Chapter 10 - Sound and the Ears

Question 1

How are sounds created?

Answer 1

Via the vibration of objects in the environment

• compression of air into a smaller amount of space increases air pressure
• changes in air pressure are important in a sound wave

Question 2

What is rarefaction?

Answer 2

Decrease in air pressure

Question 3

What is compression?

Answer 3

Increase in air pressure

Question 4

What is a sine wave?

Answer 4

The cyclical change in pressure generated by creating a vibration

Question 5

How can we describe a pure tone?

Answer 5

It contains a single sine wave

Question 6

What happens when multiple sine waves are superimposed on each other?

Answer 6

It creates a complex tone

Question 7

What is the frequency of a pure tone the physical dimension related to?

Answer 7

Pitch

• highness or lowness of a sound
• measured in Hz
  
  • cycles per second (ex. 200 Hz = 200 cycles per second)
• higher Hz = higher pitch

Question 8

What is the normal hearing range of humans?

Answer 8

20-20,000 Hz

Question 9

What is the amplitude of a pure tone the physical dimension related to?

Answer 9

Loudness

- measured in dB

Question 10

What is the baseline atmospheric pressure, as well as the minimum humans can perceive?

Answer 10

0 dB(?)

Question 11

What is the threshold of pain?

Answer 11

130 dB
- mechanisms of protection to loud stimuli exist, but too much exposure or prolonged exposure can lead to permanent hearing loss/damage

Question 12

Is absolute threshold linear?

Answer 12

No; it varies as a function of frequency

Question 13

What is the peak sensitivity in human hearing attributed to?

Answer 13

Human speech

Question 14

What are some ways we can measure hearing perception?

Answer 14

1. absolute thresholds - press a button when you can hear the sound
2. equal loudness contours - indicate when two sounds are perceived to be of equal loudness

Question 15

Why are sounds in our environment more difficult to describe?

Answer 15

they are not pure tones, therefore they have no clear amplitude or frequency
- however, we can break these down into simple sine waves using Fourier analysis

Question 16

What does Fourier analysis do?

Answer 16

Breaks down complex sound waves into simple sine waves (these all sum together to create complex sound waves)

• a mathematical procedure that describes how any signal can be decomposed into component sine waves of different frequencies
• the broken down components are called sinusoids

Question 17

What is the fundamental frequency?

Answer 17

The lowest frequency component of the complex waveform

- determines the perceived pitch

Question 18

What are harmonics?

Answer 18

Frequencies that are multiples f the fundamental

- ex. 1st harmonic = 1x fundamental, 2nd = 2x fundamental

Question 19

What is the most complex sound we will encounter?

Answer 19

A square wave

Question 20

What is a square wave?

Answer 20

The sum of infinite sinusoids with odd-integer harmonics

Question 21

What is timbre?

Answer 21

The difference in sound quality between two sounds with the same pitch and loudness

• for complex sounds, timbre is mainly due to differences in the relative amplitudes of the sounds’ harmonics
• ex. playing an a on the violin and on the flute sound different even though they are playing the same note
• timbre refers to the shape of the Fourier spectrum (how the complex sounds are broken down)

Question 22

What are the three main sections of the ear?

Answer 22

1. outer -
   - consists of the pinna and ear canal
   - funnels sound waves into the auditory canal (causes amplification as it moves into a narrower space) and protects the organs that encode for sound (mainly the tympanic membrane, or eardrum)
2. middle ear -
   - consists of the tympanic membrane and the osicles
   - vibrate in response to changes in air pressure
3. inner ear -
   - consists of the cochlea, auditory nerve and Eustachian tube

Question 23

What do the ossicles do?

Answer 23

They transmit sound energy from the tympanic membrane to the cochlea

Question 24

Name the ossicles

Answer 24

1. malleus
2. incus
3. stapes (this one presses against the oval window of the cochlea)

Question 25

How do the ossicles work together to amplify sound?

Answer 25

The malleus moves forwards and backwards and has a hinge connected to the incus, moving that as well (this serves to amplify the sound like a bike changing gears) and the incus is attached to the stapes by a lever, further increasing the sound

Question 26

What does fluid in the inner ear serve to do?

Answer 26

It changes the properties of the sound waves (reducing amplitude by 30 dB) so the pressure needs to be increased before reaching the ear

Question 27

What is one protective mechanism to prevent hearing loss in the ear?

Answer 27

Due to two small muscles (tensor tympani and stapedius) which reduce the amplification in the middle ear by contracting (but this takes about 200ms), keeping the ossicles from moving freely

Question 28

Describe the cochlea

Answer 28

• it is a coiled, tapered tube within the temporal bone of the head
• partitioned into three chambers
  1. the vestibular canal
  2. the tympanic canal
  3. the cochlear duct

Question 29

How does the cochlea perform a kind of Fourier analysis when filtering sound?

Answer 29

The movement of the oval window causes different areas of the cochlea to move

• the base of the cochlea needs high frequencies to vibrate, the middle needs middle frequencies and the base needs low frequencies
• three physical properties of sound are all encoded by the cochlea

Question 30

What is the basilar membrane responsible for?

Answer 30

frequency selectivity

Question 31

How does the ear encode vibration?

Answer 31

Using the organ of corti (the retina of the ear)

• it transduces the vibration of air into neural responses
• the hair cells move because of the vibrations of air - this causes action potentials to fire (the higher the amplitude, the more APs fire)
• 4 rows of hair cells

Question 32

What are the different types of hair cells?

Answer 32

a) 1 row of inner hair cells = afferent - type 1 nerve fibres
- transduce pressure energy into electrical signals
b) 3 rows of outer hair cells = efferent - type 2 nerve fibres
- serve to amplify and sharpen the response of inner hair cells (like horizontal cells in the eye)

Question 33

How do the inner hair cells work?

Answer 33

As sound comes in, the basilar membrane goes up, pushing the hair cells onto the tectorial membrane (the sheer force is what is being detected by the hair cells)

• each hair cell is linked by tiplinks, which, when pulled, open up ion channels (increasing the membrane potential)
• which hair cells are disturbed will encode the frequency (ex. near the entrance of the cochlea encodes higher frequency)
• number of APs determines amplitude

Question 34

What do inner hair cells synapse with?

Answer 34

afferent type 1 nerve fibers

- carries information about frequency and amplitude of incoming sounds towards the brain

Question 35

What are the outer hair cells responsible for?

Answer 35

motile responses

- they magnify the movement of the basilar membrane in regions with characteristic frequencies responding to the sound