Basic Acoustics

October 12, 2012

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.

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

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?

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

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

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?

Place and Volume:a schematic

mouth

lips

glottis

pharynx

Place and Volume:a schematic

glottis

• Voicing occurs when air flows through the glottis

airflow

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

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

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

Place and Volume:a schematic

glottis

• (Pbelow - Pabove) 0

• Airflow across the glottis will cease

• Voicing will stopPbelow

Pabove

stop closure

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

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

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

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.

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.

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.

Pin

Fad

Fad

• How is sound transmitted through the air?

• Recall our bilabial trill scenario:

Acoustics: Basics

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?

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…

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

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

Compression Wave

area of high pressure

(compression)area of low pressure

(rarefaction)

• Compression waves consist of alternating areas of high and low pressure

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

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?

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

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

Waveforms• A waveform plots amplitude on the y axis against time on the x axis.

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)