Lecture 11 Perception: Hearing - Physiology and Psychoacoustics Flashcards
What is sound?
- A change in local air pressure produced by the vibration of an object
- Travels cyclically through the medium, i.e., as a wave
- Longitudinal wave: particles move parallel to direction of wave
- Transmitted, reflected, absorbed
- Most of the sounds we hear in the environment are complex, meaning they are composed of multiple sound waves with different frequencies and amplitudes rather than a single pure tone
wave properties: amplitude
power of energy (height)
wave properties: frequency
cycles per second (Hz) (how many times does a wave oscillate)
wave properties: wavelength
length of one cycle, distance between the peaks
Wave properties: speed
distance travelled per unit time
Sound wave frequency relation to period
reciprocal
Time taken (period of the wave) = 1/100 = 0.01 seconds
Speed of sound in air
340 m/s
speed of sound in water
1500 m/s
speed of sound in steel
5000 m/s
Similarities between sound and light waves
- Both obey the wave equation, c = fλ
- Both can be reflected, refracted and absorbed
- Pure tone comparable to monochromatic light (a single-wavelength light (e.g., 10 Hz vs. 600 nm)
- Pure tones and monochromatic light are rare (pure tones are sound waves with a single frequency, monochromatic light is light with a single wavelength (or frequency)
- Natural sound and images comprise a spectrum of frequencies or wavelengths
differences between light and sound waves
- Sound needs a medium, light can travel in space
- Sound wave: longitudinal; Light wave: transverse
- Light is faster than sound
- Light is classified by wavelength, sound by frequency
- Light wavelengths are much shorter
- sound audible range: 1.7 cm - 17 metres
- light visible range: 400-700 nm (nm: one-billionth of a meter)
What does a higher amplitude in a sound wave correspond to
louder sound
what is a high frequency in a sound wave perceived as
higher-pitched sound
fourier analysis
separating complex waveforms into their sinusoidal components
Fourier analysis: analysis meaning
breaking waves down
Fourier analysis: synthesis meaning
adding waves up
Fourier spectrum
Amount of energy at each frequency component of the complex waveform
Harmonic spectra
Fundamental frequency: Lowest frequency component in the spectrum
timbre: sound quality conveyed by all harmonic and higher frequencies
Sound qualities: timbre
harmonic structure
what does a high frequency correspond to
short wavelength
How does the size of an animal relate to the frequency
The larger the animal the lower frequencies it can hear
What is an ultrasound
very high frequency sound
Audible intensity frequency range: graph explained
Sound pressure level measures the intensity or loudness of a sound, frequency is pitch or tone,
6 dB increase in audio intensity level
doubling of sound pressure
10 dB increase in audio intensity level
doubling of loudness
Physiology of the auditory system: 3 tiny bones function
malleus
incus
stapes
Transmit the sound from the Tympanic membrane to the inner ear on the other side, amplifying the sound
Physiology of the auditory system: cochlea importance
Cochlea connected to the auditory nerve which is taking the input away from the ear to the brain by the auditory nerve.
where hearing actually happens
Sound wave gets converted to a neural signal for the brain inside the cochlea.
Physiology of the auditory system: outer ear
- Funnels sound toward middle and inner ear
- Enhances frequencies between 2000-6000 Hz
- Protects the tympanic membrane (eardrum)
Physiology of the auditory system: Middle ear
Sound wave is amplified
- Lever action of ossicles increases sound pressure by one third
- Tympanic membrane vibrates the wave
- The malleus moves up and down
- The stapedius is going to bang on the oval window
- Sound pressure has been increased by 1/3
- Surface areas of tympanic membrane is 18 times that of the oval window - lager to a small surface means an increase in pressure
Why sound waves need to be amplified in mid ear
aids sound wave travel through fluid-filled chambers in inner ear - needs more energy to pass to move through the ear effectively
middle ear: Acoustic reflex
- Protects the inner ear from sustained intense sounds
- Tensor tympani and stapedius muscles tense up to prevent ossicle movement
Physiology of the auditory system: inner ear
- Cochlea: Fluid-filled, spiral-shaped structure - looks like a snail shell
- 4 mm coiled up, 10 times longer when uncoiled
- Helicotrema and round window: relive sound pressure when sound is intense
Physiology of the auditory system: inner ear cochlea unrolling
3 canals: Tympanic, vestibular, middle
* Canals separated by two membranes
* Reissner’s membrane
* Basilar membrane: made of stiff fibers,
base of cochlear partition
Physiology of the auditory system: inner ear cochlea unrolling - organ of corti
- Sits atop the basilar membrane
- Made of hair cells and auditory dendrites
- Concerts movements of cochlear partition into neural signals
Physiology of the auditory system: inner ear cochlea unrolling - tectorial membrane
lappy gelatinous membrane on top of hair cells
produces a shearing movement in response to sound - goes back in fourth along the stereocilia, stimulating them causing the hair cells to pivot
Physiology of the auditory system: inner ear cochlea unrolling - stereocilia and tip links
tip links connect to stereocilia, so that hair cells bend together
Bending of stereocilia opens ion channels,
causing depolarization (K+ in, Ca2+ in)
* Auditory nerve fibers stimulated
* Mechanoelectrical transduction
Physiology of the auditory system: inner ear cochlea unrolling - hair cells
- convert stimulus energy to neural energy by the auditory nerve that is connected to them
- The tectorial membrane is stimulating them
- Any damage is irreversible
- One a hair cell dies it can’t regrow
- Fast and very sensitive
- Inner hair cells: 1 row, 3500
- Outer hair cells: 3 rows, 10,500
- The bristles is where the change is happening from mechanical to another
Mechanoelectrical transduction
- Airborne sound (pinna to eardrum)
- Eardrum vibrates (tympanic membrane)
- Lever action of the ossicles (malleus-incus-stapes)
- Oval window vibrates
- Fluid-borne pressure waves move through the vestibular canal
- Middle canal displaced (up and down)
- Basilar membrane displaced, tectorial membrane shearing movement
- Stereocilia of hair cells stimulated
- Tip links open ion channels, K+ in
- Depolarization: neurotransmitter released through hair cell synapses
- Auditory nerve action potentials
- Signal sent to brain
Amplitude encoding in the cochlea: louder sound causes
tympanic membrane & oval window to vibrate farther
larger bulge in vestibular canal
larger movement of cochlear partition (up and down)
more forceful shearing of tectorial membrane
hair cells pivot farther
more neurotransmitter release
auditory nerve fibers fire faster, i.e., higher firing rate
encoding in the cochlea: frequency place code
Stiffer, thicker, narrower at the base (near the oval window)
Flexible, thinner, wider at the apex
Higher frequencies displace the base more, lower frequencies displace the apex
Location at which cochlea is most active
corresponds to frequency of sound
Frequency tuning in the cochlea
Frequency place code and characteristic frequency
Frequency to which an auditory nerve (AN) fiber is most sensitive
AN fibers correspond to specific hair cells and prefer specific frequencies (5-30 AN per hair cell)
Place code & outer hair cells
outer hair cells sharpen frequency tuning
- Inner hair cells (3500) send signals to afferent auditory nerve fibers and brain
- Outer hair cells (10,500):
- receive signals from brain via efferent auditory nerve fibers
- increase stiffness of cochlear partition depending on frequency
- increase the sensitivity of that part of the cochlea
frequency: temporal code
Based on the timing of neural firing
Phase locking: Neuron fires at a distinct point in the period (i.e., cycle) of the wave
If firing rate = 100 times per second, sound wave contains 100 Hz component
Temporal code no good above 1000 Hz because
AN fiber can’t fire more than 1000 times per second
Frequency: temporal code - volley principle
Volley principle: Neurons fire at distinct points in
the period of the wave, but not at every period
Auditory pathway: Auditory nerve fibers
cranial nerve VIII (vestibulocochlear nerve)
Auditory pathway: Cochlear nucleus:
Onset selective, coincidence-of-onset selective, lateral inhibition
contralateral after this point
Auditory pathway: Superior olive
Timing information; first site where input from both ears is combined
Auditory pathway: inferior colliculus
mostly contralateral connections
Auditory pathway: Medial geniculate nucleus
More efferent than afferent fibers (feedback; like LGN)
Auditory pathway: Primary auditory cortex (A1):
Analogous to primary visual cortex (V1)
Auditory pathway: Belt area, parabelt area
Analogous to association cortices, complex sounds
what do all structures in auditory pathway show
tonotopic organization (i.e., frequencies are spatially organized)
Key facts about loudness in relation to frequency
Loudness depends on frequency
All frequencies do not sound equally loud
At a given intensity (dB), some frequencies sound louder than others
What is the equal loudness curve
A graph showing the loudnesses of different
combinations of frequency and intensity
A graph showing those combinations of frequency and intensity that
correspond to a given loudness
perceived loudness in relation to intensity
perceived loudness increases more slowly than intensity
double intensity does not equal double loudness
Hearing loss key point
Usually not total loss, but audibility thresholds increase (e.g., from 20 dB to 40 dB for high frequencies)
Hearing loss: conductive hearing loss
ossicles lose mobility
Hearing loss: conductive hearing loss otitis media
middle ear filled with mucus, ossicles move less, less amplification; thresholds raised by
50 dB; common in children
Hearing loss: Otosclerosis
abnormal growth of ear bones; surgery needed
Hearing loss: Sensorineural hearing loss
Metabolic vs. sensory (cochlear fluid environment vs. hair cell injury)
* Diabetes, ototoxic drugs, viral infection, genetic mutations, noise exposure
* Noise: outer + inner hair cell loss (affects volley principle, high frequencies)
* earphones
* Age: Presbycusis
* Over the age of 20 years, can’t hear over 15,000 Hz
