Audition Theory

What is the human ear transducer?

\
* - changes energy from one form to another

What are audition/hearing sensitive to?

\
* Changes in frequency and intensity over time
* Speech sounds

What’s the anatomy of the audition theory?

\
* Outer Ear
* Middle Ear
* Inner Ear

\
__Outer Ear (Air Conduction) - Structures__

\
\
* Pinna
* EAM (canal)
* TM (eardrum)
* Vibrates and is extremely sensitive to changes in pressure
* Can respond to a wide range of frequencies

\
__Outer Ear (Air Conduction) - Functions__

\
* Protect ear
* “Funnel” sound into the ear
* Pinna and EAM:
*  high frequency sounds boosted 10-15dB for sounds between 2500-5000 Hz

\
__Middle Ear (Mechanical Conduction) - Structures__

\
* Ossicles: Malleus, Incus, and Stapes
* Eustachian Tube

__Middle Ear (Mechanical Conduction) - Functions__

* Protect ear (via acoustic reflex via muscle contraction → damps, amplifies sounds greater than 80 dB)  
* Tensor Tympani
* Stapedius muscle
* Equalize air pressures (via Eustachian tube)
* Transmit sounds to cochlea 
* Impedance matching – differences in surface area between TM and oval window, lever action of ossicles boosts by 2dB, shape of TM 

\

\
__Inner Ear (Fluid Conduction) - Structures__

\
* Cochlea (hearing)
* Organ of Corti (sensory organ of hearing)

__Inner Ear (Fluid Conduction) - Functions__

\
* Convert mechanical energy into electro-chemical signals to be sent to the brain 
* Analyzes frequencies and intensity
* Assists with balance (semicircular canals/vestibule)

\
__Inner Ear (Fluid Conduction) - outer hair cells__

\
* Roof or tectorial membrane
* In response to sound, which amplifies and improves the tuning of the basilar membrane
* Improve frequency and auditory sensitivity

\
__Inner Ear (Fluid Conduction) - inner hair cells__

\
* Release the neurotransmitters 

\
__Inner Ear (Fluid Conduction) - basilar membrane__

\
* Stiffer at base
* More flexible at apex 
* Lower frequencies

\
__Traveling Wave Theory__

\
* Wave peak → excitation of hair cells
* Traveling waves
* Basilar membrane displacement as a function of frequency

The Cochlea

\
* Tonotopic organization
* Place-frequency mapping along the basilar membrane

Acoustic Cues to Speech Perception

\
* Information used to identify speech sounds
* Variability in the speech signal 
* Humans as pattern detectors
* Varies on rate of speech, context, sounds around it 
* Not a one to one correspondence
* Even if you don’t fully understand the sounds or its a noisy environments, you can still pick up the context 
* Redundancy

Acoustic Cues to Speech Perception - physiological

both ears stimulated

Acoustic Cues to Speech Perception - acoustic cues

acoustic cues

Acoustic Cues to Speech Perception - lexical

 knowing where the sounds fall in a word

Acoustic Cues to Speech Perception - word prediction

\
knowing that there is another word coming

Acoustic Cues to Speech Perception - Cues for vowels

\
* Voicing
* High intensity
* Long duration
* Formant frequency ratios (F2 to F1)

Acoustic Cues to Speech Perception - Categorical Perception (consonants)

\
* All sounds that are differing in one acoustic aspect are perceived as the same phoneme until a boundary is reached
* /d/ and /t/ are pre acoustically similar until the voice onset time, where then we can tell the difference 
* Alveolars are similar 

Acoustic Cues to Speech Perception - Cues for Semi-Vowels (Liquids & Glides)

\
* Relatively high intensity 
* Formant transitions
* Transitions are faster than they are in diphthongs and vowels 

Acoustic Cues to Speech Perception - Cues for Nasals

\
* Decreased intensity
* Dampened sounds 
* Nasal murmur (resonance

Acoustic Cues to Speech Perception - Cues for Stops

\
* Silence
* Burst (if possible)
* Formant transitions in neighboring vowels 
* Formant durations
* 40-60 m/sec stop

Acoustic Cues to Speech Perception - Cues for Fricatives

\
* Relatively low intensity
* Noise-center frequency
* Longer in duration

Acoustic Cues to Speech Perception - Cues for Affricates

\
* Long in duration 

Acoustic Cues to Speech Perception - Additional Cues

\
* Multiple cues often available
* Coarticulation may aid in perception

__Types of Hearing Loss__

\

* Conductive
* Sensorineural

Conductive

* outer/middle ear

Sensorineural

* inner ear

\
How might hearing loss affect speech perception?

\
* Chief complaint
* Effects of hearing loss for children vs. adults
* The speech signal and how to deliver it
* Audibility
* Sound quality 

\
__Hearing Loss and Impact on Bandwidth__

* Normal listeners are at 1000 Hz
* Respond to a bandwidth of about 100-150 Hz around this frequency
* In SNHL, 3-4 times larger bandwidth
* This impacts frequency resolution that limits ability to discriminate between phonemes

\

\
__What Cues Can Be Used?__ 

\
* Difficult to analyze stops and fricatives (high frequency)
* Listen for formal transitions (typically present in low frequencies)
* Listen for duration
* Less use of F2 ie. place for identification 
* However, can use visual cues for place 

\
__Cochlear Implants__

\
* Deliver sound by converting electrical current and directly stimulating CNVIII
* External microphone
* Speech processor, transmitter
* Implant
* External, receiver 

What can cochlear impacts detect?

* a wide range of frequencies, arranged in bandwidths
* Must be able to detect small changes in frequency to distinguish between different vowel formants
* Must be able to determine where the formants are at a given time
* Candidacy (changing)
* > 1 year of age
* Medically, psychologically suited
* Post surgery - device “turned on”, programmed
* Auditory training required 

\

\
__Auditory Training__ 

\
* Often starts with counting numbers of sounds
* Suprasegmentals present in low frequencies, therefore, more audible 
* Use them as a bridge to phoneme and word recognition