How does hearing loss initially manifest?
- Difficult to hear in noisy environment
- May require to lip read
- Gets worse as you get older
What is sound?
Vibrations of objects --> which set up pressure waves in the surrounding air
Elastic property of air --> allows these pressure waves to propagate (spread)
What is the shape of sound?
What are the 2 parts of a sound wave?
- Sine Wave
- Compression & Rarefaction
During pure tones, what is frequency related to?
- Frequency of the tone is directly related to the pitch of the sound
Amplitude is related to perceived loudness
If a sound has a particular frequency. It is a ....
Pure Tone Sound
What is the relationship between frequency & pitch + amplitude & loudness?
- Logarithmic Relationships
Double in Frequency --> Increases Pitch by 1 Octave
Double in Amplitude --> Increase Loudness by 6dB
What is a property of sounds with a clear pitch?
Frequencies are related
- They are all multiples of a fundamental frequency
What are sounds that have all integer multiples of a 'fundamental frequency' called?
What does the sound sound like?
- Harmonically Related
- Sounds like a sound with a clear pitch
(e.g. lowest common denominator)
What makes up complex sounds?
- Frequency components (i.e. different sine waves)
What dies a Fourier analysis help us do?
- Plot the amplitude (& phase) for each frequency component of a sound
- This gives us a sound spectrum
- Giving us a compact representation for signals that contain oscillations
How do you do a fourier analysis?
What is the graph you get called?
- Convert PRESSURE as a function of TIME
- to AMPLITUDE as a function of DIFFERENT FREQUENCIES
- Sound Spectrum
What is the advantage of using a Fourier Analysis?
Gives you a compact respresentation of signals that contains oscillations
(graphs contain lots of complicated mini-waves --> thus it helps you to not have to track sound every second --> you can present sound as different frequencies with the amount of each frequency)
What is meant by narrowband?
- Sound contains energy at a narrow range of frequencies
(Pure tone is an extreme example of a narrowband)
These sounds are periodic and may evoke an identifiable pitch
What is meant by broadband sounds?
- Contain energy at a lot of different frequencies
- e.g. noises & clicks (natural broadband sounds)
What does a spectrogram do?
Divides sounds up into short-time segments --> then calculates a spectra for each time segment in turn
- This helps you track changes of sounds over time (as a sound spectrum only gives you it in one split moment)
Ear effectively converts sound into a spectrogram
What are the 3 dimensions of a spectrogram and how are they represented?
- Time (x-axis)
- Frequency (y-axis)
- Energy (colour)
What does the outer ear consist of?
- Pinna (odd bumps and folds)
- Ear Canal
Which part of the ear is considered to be unique to each person (like a finger print)?
- Pinna (bumps and folds on it)
Which part of the external ear changes the nature of sound in a subtle way?
What is the function of a pinna?
Collect all the sound that hits it
Directs the sound into the ear canal (thus acts to amplify sound)
How is the pinna involved in sound localisation?
- Sound hits the ear --> bounces around in a complex way
- Depending on where it came from --> sound bounces in a different way
(Causing subtle changes to nature of the sound)
How is the outer ear involved in localisation?
- Incoming sounds --> are filtered by outer ear --> in a way that depends upon location of the sound source in the vertical plane (elevation)
- It is also important in horizontal plane (but only if you have hearing loss in one ear)
What happens to the frequency if a sound is presented to you at a higher inclinication level?
- Higher Frequency
What happens to the frequency if a sound is presented to you at a lower inclinication level?
- Lower Frequency
What would happen if you had no pinna?
- Would not be able to localise sound above, below, infront or behind you
Name the 2 main functions of the outer ear.
Sound Amplification (transforms acoustic energy at the tympanic membrane - pinna transfer function)
Sound Localisation (pinna transfers function depending upon sound source position in vertical plane)
Collects & funnels sounds to eardrum but does so differently depending on sound source location
Name the 3 bones in the middle ear.
Why is 99.9% of sound reflected from cochlea?
What is the conseuqence of this?
- Due to impedence of fluid in the cochlea
30dB sound loss due to impedence mismatch between air & fluid
By how much do the middle ear bones overcome the loss of sound?
What are the 3 methods used to undergo impedence matching?
- Reduce Area
- Increase Force
- Buclking of Ear Drum
How does the middle ear reduce area?
By how much does this increase sound pressure?
- Area ratio of the ear drum : stapes footplate (20:1)
(tympanic membrane is much bigger than the footplate of the stapes --> so focused onto a smaller area)
How does the middle ear increase force?
By how much does this increase sound pressure?
- Lever action of the ossicles (1.3:1)
What is the third method that causes the increase in sound pressure?
By how much does this increase sound pressure?
- Buckling of the Ear drum
- 2x increase in pressure
When the ear drum vibrates it also buckles (complex) --> this is equivalent to a secondary lever effect --> causing an increase in pressure
How does the middle ear amplify different frequencies?
- It does not amplify all frequencies to the same level
At what frequency does it amplify well?
1 kilohertz (around level of human speech)
- However higher & lower frequencies --> it is not as good
Which muscle in which reflex is activated to protect hearing?
- Stapedius Muscle
- Contracts in the Middle Ear Reflex
Contract --> oscilles unable to move (amplification not allowed)
Relax --> oscilles able to move (amplification allowed)
The reflex controls the amount of amplification
How does the middle ear enhance sound?
- Enhances sound transmitted to inner ear
- Done in a controlled & frequency-dependent way
What are the dimensions of the cochlea?
- 1cm wide
- 5mm from base to apex
- Snail-shaped (coiled)
How does the cochlea connect to the middle ear?
- Via the footplate of the stapes --> which rests against the oval window
After the fluid in the cochlea moves --> what moves?
- Round window
Name the 3 layers in the cochlea. Where are they in relation to each other?
- Scala Vestibuli (top)
- Scala Media (middle)
- Scala Tympani (bottom)
Where do hair cells sit?
Down what do they send signals?
What do they respond to?
- Sit on the basilar membrane
- Respond to movements of fluid (created by sound vibration)
- Send signals down the auditory nerve (cranial nerve VIII)
What is found in the 3 layers of the cochlea?
Scala Vestibuli & Scala Tympani --> contains PERILYMPH
Scala Media --> contains ENDOLYMPH
What is the composition of endolymph?
- High K+
What is the composition of perilymph?
- Low K+
What maintains the composition of the endolymph?
- Very High K+ Concentration
- Unusual in the body
- Due to stria vascularis
Sits in the lateral wall
- Actively increases K+ concentration of endolymph
What would happen if K+ dropped in endolymph?
- Complete Deafness
- Needs to be maintained at a high level by stria vascularis (lateral wall of scala media)
What is the membranous labrinyth made up of?
What does it comprise of?
- Semi-Circular Canals + Otolithic Organs
- Contains endolymph
Why do syndromes/diseases affecting balance and vestibular apparatus affect hearing?
- Fluid environment of the vestibular apparatus is continuous with that of the cochlea
- Problems usually come together (e.g. mania syndrome)
What do vibrations do to the basilar membrane?
- Causes complex deflections
- Complicated waveform
What determines where the peak/centre of the fixed point on the basilar membrane will be?
Depends on the frequency (stapes will vibrate at a certain frequency causing fluid to vibrate at that frequency causing the peak at a certain point)
What is the wave from the ovale window (stapes) called?
- Travelling Waveform
What deteremines where the wave peaks are on the basilar membrane?
- Sound Frequency
Where is the wave peak during low frequency sounds?
Close to Apex (i.e. centre)
Where is the wave peak during high frequency sounds?
Close to Base (i.e. near ovale window)
What happens if there is a complex sound? (in terms of basilar membrane)
- More than 1 frequency
- Therefore different energies
- Movements of the basilar membrane will be at multiple different points
What is the equation that links stiffness (k), mass (m) & frequency (w)?
w2 = k/m
Where is the basilar membrane most stiff?
Base (near ovale window)
What happens in terms of stiffness from base to apex?
Stiffness decreases as you go from the base to the apex
How does mass change as you go from the base to the apex?
Base --> Little Mass
Apex --> Lots of Mass
- The mass increases as you go towards the apex
Where does low frequency sound produce movement on the basilar membrane?
Apex of the Basilar Membrane
Where does high frequency sound produce movement on the basilar membrane?
Base of the Basilar Membrane
What two properties of an object determine the frequency they are likely to vibrate at?
- Stiffness (rigidity)
What is the mass & rigidity like at the apex?
- Wide & Floppy
Large Mass & Low Rigidity
What is the mass & rigidity like at the base?
- Narrow & Rigid
Small Mass & High Rigidity
What frequency does the base 'like'?
What determines the frequency of the Basilar Membrane vibration?
What is the stiffness & mass like at the base?
- Less Mass
What is the stiffness & mass like at the apex?
- Less Stiff
- More Mass
What is the basilar membrane like?
- Acoustic Prism
- Takes in light --> splits it up into lots of different frequencies
What is the component of the basilar membrane that responds to sound called?
- Organ of Corti
What does the Organ of Corti do?
- Converts movement of the basilar membrane into electrical activity
What does the up-down movement of the basilar membrane cause?
Tectorial Membrane --> to slide sideways over the membrane
- Causes sideways displacement of the hair cell bundles in the cochlear hair cells
(sliding of tectorial membrane to one side relative to basilar membrane pushes hairs in a particular direction (towards depolarisation or hyperpolarisation)
How are the adjacent stereocilia joined up?
How does transduction by the hair cells take place?
Stereocilia of the hair cell bundles connected via tip links
Movement of the bundle changes the tension on the tip links --> causing opening/closing stretch sensitive K+ channels
K+ Channel Opening --> causes influx of K+ from endolymph --> causing depolarisation of the hair cell membrane
- This causes opening of voltage-gated Ca2+ channels --> increasing probability of transmitter release
How does the hair work?
- Hairs vary in length
- Connected to a stretch-sensitive K+ channel
- If they move in one way --> distance between hair either increases/decreases --> causing an increase/decrease in tension in the tip-links --> thus opening/closing K+ channels
What happens if the distance between hair cells decrease, therefore tension in tip-link decreases?
What happens if the distance between hair cells increases, therefore tension in tip-link increases?
- Increase in tension
- Open K+ channels
What happens when K+ channels open?
K+ flood in from endolymph
- This causes voltage-gated Ca2+ channel to open --> causing an increase in intracellular Ca2+
- This causes depolarisation
- Causing NT release (glutamate)
What are the 2 types of hair cells?
What are they both connected to?
- Inner Hair Cells (IHCs)
- Outer Hair Cells (OHCs)
Both connected to Auditory Nerve Fibres
What is the function of the Inner Hair Cells (IHCs)?
- Respond to vibration of the basilar membrane
- Perform a process of mechano-electrical transduction (convert mechanical energy to electrical)
- They release NT onto Auditory Nerve Fibres (electro-chemical transduction) --> transmitting information along auditory nerve
What happens at low frequencies in terms of membrane potential?
Membrane potential of the hair cell --> follows every cycle of the stimulus (AC response)
Fluctuate rapidly (but can keep up)
What happens at high frequencies in terms of membrane potential?
- Membrane potential is unable to follow individual cycles
- Therefore remains depolarised throughout duration of the stimulus (DC response)
Cannot keep up anymore with the rapid fluctuations (around 4000 hertz) --> thus cell will depolarise and not change (frequency too high)
What happens at intermediate frequencies?
- Membrane potential exhibits a mixed AC + DC response
What is the function of Outer Hair Cells (OHCs)?
- Electric-mechanical transduction
- They contract --> feed energy back into the basilar membrane
Electric signals feedback into it to cause mechanical mvoement so that it moves the basilar membrane --> causing positive feedback (IHCs and OHCs propogate each other)
- Takes small signals/vibrations and amplifies them to make movement of BM even bigger
What are the important implication of OHC function?
- Process is responsible for sensitivity of hearing
- Vulnerable to damage by NOISE or OTOTOXIC DRUGS
What do OHCs contain which allows them to change their shape/length when they are depolarised?
What are the OHCs connected to and why is it important?
Connected to Basilar Membrane
- Therefore causes movement of the BM
- This amplifies vibrations of the basilar membrane
Generally, what are the active cochlea's 2 main functions?
- Increase Sensitivity
- Increase Tuning
How do OHCs increase sensitivity & tuning?
Increase gain in the cochlea by boosting vibrations --> this also results in narrow tuning
What happens if OHCs are not working (i.e. just passive sound)?
Most of the Adjacent Basilar Membrane will move (displace) too at a similar level
- Therefore will not be tightly tuned
Not much difference between different points
- Reduced sensitivity
- Reduced tuning
What happens if OHCs are not working? (generally)
- Remove the Tuning
- Less Sensitive
- Need louder sounds for auditory nerve fibres to respond (cause there is no 'help')
Why does a healthy cochlea 'generate sound'?
- Due to active mechanical feedback (electro-mechanical transduction) of the OHCs
How is the theory about OHCs incorporated in baby hearing tests?
OHC amplification system --> technically produces sounds that were not put in at all (produces own sounds)
- Therefore if you feed 2 frequencies --> you should get a third if the OHC is working fine
- If it is not there --> problem with hearing --> need further checks
What are the effects of antibiotics on hearing?
Antibiotics can affect Organ of Corti
- Cause wiping out of the hairs on the hair cells
- Need to do hearing test after to ensure they are okay
What can cause damage to Outer Hair Cells?
Specifically what antibiotic can cause hair damage (on OHCs)?
Why is this AB usually given?
- Often given to children for juandice & other conditions --> but too much causes hair cell destruction
What are the outer & middle ear important for?
Transmitting sound to the inner ear
- In a form compatible with the fluid environment of the inner ear
What is special about the basilar membrane?
- Tuned to sound frequency
Splits complex sounds into constituent components (e.g. acoustic prism)
What are inner hair cells?
- Mechano-electrical transducers
- Convert mechanical deflections into electrical potentials
What are OHCs?