling of a “sinewave” has the following pressure values:
It looks like this (in Excel):
Write the values on the board;
Then calculate RMS amplitude before going to next slide.
Work it all out by hand, slowly.

7. RMS calculations
To calculate RMS amplitude for this sound, first square the values of each sample:
Then average all the squared values
( ) / 9 = 5/9 = .555
Then take the square root of the average
RMS amplitude = .745

8. Another example What about the RMS amplitude of this sound wave?
It looks like this (in Excel):

9. More Complex Waveforms
The following waveforms all have the same peak-to-peak amplitude:

10. Intensity Two related concepts are acoustic power and intensity.
Power is just the square of amplitude.
P = A2
The intensity of a sound is its power relative to the power of some reference sound.
Intensity is usually measured in decibels (dB).
Decibels is a measure of intensity with reference to the quietest sound human ears can hear.

11. Some Numbers
The intensity of a sound x can be measured in bels, where a bel is defined as:
= log10 (x2 / r2)
r2 is the power of the reference sound
x2 is the power of sound x.
A decibel is a tenth of a bel.
Some typical decibel values:
30 dB Quiet library, soft whispers
40 dB Living room, refrigerator
50 dB Light traffic, quiet office
60 dB Normal conversation

12. Numbers, continued Some typical decibel values:
70 dB Vacuum cleaner, hair dryer
80 dB City traffic, garbage disposal
90 dB Subway, motorcycle, lawn mower
100 dB Chain saw, pneumatic drill
120 dB Rock concert in front of speakers, thunderclap
130 dB Pain threshold
140 dB Gunshot blast, jet plane
180 dB Rocket launching
Bold = sustained exposure can cause hearing damage
Red = pain, immediate damage

13. Intensity Interactions
Perceived loudness depends on frequency, as well as amplitude.
Mid-range frequencies sound louder than low or extremely high frequencies.
100 Hz
250 Hz
440 Hz
1000 Hz
4000 Hz
10000 Hz

14. Sonority Loudness is also a highly context-dependent measure.
Can vary wildly within speaker, from speaker to speaker, from room to room, and across speaking contexts.
However, all things being equal, some speech sounds are louder than others.
Course in Phonetics:
“The sonority of a sound is its loudness relative to that of other sounds with the same length, stress and pitch.”

15. From Ladefoged

16. A Sonority Scale low vowels high vowels glides high sonority liquids
nasals
fricatives
stops
high sonority
low sonority
Write this on the board

17. Sonority and Syllables
An old idea (e.g., Pike, 1943): syllables are organized around peaks in sonority.
This is the Sonority Sequencing Principle (SSP).
Example: [bæd] is a well-formed syllable in English.
[æ]
[b] [d]
high sonority
low sonority

18. Sonority and Syllables
An old idea (e.g., Pike, 1943): syllables are organized around peaks in sonority.
This is the Sonority Sequencing Principle (SSP).
Example: [blænd] works well, too.
[æ]
[l] [n]
[b] [d]
high sonority
low sonority

19. Technical Terms sonority peak [æ] [l] [n] [b] [d] high sonority
low sonority

20. Technical Terms The sonority peak forms the nucleus of the syllable.
[æ]
[l] [n]
[b] [d]
nucleus
high sonority
low sonority

21. Technical Terms The sonority peak forms the nucleus of the syllable.
The sounds that precede the nucleus form the syllable onset.
[æ]
[l] [n]
[b] [d]
onset
high sonority
low sonority

22. Technical Terms The sonority peak forms the nucleus of the syllable.
The sounds that precede the nucleus form the syllable onset.
The sounds that follow the nucleus form the syllable coda.
[æ]
[l] [n]
[b] [d]
coda
high sonority
low sonority

23. Technical Terms The sonority peak forms the nucleus of the syllable.
The sounds that precede the nucleus form the syllable onset.
The sounds that follow the nucleus form the syllable coda.
Together, the nucleus and coda form the syllable rhyme.
[æ]
[l] [n]
[b] [d]
rhyme
high sonority
low sonority

24. Syllables “defined”
“Syllables are necessary units in the organization and production of utterances.” (Ladefoged, 1982)
The construct of a “syllable” can account for a variety of interesting phonological patterns:
Vowel reduction in unstressed syllables in English.
Fricatives and stops devoice at the end of syllables in German, Russian (and many other languages)
Place contrasts disappear in coda position in Japanese (and many other languages)
Voiceless stops are aspirated at the onset of stressed syllables in many Germanic languages.

25. Back to Stress
Stress is a suprasegmental property that applies to whole syllables.
Stressed syllables are higher in pitch (usually)
Stressed syllables are longer (usually)
Stressed syllables are louder (usually)
Stressed syllables reflect more phonetic effort.
The combination of these factors give stressed syllables more prominence than unstressed syllables.

26. Stress: Pitch
(N)
(V)
Complicating factor: pitch tends to drift downwards at the end of utterances

27. Stress: Intensity (N) (V)
Perception of stress is highly correlated with the area under the intensity curve

28. “Phonetic Effort”
Voiceless stops are more aspirated at the onset of stressed syllables in English
Vowels are often reduced to in unstressed syllables in English.
There is less coarticulation across syllable boundaries in stressed syllables.
X-Ray microbeam study (deJong et al., 1993); two utterances:
I said put the TOAST on the table, not the napkins!
I said PUT the toast on the table, don’t throw it!

29. X-ray microbeam data

30. Varying Levels of Stress/Prominence
English has both primary and secondary stress.
Example: “exploitation”
vowel X X X X
full vowel X X X
stress X X
tonic accent X