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When things happen to objects they emit vibration energy and different objects different types of sound energy. Why?
- Because physical objects differ in their mass and elasticity.
- The mass of the object and stiffness of spring determine the frequency of vibration.
- Vibration frequencies are cues to the physical properties of objects.
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What happens?
1. Change displacement...
2. Higher stiffness...
3. Higher mass...
- 1. same frequency, but different amplitutde
- 2. higher stiffness --> higher frequency
- 3. Higher mass --> lower frequency
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What wave provides a good description of the vibration behaviour of many objects?
Sine wave
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What 3 parameters are required to describe any sine wave?
- Amplitude
- Frequency (measured in Hertz/Hz - number of times per second)
- Phase
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A single sine wave produces a...
pure tone
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In reality, when most objects vibrate, they
produce vibrations at many frequencies.
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Real objects vibrate at a __ __ (__) and at ___ of that frequency.
- Fundamental resonant frequency (F0)
- harmonics (modes)
- eg. 2xF0, 3xF0, 4xF0
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Two instruments can play the same note but have different sounds. What is this quality of sound called?
Timbre (or texture)
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Real objects can vibrate in two or three dimensions each at fundamental and higher frquencies. This complex pattern of vibrations is expressed in a...
Amplitude Spectrum of Frequencies
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What two things are responsible for the sound something makes (tibre)?
- Amplitude spectrum of frequencies
- damping (how fast a sound dies away)
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Briefly describe how sound travels through a medium.
- Vibration causes local increase in concentration of molecules (air pressure) in one place (condensation) and reduces it (rarefaction) elsewhere.
- Molecules from high-pressure region move to low-pressure region. Causes a travelling wave.
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In an open space, sound energy disperses out. The same energy is therefore spread over a greater space the further you go out. What is the law related to this?
- Inverse square law
- Amplitude of vibration is decreased in proportion to the square of the distance
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When sound is propagated through air, normal vibrations of air molecules will reduce the __ of the wave. This affects __ frequencies much more than __ frequencies. Example: __
- coherence
- high
- low
- Eg. Thunder --> near it you can hear the 'crack' but further away you hear the rumble.
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Because of the huge range of amplitudes the human ear can hear, we use a ___ scale to express a particular sound presure level relative to the __ __ we can hear. This is typically measured in __.
- logorithmic scale (log1=0, log10=1, log100=2)
- lowest pressure (usually 20 micropascal)
- decibles
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To standardise the scale, we usually use __ __ as the lowest pressure we can hear. We therefore call it __ __.
- 20 micropascals
- dB SPL (SPL = standard pressure level)
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What are the three sections of the ear and the main structures within them?
- Outer: Pinna, Meatus (auditory canal)
- Middle: Tympanic membrane (ear drum), the ossicular chain (malleus, incus, stapes)
- Inner: choclea, semicircular canals
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Describe what happens in the outer ear.
- Vibrations channeld by pinna and meatus to the ear drum.
- Air pressure difference between the two sides of membrane cause it to move in and out with sound.
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Describe the structure of middle ear.
- Ear drum is attached to the malleus,
- which is attached to the incus,
- which is attached to the stapes,
- which interfaces with the oval window of cochlea.
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What is the phrase given to the functionality of the ossicular chain? Describe how it works.
- Impedance matching devise
- Needed because cochlea is filled with fluid (perilymph) that has much higher impedance than air. (ie need more energy for sound to energy to pass through)
- So... 3 main functionalities:
- 1. ear drum area much bigger than face of the stapes, meaning ossicles concentrate same force over much smaller area
- 2. Hinge-like ossicular chain provides some leverage to further amplify the movement of the stapes.
- 3. Muscles interfacing with ossicles allow a reflex to partially disengage the stapes during loud sounds (the acoustic reflex). But this reflex is slow so it won't protect your ears from sudden loud sounds.
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Describe the structure of the cochlea.
- Embedded in the temporal bone
- spiral of 2.5 turns and length of 3.5cm
- 3 compartments:
- scala vestibuli
- scala tympani (at bottom)
- scala media
- Scala vestibuli and tympani interconnect at apex at the helicotrema and are filled with perilymph.
- Scala media is filled with endolymph which is rich in K+ ions.
- Basilar membrane sits in-between scala vestibuli and tympani.
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Describe how the cochlea works up until the basilar membrane.
- Sapes moves --> presure moves perilymph inside cochlea to move --> causes basilar membrane to move.
- Perilymph does not compress so makes round window bulge outwards as oval window is deflected inwards.
- Different frequencies cause maximal vibration of membrane at different locations (tonotopy).
- Low freq - near apex; High freq - near base
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Why do different frequencies cause different levels of movement at different locations in the basilar membrane?
- 1. Stiffness of BM changes along its length (stiff at base)
- 2. Perilymph has inertia - harder to produce movement for high frequencies --> high freq movements are attenuated as distance increases away from oval window
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How does the cochlea work? Explain what happens after BM moves.
- BM makes Organ of Corti move, which is in scala media
- Shearing motion between organ of corti and the tectorial membrane
- Moves stereocilia of auditory hair cells on organ of corti pointing up towards scala vestibuli
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After stereocilia of auditory hair cells move, what next?
- Stereocilia deflect
- tip links open ion channels
- K+ (from scala media) enter cell, depolarising it
- causes glutamate release
- triggers action potential in spiral ganglion cells
- Long axons from ganglion cells travel through auditory nerve (the VIIIth cranial nerve) to cochlea nucleus of brainstem
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How are the vibrations still tranduced into an analogue electrical signal?
- Because deflection of sterocilia changes extent of hair cell depolarisation by changing number of open ion channels
- this analogue depolarisation is transmitted to brain via discrete action potentials in ganglion cells.
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