Topic 5 - Auditory System Flashcards
Sound Definitions
Physical definition - sound is created by pressure changes in the air or other medium
Perceptual definition - sound is the experience we have when we hear
Sound waves
Pattern of air pressure changes
How speakers work
- The diaphragm of the speaker moves out, pushing air molecules together called condensation
- The diaphragm also moves in, pulling the air molecules apart called rarefaction
- The cycle of this process creates alternating high- and low-pressure regions that travel through the air
Pure tone
created by a sine wave (think whistle or high flute notes)
Frequency
number of cycles per second that the pressure changes repeat
Measured in Hertz (Hz) - 1 Hz is 1 cycle per second
Amplitude
the size of the pressure changes - difference in pressure between high and low peaks of wave
- Perception of amplitude is the loudness of a noise
- Measured in Decibel (dB)
dB = 20 x logarithm (p/p0)
Complex periodic sounds
Both pure and some complex tones are periodic tones
Complex Tones
not even - made up of a number of pure tone components added together
Harmonics
Harmonic - the pure tone components
First harmonic - a pure tone with frequency equal to fundamental frequency
Higher harmonics - pure tones with frequencies that are whole-number multiples of the fundamental frequency
Fundamental Frequency
the repetition rate that is 200 times per second, and is called the first harmonic
Frequency spectra
see notes for diagram
Additive synthesis
process of adding harmonics to create complex sounds
Perceptual Aspects
Loudness
Audibility curve
Pitch
Timbre
Pitch
Missing fundamental
Tone height - increasing pitch that accompanies increases in a tone’s fundamental frequency
Tome chroma - notes with the same letter (on a piano) (also octaves)
Effect of the missing fundamental - pitch remains the same when fundamental or other harmonics are removed, but timbre changes (periodicity pitch)
Aperiodic sounds
sound waves that do not repeat (for example, slamming a door)
Range of hearing
Human hearing range = 20-20,000 Hz
humans are most sensitive to 2,000 to 4,000 Hz
The outer ear
Pinnae
Auditory Canal
Middle Ear
Tympanic membrane (eardrum) Ossicles - three smallest bones in the body Malleus (hammer) Incus (anvil) Stapes (stirrup) Oval window Round window Middle-ear muscles
Inner ear
Cochlea which includes: Scala vestibuli - upper half Scala tympani - lower half Separated by the cochlear partition Organ of Corti - contains hair cells Basilar membrane and tectorial membrane Cilia One row of inner hair cells (3500), about three rows of outer hair cells (12,000)
Cilia
Processes that protrude from the tops of the hair cells
- Movement of hair cilia in one direction opens ion channels (by stretching tip links) in the hair cell, which results in the release of neurotransmitter onto an auditory nerve fibre
- Movement in the opposite direction closes the ion channels so there is no ion flow and no transmitter release
Phase locking
property of firing at the same place in the sound stimulus
Frequency
There are two ways nerve fibres signal frequency:
Which fibres are responding - Specific groups of hair cells on basilar membrane activate a specific set of nerve fibres
How fibres are firing - Rate or pattern of firing of nerve impulses
Bekesy’s Place Theory of Hearing
Frequency of sound is indicated by the place on the organ of Corti that has the highest firing rate
- Saw the basilar membrane’s vibration as a travelling wave
*see notes for more
Amplitude-modulated noise - Burns and Viemesiter
amplitude-modulated noise - level of noise was changed so loudness of noise fluctuated rapidly up and down
Phase locking - Wever’s Volley Theory
Nerve fibres fire in bursts – a ‘volley’
- Firing bursts happen at or near the peak of the sine-wave stimulus
- Thus, they are “locked in phase” with the frequency of the stimulating tone wave (aka fire in time with)
Place coding
effective for the entire range of hearing, Temporal/periodicity coding with phase locking is effective up to 4,000 Hz
- Both effect for frequencies below 4000 Hz
Harmonics (resolved and unresolved)
Lower harmonics can be distinguished by a peak - resolved harmonics
Higher harmonics create a smooth function - unresolved harmonics
Path from Cochlea to Cortex
- Cochlear nucleus
- Superior olivary nucleus (in the brain stem)
- Inferior colliculus (in the midbrain)
- Medial geniculate nucleus (in the thalamus)
- Auditory receiving area (A1, primary auditory cortex)
Activity in the cortex
- Other cortical auditory areas - the core area, belt area and parabelt area
- Neural signals travel through the core, then belt, followed by the parabelt area
- Simple sounds cause activation in the core area
- Belt and parabelt areas are activated in response to more complex stimuli made up of many frequencies
Tonotopic maps - findings
Neurons that respond better to low frequencies are on the left and those that respond best to high frequencies are on the right
Research on stroke patients and marmosets provided support for the link between perception and physiological response in A1
Hearing loss
Caused by damage to outer hair cells, inner hair cells, auditory nerve fibres
Presbycusis - hair cell damage from cumulative effects over time of noise exposure, ingestion of drugs and age-related degeneration
Auditory space
surrounds an observer and exists wherever there is sound
On average, people can localize sounds directly in front of them most accurately, and to the sides and behind their heads least accurately
Localisation by coordinates
- Azimuth coordinates - position left to right
- Elevation coordinates - position up and down
- Distance coordinates - position from observer
Binaural cues
location cues based on the comparison of the signals received by the left and right ears
Interaural disparity
Processing of interaural disparity cues occurs in the superior olivary complex (SOC) - first site of binaural interaction
Interaural time difference (ITD)
difference between the times sounds reach the two ears
Interaural level difference (ILD)
difference in sound pressure level reaching the two ears
- Reduction in intensity occurs for high frequency sounds for the far ear
- The head casts an acoustic shadow
- This doesn’t occur for low-frequency sounds
Cone of confusion
ILD and ITD are not effective for judgments on elevation since in many locations they may be zero
Monoaural cues
Single ear cues
- primary cue for localisation is Spectral Cues
Pinna Cues (shape of ear)
Reflections of high frequencies from the convolutions of the pinnae produce elevation and front-back dependent spectral transformations in the sound
- only cues for vertical sound localisation
- remember experiment with the ear mould (see notes for more)
Physiology of Auditory localisation
Two mechanisms have been proposed for source localisation:
Narrowly tuned ITD neurons and Broadly-tuned ITD neurons
Narrow:
- coincidence detectors - fire only when signals arrive from both ears simultaneously
Hearing inside rooms
- Direct sound - sound that reaches the listener’s ears straight from the source
- Indirect sound - sound that is reflected off of environmental surfaces and then to the listener
- When a listener is outside, most sound is direct; however inside a building, there is direct and indirect sound
Precedence effect - when sound appears to originate from the source that reaches our ears first (speaker and microphone)
Reverberation time - the time it takes for the sound to decrease to 1/1000th of its original pressure
Hearing in concert halls
Intimacy time - time between when sound leaves its source and when the first reflection arrives - best time around 20 ms
Bass ratio - ratio of low to middle frequencies reflected from surfaces - high bass ratios are best
Spaciousness factor - fraction of all the sound received by listener that is indirect - high spaciousness factors are best
Hearing in classrooms
Ideal reverberation time in classrooms is .4 to .6 second for small classrooms, 1.0 to 1.5 seconds for auditoriums
These maximize ability to hear voices, Most classrooms have times of one second or more
Background noise is also problematic - Signal to noise ratio should be +10 to +15 dB or more
Auditory Scene
the array of all sound sources in the environment
Auditory scene analysis
process by which sound sources in the auditory scene are separated into belonging to individual perceptions/streams
Heuristics that help organise stimuli (auditory grouping)
- Onset time - sounds that start at different times are likely to come from different sources
- Location - a single sound source tends to come from one location and to move continuously
- Similarity of timbre and pitch - similar sounds are grouped together
- Proximity in time - sounds that occur in rapid succession usually come from the same source
- Auditory continuity - sounds that stay constant or change smoothly are usually from the same source
- Effect of past experience - Melody schema - representation of a familiar melody stored in a person’s memory
Auditory Stream segregation
Compound melodic line in music is an example of auditory stream segregation
Experiment by Bregman and Campbell with the musical melodies
Good Continuation
Experiment by Warren et al.
Tones were presented interrupted by gaps of silence or by noise
- In the silence condition, listeners perceived that the sound stopped during the gaps
- In the noise condition, the perception was that the sound continued behind the noise
