Week 4 - Hearing

Question 1

The nature of sound

Answer 1

• cause by changes in air pressure
• pressure waves characterised by
  amplitude: loudness
  frequency: pitch
  phase: position within a cycle
• pure tone (sine wave) is the simplest sound wave

Question 2

Loudness of common sounds

Answer 2

Space shuttle launch - 180dB
loudest rock band - 160dB
pain threshold - 140dB
loud thunder - 120dB
loudest recorded shout - 111dB
Heavy traffic noise - 100dB
Vacuum cleaner - 80dB
Normal conversation - 60dB
quiet office - 40dB
soft whisper - 20dB
Threshold of hearing - 0dB

Question 3

Inner ear - Cochlea

Answer 3

contains auditory sensory receptors
- oval window (smaller than eardrum to amplify soundwaves)
- membrane covering opening in the cochlea
- stapes attached directly to the oval window
- filled with watery liquid that responds to vibrations from middle ear

Question 3

Complex sounds

Answer 3

• built up from series of sine waves of varying amplitude, frequency and phas
• can decompose complex sounds into their sine waves through a process called Fourier analysis
• lowest frequency is called the fundamental
• made up of harmonics - integer multiples of the fundamental

Question 4

Human hearing range

Answer 4

20-20000 Hz
typical vocal range 80-1100 Hz

Question 5

The outer ear

Answer 5

Pinna:
- increases the sound amplitude
- helps determine the direction from which a sound is coming
External auditory canal:
- provides protection
- increases the sound amplitude
Eardrum:
- vibrates in response to sound waves
- moves bones in the middle ear

Question 5

The middle ear - Ossicles

Answer 5

• Malleus
• Incus
• Stapes
  smallest bones in the human body that transmit the vibration of the eardrum into the cochlea through lever action. They also provide protection against high amplitude sounds

Question 6

Canals in the cochlea

Answer 6

• vestibular canal
• tympanic canal
• cochlear duct
  seperated by Reissner’s and Basilar membrane that vibrate in response to vibrations of the oval window

Question 7

Inner ear - hair cells

Answer 7

Vibrate from the basilar membrane. Converts vibrations into neural signals.
- Basilar membrane is about 30mm long and varies in stiffness and width along its length
- tuned to different ranges of frequency according to the location along the basilar membrane

Question 8

Central Auditory Pathways

Answer 8

• Nerve fibres from each cochlea synapse in a number of sites on the way to the primary auditory cortex
• The signal arriving at the cochlear nucleus splits and goes to each of the superior olivary nuclei

Question 9

Auditory tasks that can be performed without auditory cortex being present?

Answer 9

• the onset of sound
• changes in sound intensity
• changes in sound frequency

Question 10

Auditory tasks that cannot be performed without the cortex?

Answer 10

• discriminating the pattern of several tones
• discriminating the duration of sounds
• localising sounds in space

Question 11

The auditory cortex

Answer 11

deals with complex auditory tasks while lower structures deal with simple aspects of sound

Question 12

Frequency coding

Answer 12

• sounds are made up of a mixture of sine wave components
• the auditory system isolates and identifies the frequencies of these components
• travelling waves move along the basilar membrane and peak at different points depending on the frequency of the sound

Question 13

Binaural pitch encoding

Answer 13

structures beyond the cochlear nucleus should be contributing to pitch perception

Question 14

Human auditory range

Answer 14

0-120dB

Question 15

Mechanisms of loudness perception

Answer 15

• overall firing rates
• range of firing
  more neurons fire when a sound is more intense

Question 16

Factors that effect loudness perception

Answer 16

• sound duration (longer = louder)
• frequency (high frequency sounds are perceived to be louder)
• 3000-5000 Hz are perceived to be the loudest
• as amplitude goes up, frequency becomes smaller

Question 17

Auditory space perception

Answer 17

sound is determined by:
- horizontal direction
- vertical direction
- distance
- vision is more precise when detecting an objects location

Question 18

How do we localise sounds

Answer 18

binaural process
- interaural time difference (onset difference and phase difference)
- interaural intensity difference

Question 19

Interaural time difference

Answer 19

• Unless a sound is directly in front of or behind you, it reaches two ears at different times (onset difference)
• onset difference detected by ‘delay line’ mechanism in the brain

Question 20

Interaural Intensity difference

Answer 20

sound should be more intense in the ear closer to the sound source
- energy of sound decreases as it travel farther
- head is a barrier that reduces intensity of sound (sound shadow), this is more obvious in high-frequency sounds

Question 21

Frequency effects of auditory space perception

Answer 21

• interaural time difference is useful for localising low-frequency sounds
• interaural intensity difference is useful for localising high-frequency sounds
• neither cue works for neutral sounds (1000-3000 Hz)

Question 22

Horizontal vs vertical direction

Answer 22

• horizontal directions better than vertical through auditory cues
• pinnae more effective in distinguishing front/back than above/below
• ear positions are more freely varied along horizontal dimensions (ears are on horizontal plane, head movement better on horizontal dimension)

Question 23

Limits of auditory localisation

Answer 23

• auditory localisation cues are dependent on the distance between a sound source and ears
• difficult to distinguish locations of sounds that are equidistant to an ear (cone of confusion)

Question 24

Auditory distance perception

Answer 24

two cues:
- loudness
- energy ratio of direct and reverberant sound
utility of cues limited to:
- loudness cues tell us only about relative distance
- reverberation cues very depending on various properties of the reflection surface

Question 25

Visual Capture

Answer 25

• when we can visually perceive where a sound should be coming from, it override auditory localisation
• the same happens when we have visual information about how a stimulus should sound (the mcgurk effect)