3.2 Psychoacoustic Phenomena: Masking, Localization, and Binaural Hearing

Psychoacoustic phenomena shape how we perceive sound. Masking occurs when one sound makes another harder to hear, while localization helps us pinpoint sound sources. These processes rely on complex interactions between our ears and brain.

Binaural hearing, using both ears, enhances our ability to locate sounds and understand speech in noisy environments. It's crucial for navigating our auditory world and helps us make sense of complex soundscapes.

Masking

Types of Auditory Masking

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• Auditory masking occurs when the presence of one sound makes it more difficult to perceive another sound
• Temporal masking happens when a loud sound masks a softer sound that occurs shortly before (backward masking) or after (forward masking) the louder sound
  • For example, a loud clap can mask a softer sound like a whisper that occurs immediately before or after the clap
• Frequency masking occurs when a sound at one frequency masks a sound at a nearby frequency
  • This can happen when a loud, low-frequency sound (bass) masks a softer, higher-frequency sound (treble)

Factors Affecting Masking

• The level of masking depends on the relative loudness of the masking sound and the masked sound
  • A louder masking sound will have a greater masking effect on a softer sound
• The frequency content of the sounds also plays a role in masking
  • Sounds with similar frequency content are more likely to mask each other than sounds with different frequency content
• The temporal proximity of the sounds affects masking
  • Sounds that occur closer together in time are more likely to mask each other than sounds that are further apart

Sound Localization

Mechanisms of Sound Localization

• Sound localization is the ability to determine the direction and distance of a sound source
• Interaural time difference (ITD) is the difference in arrival time of a sound at the two ears
  • ITD helps localize low-frequency sounds below about 1.5 kHz
• Interaural level difference (ILD) is the difference in sound pressure level between the two ears
  • ILD helps localize high-frequency sounds above about 1.5 kHz
• Head-related transfer function (HRTF) describes how the shape of the head, pinnae, and torso affect the sound reaching the ears
  • HRTFs provide additional cues for sound localization, particularly for elevation and front-back discrimination

Factors Affecting Sound Localization

• The distance between the ears (head size) affects ITD and ILD cues
  • Larger head sizes lead to greater ITDs and ILDs, improving localization accuracy
• The frequency content of the sound influences which localization cues are most effective
  • Low frequencies rely more on ITD, while high frequencies rely more on ILD and HRTF cues
• The presence of reflections and reverberation in the environment can make localization more challenging
  • Reflections can create confusing or conflicting localization cues

Binaural Hearing

Benefits of Binaural Hearing

• Binaural hearing refers to the use of both ears to perceive sound
• Binaural hearing improves sound localization compared to monaural (one ear) hearing
  • The brain compares the differences in timing and level between the two ears to determine sound direction
• Binaural hearing also enhances the ability to separate sounds in a complex acoustic environment (cocktail party effect)
  • The brain can focus on a desired sound source and suppress competing sounds

Binaural Phenomena

• The precedence effect occurs when two similar sounds arrive from different directions with a short time delay
  • The brain perceives the sound as coming from the direction of the first-arriving sound, suppressing the later sound
  • This helps maintain localization accuracy in reverberant environments
• The cocktail party effect is the ability to focus on a single talker among multiple competing talkers
  • Binaural hearing helps the brain separate the desired speech from the background noise
  • Spatial separation between the target speech and competing sounds improves the cocktail party effect