How the ear works

Three parts of the ear - Outer, middle, inner

Outer Ear - amplifies incoming air vibrations

Middle Ear - transduces these vibrations into mechanical vibrations

Inner Ear - processes the sound via mechanical, hydrodynamic, and electrochemical methods.

Outer Ear Parts

Pinna-outside of ear

Concha-entry into ear

Auditory Canal-passage to the Eardrum (80mm 2 in size)

Middle Ear

Ossicles-(malleus, incus and stapes)- three bones that amplify and moderate sound levels to the Oval Window (3mm 2)

Eustachian Tube-allows to equalize pressure in Middle ear

Inner Ear

Cochlea-coiled organ that houses the Basilar Membrane

-filled with perilymph fluid that transmits sound waves

-Basilar Membrane-vibrates from the sound waves travelling through it. High frequencies are detected at the beginning of Cochlea and low freq at the end.

-Organ of Corti-translates the movement of hair cells that are stimulated by the various frequencies of sound waves into nerve impulses.

Frequency Response - Humans hear from about 20Hz to 20,000Hz.

3. How the brain interprets sound

Musical Pitch - related to frequency with A being 440 Hz, semitones in equal tempered tuning are 6% different (ratio 1.06:1)

Timbre - quality of a pitch is dictated by its harmonic content, frequencies that are multiples of the base frequency.

Sound Localization - the apparent direction or location of a sound is dictated by several factors

-onset of sound-percieved beginning of a sound from a specific direction-occurs in both ears at the same time, when delayed the ear perceives as indirect sound.

-diffraction-high frequency sounds don't diffract around your head due to the wavelengths of sounds, shadows in frequence between the ears occur.

-direct/reflected sound proportion

-Phase differences between the sounds.

-psychological information-airplanes often are above you, etc.

Doppler Shift - a change in pitch that results when the source and the listener are moving relative to each other. A shrinking of the interval between wavefronts as the source moves closer to the listener, and then expanding as it moves away.

Haas effect - a sound that arrives at one ear first dictates the apparent direction of the sound despite a 10dB difference in sound levels of the delayed sound.

The ear is most sensitive to sound at 500-5000 Hz, the frequencies of human speech.

Critical Bandwidth -difference between freq the ear can differentiate as seperate tones without dissonance. When freq. are close to each other it confuses nerve impulses and cause beat frequencies or dissonance. This distance between tones that cause beat freq increases as freq decreases. At 200Hz a 5th will be dissonant.

Difference Tones - f 2-f 1 - when combines two tones at different high freq a resultant lower freq tone is perceived, and cubic difference tones 2f 1-f 2. ex) 700Hz and 1200Hz