3.1 Auditory System and Sound Perception

Our ears are incredible sound detectors. From the outer ear to the inner ear, each part plays a crucial role in capturing and processing sound waves. The brain then takes over, turning these signals into the rich auditory world we experience.

Sound perception goes beyond just hearing. Our brains interpret pitch, loudness, and timbre, allowing us to differentiate between voices and instruments. We can even focus on specific sounds in noisy environments, thanks to our brain's amazing processing abilities.

Anatomy of the Ear

Outer Ear Structure and Function

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• Pinna collects and funnels sound waves into the ear canal (auricle)
• Ear canal amplifies sound waves and directs them toward the eardrum (external auditory meatus)
• Eardrum vibrates in response to sound waves, converting them into mechanical energy (tympanic membrane)

Middle Ear Structure and Function

• Ossicles are three tiny bones that transmit vibrations from the eardrum to the inner ear (malleus, incus, stapes)
• Eustachian tube equalizes pressure between the middle ear and the throat, preventing damage to the eardrum
• Tensor tympani and stapedius muscles contract reflexively to protect the ear from loud sounds

Inner Ear Structure and Function

• Vestibule contains the organs of balance (utricle and saccule)
• Semicircular canals detect rotational movement of the head
• Cochlea is a spiral-shaped, fluid-filled structure that converts mechanical energy into electrical signals
• Organ of Corti sits on the basilar membrane within the cochlea and contains hair cells that respond to specific frequencies
• Auditory nerve transmits electrical signals from the hair cells to the brain for processing

Sound Perception in the Brain

Auditory Cortex Processing

• Primary auditory cortex is located in the temporal lobe and processes basic features of sound (Heschl's gyrus)
• Secondary auditory cortex further processes and interprets sounds, such as speech and music
• Auditory association areas integrate auditory information with other sensory inputs and memories

Pitch and Loudness Perception

• Pitch is the perceived frequency of a sound, determined by the rate of vibration of the sound source
• Place theory suggests that different regions of the basilar membrane respond to specific frequencies, allowing for pitch discrimination
• Loudness is the perceived intensity of a sound, determined by the amplitude of the sound waves
• Stevens' power law states that the perceived loudness of a sound is proportional to the physical intensity raised to a power (usually around 0.6)

Timbre Perception

• Timbre is the quality of a sound that distinguishes it from other sounds of the same pitch and loudness
• Determined by the harmonic content and envelope of the sound wave
• Allows us to differentiate between different musical instruments or voices, even when playing the same note at the same volume

Auditory Processing

Frequency Range and Sensitivity

• Human hearing range spans from approximately 20 Hz to 20 kHz, with maximum sensitivity around 2-5 kHz
• Threshold of hearing is the minimum sound intensity that can be detected at a given frequency (0 dB SPL at 1 kHz)
• Threshold of pain is the sound intensity at which sound becomes uncomfortably loud (around 120 dB SPL)
• Equal-loudness contours (Fletcher-Munson curves) show how the perceived loudness of a sound varies with frequency at different intensities

Auditory Scene Analysis

• Process by which the brain separates and groups sounds from different sources in a complex auditory environment
• Simultaneous grouping occurs when sounds with similar characteristics (e.g., frequency, timbre) are perceived as coming from the same source
• Sequential grouping occurs when sounds that are close together in time are perceived as part of the same auditory stream
• Auditory masking occurs when one sound makes it difficult to hear another sound that is present at the same time (simultaneous masking) or immediately before or after (temporal masking)
• Cocktail party effect demonstrates the brain's ability to focus on a single conversation in a noisy environment by using spatial, spectral, and temporal cues to separate the target speech from background noise