Hearing (Foundations and Spatial Hearing)

Question 1

Outer Ear

Answer 1

• Auricle - collect sounds into ear canal (amplification)
• Auditory canal
• Pinna protects entrance to canal
• Meatus is an external canal which amplifies sound, particularly higher frequencies through resonance

Question 2

Middle Ear

Answer 2

• The Ossicles (hammer, anvil, stirrup) = small bones which change the air movement to mechanical movement
• Eardrum = vibrates with the pressure of sound waves

Question 3

Inner Ear

Answer 3

Most complex, transforms eardrum vibration into oscillations of liquid by vibrating ‘oval window’

• Cochlear = transformation from mechanical vibrations to electric nerve impulses
• Auditory nerve = interprets signals in brain
• Eustachian tube/canals = balances pressure

Question 4

Cochlea Parts

Answer 4

Cochlea partition in the middle of two parts:

• Basilar/basal membrane, start tuned to high frequencies (not linear scale)
• Reissner’s/apical membrane, close to apex, skinnier, tuned to low frequencies

Question 5

Range of Human Hearing

Answer 5

20Hz - 20,000Hz
0dB - 140dB

Question 6

Action of Cochlear

Answer 6

• Organ of Corti sits on the basilar membrane and transforms mechanical movements into electrochemical pulses (causes chemical reactions) by bending hair cell
• Equivalent to a bank of band-pass frequencies which decrease the range of frequencies
• Responds to high frequency sounds faster than low as they travel faster

Question 7

Loudness

Answer 7

• Subjective, depends on frequency and intensity
• Absolute hearing threshold varies with frequency (minimum audible field)

Question 8

Equal Loudness Contours

Answer 8

• Graphs mapping sound pressure levels (dB SPL) across the frequency spectrum (20Hz–20kHz) that are perceived as having constant loudness by the human ear
• Adjust the intensity of a sound at one frequency until it is at the same loudness as a sound at a different frequency
• Do this for different frequency combinations

Question 9

Phons

Answer 9

Unit of perceived loudness

1kHz at ndB is n phons (reference frequency)

Question 10

Critical Bands and Loudness

Answer 10

• Perceived loudness doesn’t change until critical bandwidth (of complex, not pure, sounds)
• i.e. if spread across more frequencies it is louder (excites neighbouring bands)
• Two sounds with the same energy, one with larger frequency range sounds LOUDER

Question 11

Interaural Time Difference (ITD)

Answer 11

• Sound wave has to
  travel longer way “around the head”
• Azimuth is rotation of the head, starting with facing the source of sound
• Symmetrical front to back
• Also happens e.g. with 2 microphones

Question 12

Interaural Level Difference (ILD)

Answer 12

• Shadowing/reflection of parts of the wave front
• Amplitude is smaller in the ear further away, unique to mammals with pinna (doesn’t happen e.g. between two microphones)
• High frequency sounds (above 2kHz) are more likely to be obstructed by head, low frequencies less affected

Question 13

Monaural Spectral Cues

Answer 13

• Your head and outer ear shape the sound differently depending on direction (frequency filtering)
• One ear can figure out direction

Question 14

ITDs and ILDs Across Frequencies

Answer 14

• ITDs are more useful in low frequencies (below 2 kHz)
• ILDs are more robust in high frequencies (above 2 kHz)

Question 15

How to Calculate ITDs

Answer 15

Cross correlation function across both ears, matching high and low frequencies

Question 16

Anechoic Chamber

Answer 16

• Very dry, no reverberation
• Clean plot (clusters) between ILD and ITD, can be used to separate two sound sources
• In reverberant conditions, they overlap a lot more