Week 7- The Audition System Flashcards
What are the two key reasons why sound is important?
-Communication
-To locate and identify objects
Both of these things are key to survival!
What is the stimulus for audition?
-Acoustic energy (mechanical displacement of molecules in a medium caused by changing pressure)
-Object moves displaces molecules around it and these have an effect on nearby molecules creating a ripple effect
Can sound travel in a vacuum?
-No, there must be some sort of medium (some array of molecules) that sound can disrupt in order for sound to travel
-Can travel through gas (air), liquids and solids (unique from other sensory modalities in this)
What are sound waves?
Visual representations of acoustic energy
What are the two key features of sound waves?
-Amplitude (Displacement from baseline: y axis, large disturbance in air molecules= large amplitude)
-Frequency (Is the distance between crests on x axis. Shorter time period means a higher frequency)
What two perceptual features of sound does amplitude and frequency correspond to?
-Amplitude= loudness (measured in decibels)
-Frequency = pitch (measured in hertz)
Human range for amplitude and frequency…
-We can perceive loudness across a huge range of amplitudes
-We can perceive pitch across a very specific range of frequencies (20-20,000 Hz)
Does the acoustic range for humans remain static across the lifespan?
-No, changes for both amplitude (loudness) and frequency (pitch)
-For example, extremely high pitches (greater than 17,000 Hz) are usually only perceptible by people under 25. This lead to the creation of the “sonic teenager deterrent” (blaring tone at this frequency to disperse crowds at parties)
-Diminished sensitivity to higher frequencies is considered a normal part of ageing
What is an audiogram?
A graph of the absolute threshold of hearing across frequencies
In other words what is the minimum decibel/ loudness you can present a tone of a certain pitch to someone and they will perceive it.
Individual variation in hearing
-Sensitivity differs across frequencies (as shown in audiograms) not just among the elderly but also in younger individuals
Fletcher & Munson (1933) - equal loudness contours:
-Present pure tones of various intensities
-Start at 1000 Hz and increase/ decrease frequency
-Ask participant to adjust a reference tone (always at the same frequency:1000Hz) until loudness matches the test tone (Fechner’s method of adjustment)
-Lowest equal-loudness contour represents quietest audible tone (absolute threshold for hearing)
-Highest equal-loudness contour represents pain threshold
When does the process of hearing start?
-When acoustic energy (sound waves) reaches your outer ear: pinna or auricle
Important features of the outer ear/ functions
-Shapes to collect sound and funnel into the ear canal
-Ear canal amplifies sound
-Sound then reaches ear drum (aka tympanic membrane)
What happens when acoustic energy reaches the tympanic membrane?
Vibrates:
-vibrations match frequency of incoming sound wave
-vibrations channeled through 3 ossicles (bones of the middle ear)
What are the three ossicles: how are they connected?
-Malleus (hammer) is connected to the tympanic membrane: movement of membrane moves malleus
-The malleus articulates with the incus: movement of the malleus moves the incus
-The incus articulates with the stapes: movement of the incus moves the stapes
What is the stapes connected to? What does movement of the stapes cause?
-The oval window of the cochlear
-The stapes strikes the oval window of the cochlea, sends vibrations through the fluid inside the cochlea
Cochlea
-A snail shapes, fluid filled chamber in the inner ear
-If unfurled would be 34mm long
-Two canals if the cochlea- the scala vestibuli and scala tympani. These are separated by the cochlea duct.
What is inside the cochlea duct?
-Organ of corti
-This is home of many hair cells that sit on the basilar membrane
What causes the basilar membrane to bend, what also subsequently bends?
-Sound driven vibration travels as a wave along the basilar membrane, causing it to bend
-And then so do the hairs on hair cells
What happens when the cochlear hair cells bend?
-Cochlear hair cells are mechanoreceptors- when hairs (cilia) on the cells bend, the cells fire (signal transduction)
-The synapse onto spiral ganglion cells, whose axons are part of the cochlear nerve (aka auditory branch of CNVIII)
-Signals can make there way to the brain
Why can’t sound waves directly just act on the cochlear?
-Need many steps for amplification
-Sound waves travel through air to reach the air, however, the cochlear is filled with fluid. Initiated pressure disturbances in liquid requirements more energy we therefore, need to amplify the signal before it reaches the cochlear as much as possible!
How is amplification achieved?
-The tympanic membrane is a lot bigger than the oval window (20x the SA). Because the oval window is a lot smaller it means force can be concentrated onto the cochlear giving the best chance of disrupting the fluid and setting of the change of events leading to signal transduction.
-Additionally ossicles work to transfer force applied to the tympanic window to the oval window
Acoustic reflex
-An involuntary contraction of muscles attached to the stapes and malleus
-Happens in response to intense sound stimuli (anywhere from 70-100db) depending on the individual
-Muscle contractions serve to reduce movement of the ossicles so they can’t transmit force efficiently to the cochlea
-Decreases transmission of pressure to the cochlea (15-30dB)
Note: this isn’t all that much and it takes 10-100ms to initiate the reflex so certainly doesn’t provide full protection against loud noise. Can’t protect against a sudden intense noise e.g. gunshot
What needs to happen for amplification to occur?
-Air pressure needs to be equal either side of the tympanic membrane
-This happens because the eustachian tube allows outside air from the throat to the middle ear
-Mechanism goes wrong if the eustachian tube gets blocked (infection, tumour) and results in hearing loss due to lack of amplification and intense pain
What happens if the tympanic membrane is damaged?
-Known as having a burst eardrum
-Hearing loss: no longer have equal pressure on both sides so can’t get amplification
-pain