basic acoustics
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Basic Acoustics. October 12, 2012. Agenda. The Final Exam schedule has been posted: Tuesday, December 18 th , from 8-10 am Location TBD I will look into getting that time changed… On Monday, we’ll talk about suprasegmentals Pitch, Tone, length, etc. - PowerPoint PPT PresentationTRANSCRIPT
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Basic Acoustics
October 12, 2012
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Agenda• The Final Exam schedule has been posted:
• Tuesday, December 18th, from 8-10 am
• Location TBD
• I will look into getting that time changed…
• On Monday, we’ll talk about suprasegmentals
• Pitch, Tone, length, etc.
• On Wednesday, we’ll do some suprasegmental transcription practice.
• Next Friday, we’ll cover more complicated suprasegmental structures:
• Syllables and Stress.
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Laryngeal Settings• We now know of two basic laryngeal settings for any
pulmonic egressive sound:
1. Vocal folds are adducted (brought together)
• Air from lungs makes vocal folds “trill”
• = voiced sounds
2. Vocal folds are abducted (held apart)
• Air passes through glottis unobstructed
• = voiceless sounds
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Independence• Stops can be voiced or voiceless.
• Two anatomically independent settings:
• Place of articulation
• Voiced/Voiceless
• Are these two settings aerodynamically independent of each other?
• Is it easier to make a voiced or a voiceless stop?
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Cross-linguistic Data• From Ruhlen (1976), who surveyed 706 languages
• 75% had both voiced and voiceless stops
• Of the remaining 25%...
• 24.5% had only voiceless stops
• 0.5% had only voiced stops
• voiced stops are hard
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One step further• Are some voiced stops harder than others?
• Stop inventories:
English p t k
b d g
Thai p t k
b d
Efik t k
b d
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More Cross-Language Data• From Sherman (1975), who surveyed the stop inventories of 87 languages.
• 2 languages were missing voiced bilabials
• 21 languages were missing voiced dentals/alveolars
• 40 languages were missing voiced velars
• voiced velars are particularly hard
• Why?
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Place and Volume:a schematic
mouth
lips
glottis
pharynx
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Place and Volume:a schematic
glottis
• Voicing occurs when air flows through the glottis
airflow
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Place and Volume:a schematic
glottis
• For air to flow across the glottis…
• the air pressure below the glottis must be higher than the air pressure above the glottis
• Pbelow > Pabove
Pbelow
Pabove
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Place and Volume:a schematic
glottis
• If there is a stop closure and…
• Air is flowing through the glottis…
• The air above the glottis will have nowhere to go Pbelow
Pabove
stop closure
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Place and Volume:a schematic
glottis
1. Air pressure below the glottis will drop
2. Air pressure above the glottis will rise
3. The difference between the two will decrease
Pbelow
Pabove
stop closure
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Place and Volume:a schematic
glottis
• (Pbelow - Pabove) 0
• Airflow across the glottis will cease
• Voicing will stopPbelow
Pabove
stop closure
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Place and Volume:a schematic
glottis
• The further back a stop closure is made…
• The less volume there is above the glottis for air to flow into
Pbelow
Pabove
velar stop closure
decreased volume
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Place and Volume:a schematic
glottis
• Pabove will increase more rapidly as air flows through the glottis
• Voicing will cease more quickly
Pbelow
Pabove
velar stop closure
decreased volume
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More Numbers• From Catford (1982), Fundamental Problems in
Phonetics
• Lung volume = 1840 - 4470 cm3
• During inhalation/exhalation, lung volume typically changes 500-1000 cm3
• Vocal tract volume = space between glottis and oral closure:
1. Bilabials: 120-160 cm3
2. Alveolars: 70-100 cm3
3. Velars: 30-50 cm3
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Morals of the Story• Voiced stops are hard because too much air gets pushed into the mouth, behind the stop closure
• This makes it impossible for there to be a pressure drop across the glottis.
• Voiced velars are worse, because the space above the glottis, behind the stop closure, is even smaller.
• This space gets filled up by pulmonic airflow even faster
• Independent articulatory gestures may interact aerodynamically
• They have to share the same stream of air.
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Some Leftovers• Velar trills?
• Velars often have multiple release bursts…
• due to the massiveness (and sluggishness) of the back of the tongue
• Check out an example.
• An alternate strategy to maintain voicing:
• pre-nasalization
• [mb], [nd], etc.
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Implosive Stats• Implosives often begin life as voiced stops.
• Trying to voice them completely can lead to them becoming implosives.
• Implosives are more frequently found at fronter places of articulation
Bilabial: 39 Palatal: 7
Alveolar: 36 Velar: 5
Retroflex: 1 Uvular: 1
• The lack of more posterior implosives may be due to the lack of posterior voiced stops to begin with.
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Pin
Fad
Fad
• How is sound transmitted through the air?
• Recall our bilabial trill scenario:
Acoustics: Basics
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What does sound look like?• Air consists of floating air molecules
• Normally, the molecules are suspended and evenly spaced apart from each other
• What happens when we push on one molecule?
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What does sound look like?• The force knocks that molecule against its neighbor
• The neighbor, in turn, gets knocked against its neighbor
• The first molecule bounces back past its initial rest position
initial rest positionCheck out some atomic bomb videos…
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What does sound look like?• The initial force gets transferred on down the line
rest position #1
rest position #2
• The first two molecules swing back to meet up with each other again, in between their initial rest positions
• Think: bucket brigade
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Compression Wave• A wave of force travels down the line of molecules
• Ultimately: individual molecules vibrate back and forth, around an equilibrium point
• The transfer of force sets up what is called a compression wave.
• What gets “compressed” is the space between molecules
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Compression Wave
area of high pressure
(compression)area of low pressure
(rarefaction)
• Compression waves consist of alternating areas of high and low pressure
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Pressure Level Meters• Microphones
• Have diaphragms, which move back and forth with air pressure variations
• Pressure variations are converted into electrical voltage
• Ears
• Eardrums move back and forth with pressure variations
• Amplified by components of middle ear
• Eventually converted into neurochemical signals
• We experience fluctuations in air pressure as sound
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Measuring Sound• What if we set up a pressure level meter at one point in the wave?
Time
pressure level meter• How would pressure change over time?
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Sine Waves• The reading on the pressure level meter will fluctuate between high and low pressure values
• In the simplest case, the variations in pressure level will look like a sine wave.
time
pressure
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Other Basic Sinewave concepts• Sinewaves are periodic; i.e., they recur over time.
• The period is the amount of time it takes for the pattern to repeat itself.
• The frequency is the number of times, within a given timeframe, that the pattern repeats itself.
• Frequency = 1 / period
• usually measured in cycles per second, or Hertz
• The peak amplitude is the the maximum amount of vertical displacement in the wave
• = maximum/minimum amount of pressure
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Waveforms• A waveform plots amplitude on the y axis against time on the x axis.
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Complex Waves• When more than one sinewave gets combined, they form a complex wave.
• At any given time, each wave will have some amplitude value.
• A1(t1) := Amplitude value of sinewave 1 at time 1
• A2(t1) := Amplitude value of sinewave 2 at time 1
• The amplitude value of the complex wave is the sum of these values.
• Ac(t1) = A1 (t1) + A2 (t1)