Sonorant Acoustics

March 22, 2012

On the Horizon• Today: acoustics of sonorants

• Tuesday: more sonorant and stop acoustics

• plus an introduction to the motor theory of perception

• Next Thursday:

• stop + fricative acoustics

• The Tuesday after that:

• Articulatory physiology

• Static palatography demo

• I need a volunteer for the demo!

Extremes• Not all music stays within a couple of octaves of middle C.

• Check this out:

• Source: “Der Rache Hölle kocht in meinem Herze”, from Die Zauberflöte, by Mozart.

• Sung by: Sumi Jo

• This particular piece of music contains an F6 note

• The frequency of F6 is 1397 Hz.

• (Most sopranos can’t sing this high.)

Implications• Are there any potential problems with singing this high?

• F1 (the first formant frequency) of most vowels is generally below 1000 Hz--even for females

• There are no harmonics below 1000 Hz for the vocal tract “filter” to amplify

• a problem with the sound source

• It’s apparently impossible for singers to make F1-based vowel distinctions when they sing this high.

• But they have a trick up their sleeve...

Singer’s Formant• Discovered by Johan Sundberg (1970)

• another Swedish phonetician

• Classically trained vocalists typically have a high frequency resonance around 3000 Hz when they sing.

• This enables them to be heard over the din of the orchestra

• It also provides them with higher-frequency resonances for high-pitched notes

• Check out the F6 spectrum.

How do they do it?

• Evidently, singers form a short (~3 cm), narrow tube near their glottis by making a constriction with their epiglottis

• This short tube resonates at around 3000 Hz

• Check out the video evidence.

more info at: http://www.ncvs.org/ncvs/tutorials/voiceprod/tutorial/singer.html

Singer’s Formant Demo

QuickTime™ and aVideo decompressor

are needed to see this picture.

Overtone Singing• F0 stays the same (on a “drone”), while singer shapes the vocal tract so that individual harmonics (“overtones”) resonate.

• What kind of voice quality would be conducive to this?

Vowels and Sonorants• So far, we’ve talked a lot about the acoustics of vowels:

• Source: periodic openings and closings of the vocal folds.

• Filter: characteristic resonant frequencies of the vocal tract (above the glottis)

• Today, we’ll talk about the acoustics of sonorants:

• Nasals

• Laterals

• Approximants

• The source/filter characteristics of sonorants are similar to vowels… with a few interesting complications.

Obstruents and Sonorants• Sonorants are so called because they:

• allow air to flow freely through vocal tract so that resonance (of voicing) is still possible

• = consonants that resonate

• In contrast, obstruents:

• obstruct flow of air through the vocal tract so much that voicing is difficult to maintain

• include stops, fricatives, affricates.

• Sonorants do restrict the flow of air in the vocal tract more than vowels…

• but not as much as obstruents.

Damping• One interesting acoustic property exhibited by (some) sonorants is damping.

• Recall that resonance occurs when:

• a sound wave travels through an object

• that sound wave is reflected...

• ...and reinforced, on a periodic basis

• The periodic reinforcement sets up alternating patterns of high and low air pressure

• = a standing wave

Resonance in a closed tube

t

i

m

e

Damping, schematized• In a closed tube:

• With only one pressure pulse from the loudspeaker, the wave will eventually dampen and die out.

• Why?

• The walls of the tube absorb some of the acoustic energy, with each reflection of the standing wave.

Damping Comparison• A heavily damped wave wil die out more quickly...

• Than a lightly damped wave:

Damping Factors• The amount of damping in a tube is a function of:

• The volume of the tube

• The surface area of the tube

• The material of which the tube is made

• More volume, more surface area = more damping

• Think about the resonant characteristics of:

• a Home Depot

• a post-modern restaurant

• a movie theater

• an anechoic chamber

An Anechoic Chamber

Resonance and Recording• Remember: any room will reverberate at its characteristic resonant frequencies

• Hence: high quality sound recordings need to be made in specially designed rooms which damp any reverberation

• Examples:

• Classroom recording (29 dB signal-to-noise ratio)

• “Soundproof” booth (44 dB SNR)

• Anechoic chamber (90 dB SNR)

Spectrograms

classroom

“soundproof” booth

Spectrograms

anechoic chamber

Inside Your Nose• In nasals, air flows through the nasal cavities.

• The resonating “filter” of nasal sounds therefore has:

• increased volume

• increased surface area

• increased damping

• Note:

• the exact size and shape of the nasal cavities varies wildly from speaker to speaker.

Nasal Variability• Measurements based on MRI data (Dang et al., 1994)

Damping Effects, part 1

[m] [m]

• Damping by the nasal cavities decreases the overall amplitude of the sound coming out through the nose.

Damping Effects, part 2• How might the power spectrum of an undamped wave:

• Compare to that of a damped wave?

• A: Undamped waves have only one component;

• Damped waves have a broader range of components.

100 Hz sinewave

90 Hz sinewave

110 Hz sinewave

+

+

Here’s Why

The Result

90 Hz +

100 Hz +

110 Hz

• If the 90 Hz and 110 Hz components have less amplitude than the 100 Hz wave, there will be less damping:

Damping Spectra

light

medium

Damping Spectra

heavy

• Damping increases the bandwidth of the resonating filter.

• Bandwidth = the range of frequencies over which a filter will respond at .707 of its maximum output.

• Nasal formants will have a larger bandwidth than vowel formants.

Bandwidth in Spectrograms

The formants in nasals have increased bandwidth, in comparison to the formants in vowels.

F3 of [m] F3 of

Nasal Formants• The values of formant frequencies for nasal stops can be calculated according to the same formula that we used for to calculate formant frequencies for an open tube.

• fn = (2n - 1) * c

4L

• The simplest case: uvular nasal .

• The length of the tube is a combination of:

• distance from glottis to uvula (9 cm)

• distance from uvula to nares (12.5 cm)

• An average tube length (for adult males): 21.5 cm

The Math

12.5 cm

9 cm

fn = (2n - 1) * c

4L

L = 21.5 cm

c = 35000 cm/sec

F1 = 35000

86

= 407 Hz

F2 = 1221 Hz

F3 = 2035 Hz

The Real Thing• Check out Peter’s production of an uvular nasal in Praat.

• And also Dustin’s neutral vowel!

• Note: the higher formants are low in amplitude

• Some reasons why:

• Overall damping

• “Nostril-rounding” reduces intensity

• Resonance is lost in the side passages of the sinuses.

• Nasal stops with fronter places of articulation also have anti-formants.

Anti-Formants• For nasal stops, the occlusion in the mouth creates a side cavity.

• This side cavity resonates at particular frequencies.

• These resonances absorb acoustic energy in the system.

• They form anti-formants

Anti-Formant Math• Anti-formant resonances are based on the length of the vocal tract tube.

• For [m], this length is about 8 cm. 8 cm

• fn = (2n - 1) * c

4LL = 8 cm

AF1 = 35000 / 4*8 = 1094 Hz

AF2 = 3281 Hz

etc.

Spectral Signatures• In a spectrogram, acoustic energy lowers--or drops out completely--at the anti-formant frequencies.

anti-formants

Nasal Place Cues• At more posterior places of articulation, the “anti-resonating” tube is shorter.

• anti-formant frequencies will be higher.

• for [n], L = 5.5 cm

• AF1 = 1600 Hz

• AF2 = 4800 Hz

• for , L = 3.3 cm

• AF1 = 2650 Hz

• for , L = 2.3 cm

• AF1 = 3700 Hz

[m] vs. [n]

• Production of [meno], by a speaker of Tsonga

• Tsonga is spoken in South Africa and Mozambique

[m] [e] [n] [o]

AF1 (m)

AF1 (n)

Nasal Stop Acoustics: Summary

• Here’s the general pattern of what to look for in a spectrogram for nasals:

1. Periodic voicing.

2. Overall amplitude lower than in vowels.

3. Formants (resonance).

4. Formants have broad bandwidths.

5. Low frequency first formant.

6. Less space between formants.

7. Higher formants have low amplitude.

Perceiving Nasal Place• Nasal “murmurs” do not provide particularly strong cues to place of articulation.

• Can you identify the following as [m], [n] or ?

• Repp (1986) found that listeners can only distinguish between [n] and [m] 72% of the time.

• Transitions provide important place cues for nasals.

• Repp (1986): 95% of nasals identified correctly when presented with the first 10 msec of the following vowel.

• Can you identify these nasal + transition combos?

Nasalized Vowel Acoustics• Remember: vowels are often nasalized next to a nasal

stop.

• This can obscure formant transitions.

• The acoustics of nasalized vowels are very complex.

• They include:

1. Formants for oral tract.

2. Formants for nasal tract.

3. Anti-formants for nasal passageway.

• Plus:

• Larger bandwidths

• Lower overall amplitude

Nasal Vowel Movie

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Chinantec• The Chinantec language contrasts two degrees of nasalization on vowels.

• Chinantec is spoken near Oaxaca, Mexico.

• Check out the X-ray video evidence….

Oral vs. Partly Nasal

• Note: extra formants + expanded bandwidth…

• Tends to smear all resonances together in the frequency dimension.

Partly vs. Wholly Nasal

!Xoo Oral and Nasal Vowels