• The Hearing Mechanism
• Critical Bands
• Binaural Hearing and Localization

Hearing

The Hearing Mechanism

Figure 14: The hearing mechanism (from Rossing, 1990).

1. The Outer Ear:
   
   
   • Pinna and auditory canal.
   • Auditory canal can be analyzed (to first approximation) as a open-closed pipe.
   • Both the pinna and auditory canal impose filtering characteristics based on unique shapes.

   
   
   

2. The Middle Ear:
   
   
   • Eardrum and ossicles.
   • Ossicles consist of three bones shaped like a hammer, anvil, and stirrup (not really, but that is the convention everyone uses).
   • Ossicles act as a mechanical transformer, converting pressure on the eardrum to pressure on the oval window with a step-up factor of up to 30.
   • Most of the step-up factor is due to area differences (eardrum vs. oval window) ... about 20 times.
   • Lever action of the ossicles accounts for another 1.5 factor.

3. The Inner Ear:
   
   
   • Semicircular canals control balance.
   • Cochlea transforms pressure variations to neural impulses.
   • Approximately 30,000 hair cells along basilar membrane, which itself is about 37 millimeters long.
   • Each hair cell has many cilia or hairs which bend with the vibrations of the basilar membrane.
   • High-frequency detection occurs along rigid section of the basilar membrane closest to the oval window.
   • Low-frequency detection (greatest low-frequency amplitude ripples) occurs near far end of the basilar membrane.
   • Bone conduction to inner ear is also significant.
   • Auditory nerve fibers are ``tuned'' to different center frequencies.

Critical Bands

• Regions of amplitude envelope ``distinction'' along basilar membrane.
• Determined by psychoacoustic experiments, their values typically vary depending on the type of response being measured.
• About 24 critical bands along basilar membrane.
• Each critical band is about 1.3 mm long and embraces about 1300 neurons.

Binaural Hearing and Localization

• IID: Interaural intensity difference is dominant for frequencies greater than about 4000 Hz.
• ITD: Interaural time difference is dominant for frequencies less than about 1000 Hz.
• HRTFs: Head-related Tranfer Functions are commonly used in 3D reproduction systems.
• Need a separate HRTF for each ear and for each position. Active research to find ways to interpolate and compress the data.
• ``The Precedence Effect''.