1. Hearing. (Perception of Sound)
We use four characteristics to describe how we perceive sound:
pitch, loudness, tone quality (timbre), duration
Pitch: how “high” or “low” we perceive a sound to be.
(Directly related to how we perceive frequency.)
Combinations of notes that are “pleasing” to the ear have frequencies that are related by a simple whole-number ratio (Pythagoras)
2) Loudness: how we perceive the amplitude of the sound wave.
The unit of sound loudness is the decibel (dB)
Quiet: 30 dB; Moderate: 50 dB; Noisy: 70 dB; Very loud: 90 dB; Problems: 120 dB
3) Tone quality: how we distinguish sounds of the same pitch and loudness (how we perceive the qualities of the waveform)
4) Duration

2. Range of hearing. (some remarkable facts)
Human hearing
Human vision
frequency range ( f )
20Hz – 20kHz
4x1014 Hz – 7.5x1014 Hz
(7 million different colors)
ratio
103 (>9 octaves)
7.5/4 (<1 “octave”)
intensity range
10-12 W/m2 - 1W/m2
1012

3. Structure of the ear
Inner ear
Outer ear
Middle
ear

4. Structure of the ear Outer ear Middle ear Inner ear Auricle (pinna)
(hammer anvil stirrup)
(eardrum)
Auditory canal

5. Sections of the human ear
Outer ear:
Collects and channels sound to middle ear
Middle ear
Transform energy from sound wave into internal vibrations of bone
structure then to a compressional wave passed to inner ear
Inner ear
Transforms compressional wave to nerve impulses
Outer Ear
Ear flap (Pinna):
Protects the middle ear from damage
Helps to collect the sound
Determines source of sound
Is sensitive to high-frequency sounds
Auditory canal (meatus):
Acts as a pipe resonator
Amplifies sound in range of 2-5 kHz

6. Middle ear Ear drum (tympanic membrane)
Is comprised of circular and radial fibers
Is kept taut by the tensor tympani muscle
Changes the pressure variations of incoming sound waves into mechanical vibrations
Three tiny, interconnected bones (ossicles):
Hammer (malleus – club like)
Anvil (incus – tooth like)
Stirrup (stapes)
The Eustachian tube
Connects the middle ear to the oral cavity
Equalizes pressure outside and inside middle ear during rapid pressure changes

7. Ossicles Acoustic reflex Positioned in air filled cavity
Change a small pressure exerted by the ear drum into a great pressure (up to 30 times) on the oval window of the inner ear
The lever action of the bones contributes a factor of 1.5
The factor of 20 comes from the difference in the window area of the ear drum and the oval window
Acoustic reflex
Ossicles protect the inner ear from very loud noises and sudden pressure changes
Loud noises trigger two sets of muscles: one tightens the eardrum and the other pulls the stirrup away from the oval window

8. Inner ear Semi-circular canals Semi-circular canals Cochlea
Auditory nerve
Semi-circular canals
Fluid filled, no role in hearing
The body's horizontal and vertical detectors
Detect accelerated movement and help in maintaining balance

9. Cross section of the cochlea

10. Cochlea Snail shaped stretched to 3cm
Fluid filled (a fraction of a drop) and surrounded by rigid bony walls
Comprises three chambers:
scala vestibuli (field with perilymph – similar to spinal fluid)
scala tympani (field with perilymph – similar to spinal fluid)
coclear duct (field with endolymph – similar to fluid within cells)
Chambers are separated by two membranes:
Reissner’s membrane
basilar membrane
Organ Corti
Positioned on basilar membrane
Contains rows of tiny hair cells attached to nerve fibers
A single row of inner hair cells contains about 4000 cells
There are about 12,000 outer hair cells in several rows
The inner hair cells are mainly responsible for transmitting signals to the auditory nerve fibers
The outer hair cells are biological amplifiers
Each hair cell has many hairs (stereocilia), which bend when the basilar membrane responds to sound
In response to a sound hair cells push against the tectoral membrane, selectively amplifying the vibration of the basilar membrane

11. Basilar membrane Positioned inside cochlear
Separates two liquid-field tubes (scala vistibuli and scala tympani)
Is a base for the sensory sells (several thousand sells)
Performs frequency dispersion of sound (frequency analysis)
High frequencies lead to maximum vibrations at the basal end of the cochlear coil, where the membrane is narrow and stiff
Low frequencies lead to maximum vibrations at the apical end of the cochlear coil, where the membrane is wider and more compliant
“Each part of the basilar membrane, together with the surrounding fluid, can therefore be thought of as a "mass-spring" system with different resonant properties: high stiffness and low mass, hence high resonant frequencies at the near end, and low stiffness and high mass, hence low resonant frequencies, at the far end.”

12. Cochlear implant
(bionic ear)

13. Signal processing in the auditory system
In the peripheral auditory system (ears themselves)
In the auditory nervous system (brain)
In ears the acoustic pressure signal is transformed into mechanical vibrations pattern on the basilar membrane and then this pattern is represented by a series electric pulses transmitted by the auditory nerve.
Some sounds are heard through vibrations of the skull that reach the inner ear such as humming or clicking one’s teeth
Hearing the voice is from two pathways, hence a recording of one’s voice always sounds “unnatural”
Binaural hearing and localization
At high frequencies (above 4000 Hz) localization is due to intensity difference at two ears. (High frequency – low wavelength – shadow effect.)
At low frequencies (below 1000 Hz) localization is due to time difference between sound traveling to two ears. (Due to diffraction at low frequency there is no shadow.)
Measuring sensations: psychophysics
Fechner law:
As stimuli are increased by multiplication sensation increase by addition
(logarithmic scale)