Speech Hearing Science

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Complex Aperiodic Waveform

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Set of sound waves whose periods and amplitudes are random

noise generated by a chaotic sound source

Frequency represented as range of bandwidth/amplitude as an average

Ongoing sound wave

Soundwaves are unique and unpredictable

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Complex Periodic Waveform

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series of simple periodic sounds (harmonics) who frequencies have a common denominator (fourier series/harmonic series)

frequencies have amplitudes that decrease in predictable way

many soundwavese

each sound wave made up of same complex pattern of multiple subordinate sound waves with mathematically related frequencies

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Fundemental frequency

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lowest frequency/first harmonic (F0)

has highest amplitude

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Response of sound to impedance

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Sound encountering change in medium can be absorbed, reflected, transmitted, diffracted

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Absorbed

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occurs as sound dissipates when energy is lost due to friction between molecules

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Transmitted sound

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energy that successfully passes from one medium to another without being affected

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Diffraction (beding of sound)

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occurs whenwavelength of sound is greater tha size of object being encountered

- low frequency sounds diffract better than high frequency sounds (due to larger wavelengths)

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Reflection

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Sound encounters a high impedance

Occurs when sound wavelength is smaller than object that it encounters and results in shadow of sound

- high frequency sounds are reflected more often than low frequency sounds due to their short wavelengths

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Interference

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When one sound encounters another

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Constructive interference

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- occurs when one area of condensation encounters another area of condensation

- occurs when an area of rarefaction encounters another area of rarefaction

- Constructive interference results in an increase in sound pressure up to a doubling of amplitude.

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Destructive interference

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occurse when on area of condensation encounters an area of rarefaction or vice versa

- Destructtive interference results in a decrease in sound pressure and may result in complete cancellation of sound

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Sound Field

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Places where sound is occuring

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Near field

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- close to sound source, within one metere

- typically no reflections

- no inverse square law application in a near field

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Far field

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Area greater than one meter from sound source

- inverse square law applies

2 type of far field

- free field: area with no reflected surfaces for sound to encounter

- diffuse field: area in which reflective surfaces are present

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Sound intensity

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power per square meter (watts/m2)
work or energy flow, per unit of area per unit of time

how much energy is in wave at any point in time

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Sound pressure

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force per square meter (pascals)

amplitude of a sound wave at any distance from the sound source

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Resonant frequency

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the frequency at which a vibrating object will naturally oscillate

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Free vibration

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if no loss of energy occurs, once object is set into motion it will continue to oscillate at the same frequency indefinitely

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Forced vibratation

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one vibrating object can set another object into vibration if they both have similar natural resonant frequencies

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Resonator

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objet that vibrates in response to another vibration (mechanically or acoustically)

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Vocal tract

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acoustic resonator

As air flows through vocal tract , it will resonate in the air-filled cavities of the vocal tract

Each cavity will produce resonant frequencies whose value depends on the size and shape of cavities

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Filters

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devices used to reject some elements while allowing others to pass

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bandwidth

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frequency or range of frequencies which the resonator will respond best

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vocal tract as filter

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flters sounds produced by VFs

filtering results are the distinctive speech sounds of human languages

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Low pass Filter

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passes low frequencies, attenuates high frequencies

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High pass filter

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passes high frequencie, attenuates low frequencies

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Bandpass:

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passes frequencies within a range defined by upper and lower cut-off frequencies

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Band Reject

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Passes frequencies above and below the cut-off frequencies

attenuates the frequencies between the cut-off frequencies

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Basilar Membrane As Resonator Filter

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each location along basilar membrane has a different ronsant frequency

Different frequencies entering cochlea will cause vibratory response in basilar member at the place along its length that has the same resonance frequency as the stimulus sound.

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Vocal folds

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sound source

produces complex periodic sound comprising a series of harmonic frequencies

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Vocal tract

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resonator

series of air filled cavities that have their own natural resonating frequencies (RFs)

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Source Filter Theory

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Respiratory system: power source

Laryngeal system: Sound Source

Articulatory system: Sound Filter

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Power Sources

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Respiratory system produces controlled epiration that powers speech

provides subglottal pressure needed for VFs to vibrate and produce phonation

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Sound Source

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Laryngeal system powered by airflow from respiratory system

causes vibrations that serve as basis for boiced speech sounds

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Sound Filter

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Supralaryngeal system open and closes to let air out in greater and lesser quantities--thus producing vowels and consanants

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Filtering

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air flowing through tract starts to reverberate at particular freauencies that are determined by length and diameter of cavities involved

reverberations call resonances or formant frequencies

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Harmonics

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sounds waves that compose complex, periodic sound of voicing

lowest frequency harmonic = F0 (fundamental frequency of phontation

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Formants

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sound waves produced in the vocal tract as air flows through and resonates within various cavities.

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Harmonics with frequencies closest to the formant frequencies will be

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amplified

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Vowels

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Have more distinct formants than consonants

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Voiced sounds

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more distinct formants than voiceless sounds

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haromonic frequencies

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change independently of formant frequencies and vice versa

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Spectrogram

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is a graph that represents time of abscissa

frequency on the ordinate

amplitude as a function of darkness on a grayscale