PSYCHOACOUSTICS: Binaural hearing, sound localization and lateralization

Question 1

what are the different dimensions to describe sound location?

Answer 1

Vertical plane (elevation) - helps us determine whether a sound is coming from above, below, or at ear level.

Horizontal plane (azimuth) - helps us determine whether a sound is coming from the left or right side.

Distance of sound - estimate the distance of a sound source.

Question 2

What are some mechanisms used to localise sound?

Answer 2

Several mechanisms contribute to sound localization:
- Interaural Intensity Difference (IID),
- Interaural Timing Difference (ITD),
- Pinna effects.

Question 3

Are Interaural Intensity Difference (IID) and Interaural Level Difference (ILD) the same thing?

Answer 3

yes

Question 4

What is the role of Interaural Timing in sound localization?

Answer 4

Interaural Timing refers to the difference in the time it takes for a sound to reach each ear. This timing difference helps the brain determine the direction of a sound source.

Question 5

What is the role of Interaural Intensity in sound localization?

Answer 5

Interaural Intensity refers to the difference in sound intensity between the ears. When a sound arrives at both ears simultaneously with equal intensity, the brain interprets this as the sound source being directly in front or behind the listener.

Question 6

How does sound localization vary with different elevation positions?

Answer 6

Sound at the same azimuth but different elevation positions reaches the pinna, creating different spectra.

The distinct feature lies in the variation of spectral notches with varied elevation positions.

Question 6

What is the process for obtaining head-related transfer functions (HRTFs)?

Answer 6

Head-related transfer functions (HRTFs) are obtained by recording the response in each of the two ear canals in response to a broadband click emitted from various spatial locations.

This recording is typically conducted in an anechoic chamber to minimize external influences and ensure accurate measurements.

Question 7

How do pinna effects contribute to sound localization?

Answer 7

• Pinna effects involve the role of the external ear (pinna) in collecting and directing sound waves toward the ear canal.
• When sound comes from the front, it first interacts with the concha bowl and ear canal, while sound from behind interacts with the back of the pinna in the helix area and lobe.
• These interactions help the brain discern the direction of sound sources.

Question 8

How does sound at the same elevation level but different horizontal location affect the ears?

Answer 8

It creates differences in both time and level between the ears.

Question 9

How are head-related transfer functions (HRTFs) utilised?

Answer 9

• HRTFs help create realistic virtual sound environments by simulating how sounds are perceived from different directions and distances.
• They are applied to audio signals to accurately mimic listener experiences in various applications like virtual reality, gaming, and audio engineering.

Question 10

How does the brain interpret sound arriving at the ears at different times and intensities?

Answer 10

It perceives the sound source as originating from one side of the listener.

This is due to the interaural time difference (ITD) and interaural level difference (ILD), where sound arriving from the side reaches each ear at different times and with different intensities.

Additionally, pinna effects further contribute to localization by causing sound to interact differently with the ear anatomy, enhancing the brain’s ability to determine the direction of the sound source.

Question 11

How does interaural time difference (ITD) contribute to sound localization, particularly at low frequencies?

Answer 11

At low frequencies, ITD plays a significant role in sound localization.

The time delay between when sound reaches each ear becomes more pronounced at lower frequencies, making it easier for the brain to determine the direction of the sound source in the horizontal plane.

Question 11

What are the limitations of interaural time difference (ITD) in localizing high-frequency sounds?

Answer 11

ITD is less reliable for high-frequency sounds because humans struggle to perceive time delays between the ears at higher frequencies, making it difficult to accurately localize sound sources.

0.6 ms (sound coming from the side at 90°).
frequencies greater than about 1500 Hz.

Question 12

What is the head shadow effect in sound localization?

Answer 12

The head shadow effect occurs when a sound source is on one side, causing the head to obstruct the sound wave from reaching the ear farthest away.

While low-frequency sounds can diffract around the head, high-frequency sounds are more susceptible to attenuation, resulting in a reduced level at the farther ear.

This effect can be mitigated by turning the head or body toward the sound source.

Question 13

which works best at high frequencies, IID’s or ITD’s?

Answer 13

• IID’s work best at high frequencies, this is because they have short wavelengths so they may not be able to make it all the way round to the ear furthest away
• ITD’s better for low frequencies.

Question 14

Why do we use the Duplex theory?

Answer 14

, there is overlap as most sounds are a combination of high, low and mid freq’s so we use both systems – Duplex theory

Question 15

What is the “cone of confusion” in sound localization?

Answer 15

The cone of confusion refers to a set of points in space, shaped like a cone, that radiate out from the midpoint between a person’s ears.

Within this cone, the phase delays and level differences perceived by the listener are identical, making it challenging to accurately localize sound sources.

Question 16

How does the cone of confusion affect sound localization?

Answer 16

Since the perceived interaural intensity differences (IID) and interaural timing differences (ITD) are the same within the cone of confusion, the accuracy and precision of locating sound sources decrease.

This can lead to confusion for the listener, especially when trying to determine the direction of a sound within this cone.

Question 17

Why do human ears hear better on a horizontal plane?

Answer 17

Human ears evolved to hear better horizontally because most communication and potential threats occur at that level.

• This is because usually we are spoken to from a level similar to our own head height

Question 18

What role do post-auricular muscles (PAM) play in sound localization?

Answer 18

Post-auricular muscles attached to the pinna help animals orient their ears towards the direction of sound, aiding in sound localization.

Question 18

What are the main contributors to horizontal and vertical hearing?

Answer 18

In the horizontal plane, ITD and IID are key. In the vertical plane, pinna effects are dominant.

Question 19

How does the outer ear contribute to sound collection?

Answer 19

The outer ear collects sound waves and guides them to the eardrum.

Question 20

Do humans possess post-auricular muscles (PAM)?

Answer 20

Yes, humans have post-auricular muscles, but unlike in some animals, we have evolved not to use them for sound localization.

Question 21

Can humans voluntarily control these PAM muscles?

Answer 21

Most people cannot voluntarily control or contract these muscles, although some individuals can perform minor movements, like waggling their ears.

However, the expression “pricking up one’s ears” suggests attentiveness or alertness, even though humans do not actively use these muscles for sound localization.

Question 22

whats the difference between localisation and lateralisation?

Answer 22

Localisation = out there in environment

Lateralisation = where it is heard internally e.g. left ear, right ear or heard centrally (it sound like from within your head, not in the external space). This can be made artificially by creating time delay and intensity difference between the two ears.

Question 23

what is the doppler effect?

Answer 23

Definition: The apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer

e.g. An ambulance- As the siren is moving towards the listener, waves become closer together (higher frequency)

As the siren is moving away from the listener, waves become further apart (lower frequency)

Question 24

How do bats work out the distance of the sound source i.e. how far away the sound is?

Answer 24

• Bats and dolphins emit sounds to locate food.
• They determine location and distance by measuring the time it takes for the emitted sound to reflect back to them.
• This method also helps them assess the size and density of objects.

Question 25

How do we determine the distance of a sound source?

Answer 25

Humans perceive quieter sounds as further away due to intensity spreading over distance (inverse square law), potential loss of spectral density, and sound interference during propagation.

Echoes and intensity fluctuations help gauge distance, with more echoes and smoother sounds suggesting greater distance. The direct-to-reverberant ratio (DRR) aids in distance estimation by comparing direct and reverberant sound levels.

Question 26

whats the benefit of listening with 2 ears vs 1?

Answer 26

• Binaural summation (less gain/effort required)
• Improved ability to pick speech out of noise (better SNR)
• Improved frequency selectivity
• Better discrimination and processing
• Aiding both ears also reduces auditory deprivation
• Improved localisation and better awareness of distance of sound (uses ITD, IID’s of both ears)

Question 27

How does listening with two ears compared to one affect threshold improvement?

Answer 27

Binaural summation
+3 dB improvement at threshold.
+6 dB improvement at 50 dB HL.
+9 dB improvement at 90 dB HL.