Lecture 5 - Loudness and pitch

Question 1

What is the perceptual definition of sound?

Answer 1

• sound is the experience we have when we hear

Question 2

What is the physical definition of sound?

Answer 2

• Sound is pressure changes in the air or other medium caused by the vibration of an object, these reduce in strength as they spread wider which is why we can’t hear things as well when we are further away.

Question 3

When does a pure tone occur?

Answer 3

When changes in air pressure form a perfect sinusoidal wave, increases and decreases in pressure move away from the source through space.

Question 4

What is amplitude?

Answer 4

• The difference between the increases and decreases in pressure.
  -Size and variation in air pressure (i.e. difference between peak and trough).
  -Related to perception of loudness.

Question 5

What is frequency?

Answer 5

• How many cycles of increases and decreases in pressure travel past a point in space per second.
  -Number of cycles per second (1 Hertz = 1 cycle/s).
  -Related to perception of pitch.
  -Higher frequency is a shorter wavelength.

Question 6

What are complex sounds?

Answer 6

• Sounds made up of lots of different frequencies mixed together.
• They are all made up of different frequency and different amplitude pure tones.

Question 7

What are the 3 sub-division of the ear?

Answer 7

Outer, middle and inner.

Question 8

What are the features of the outer ear?

Answer 8

• Pinnae
• Auditory canal
• Tympanic membrane (eardrum)

Question 9

What is the pinnae?

Answer 9

• Visible external parts of the ear.
• Capture sound waves and direct them down our auditory canal.

Question 10

What is the auditory canal?

Answer 10

-3cm tube-like structure.
-Protects middle ear from external things that could cause damage.

Question 11

What is the tympanic membrane (eardrum)?

Answer 11

-Cone-shaped membrane separating the outer and middle ear.
-Sound waves induce a different pressure either side of the tympanic membrane, causing it to vibrate.
-Larger amplitude sounds result in larger vibrations in the tympanic membrane.
-Higher frequency sounds result in faster vibrations in the tympanic membrane.

Question 12

What are the features of the middle ear?

Answer 12

• The ossicles: malleus, incus and stapes.

Question 13

What are the ossicles?

Answer 13

-A small cavity that contains the ossicles: malleus (hammer), incus (anvil) and stapes (stirrup).
-They vibrate in a way that amplifies the amount of vibration and sending it to the inner ear.
-The bones amplify the vibrations of the tympanic membrane and transmit them to the inner ear of the oval window.

Question 14

What are the features of the inner ear?

Answer 14

• The cochlea

Question 15

What is the cochlea?

Answer 15

-Vibration of the oval window displaces fluid in the cochlea, resulting in a change in pressure which propagates up and down the spiral structure.
-Cochlea consists of three parallel canals (vestibular, middle and tympanic).
-Auditory transduction is triggered by the motion of the basilar membrane, which separate the middle and tympanic canals.

Question 16

What is auditory transduction?

Answer 16

-Motion of the basilar membrane are translated into neural signals by structures in the Organ of Corti.
-A voltage Is generated when specialised hair cells contained within the corti are bent (as the basilar membrane moves the hair cells become bent, triggering a response which is sent to the optic nerve).
-This produces impulses in auditory nerve cells which are sent from the optic nerve to the brain.
-Hair cells are extremely sensitive.
-Overstimulation by loud sounds can damage hair cells and lead to hearing loss.

Question 17

How do we perceive loudness?

Answer 17

• Perception of loudness is related to the amplitude of sound waves.
• Unit is decibels (dB) which is a measure of the amplitude of the sound using a logarithmic scale.
  -A change of 20dB corresponds to ten-fold increase in amplitude.
  -Rate code: sound amplitude is coded in the firing rate of auditory nerve fibres.
  -Responses increase with increasing sound intensity.
  -Some fibres have high spontaneous rates and saturate rapidly, while others have low spontaneous rates and saturate slowly.
  -The larger the amplitude of the sound, the faster the nerve fibres will respond.
  -This enables us to discriminate loudness across a range of sound levels.
  -Loudness depends on amplitude (but the two are not directly proportional)
  -For a sound to be perceived as twice as loud, its amplitude needs to be increased by a factor of approximately 3.16 (10dB).
  -Our auditory systems are not equally sensitive to all sound frequencies.

Question 18

How do we perceive pitch?

Answer 18

-The lowest frequency humans can hear is 20Hz, highest is 20,000Hz.
-Place code – sound frequencies cause vibration in specific areas along the basilar membrane.
-Lower frequencies = near apex.
-High frequencies = near base.
-Timing code - Frequency is not only signalled by which auditory nerve fibers respond, but also when they respond
-Nerve responses are synchronised to changes in pressure.
-This is called phase-locking and occurs up to 4000Hz.
-The number, frequency ratios and relative amplitudes of the harmonics dictates the quality or timbre of the sound.