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

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

Structure of the ear Inner ear Outer ear Middle ear

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

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

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

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

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

Cross section of the cochlea

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

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.” http://en.wikipedia.org/wiki/Basilar_membrane

Cochlear implant (bionic ear)

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)