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.
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
What wave provides a good description of the vibration behaviour of many objects?
What 3 parameters are required to describe any sine wave?
- Frequency (measured in Hertz/Hz - number of times per second)
A single sine wave produces a...
In reality, when most objects vibrate, they
produce vibrations at many frequencies.
Real objects vibrate at a __ __ (__) and at ___ of that frequency.
- Fundamental resonant frequency (F0)
- harmonics (modes)
- eg. 2xF0, 3xF0, 4xF0
Two instruments can play the same note but have different sounds. What is this quality of sound called?
Timbre (or texture)
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
What two things are responsible for the sound something makes (tibre)?
- Amplitude spectrum of frequencies
- damping (how fast a sound dies away)
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.
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
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: __
- Eg. Thunder --> near it you can hear the 'crack' but further away you hear the rumble.
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)
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)
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
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.
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.
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.
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.
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
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
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
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
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.