Next: Sound Pressure, Intensity, and
The human auditory system is reported to be sensitive to frequencies in the range of 20-20,000 Hz. However, there is much variation among individuals and deterioration of high-frequency sensitivity is common with increasing age.
The hearing mechanism (from Rossing, 1990).
- 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.
- 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.
- Acoustic Reflex: Loud noise triggers two sets of muscles; one tightens the eardrum and the other pulls the stirrup away from the oval window.
- 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.
- Hair cell stimulation is transmitted via voltage spikes along auditory nerve fibers.
- Each nerve fiber responds over a certain range of frequencies.
- For single-frequency stimulus tones, the voltage spikes on the nerve almost always correspond to one or two periods of the tone. Although the nerve fiber does not ``fire'' at the peak of every vibration cycle in the basilar membrane, it rarely fires at any other time.
Critical bandwidth as a function of the critical band center frequency (from Rossing, 1990).
- 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.
- 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''.