Hearing: The Nature of Sound

Sound Sound, like light, comes in waves as Sound is vibration Acoustical transduction: Conversion of sound waves into neural impulses in the hair cells of the inner ear Features of sound include: Pitch: sound’s highness or lowness, which depends on the frequency of the sound wave Hertz: measure of the number of sound wave peaks per second. Hertz determines the pitch of a sound Decibels: measure of the height of a sound wave, which determines the loudness of a sound

Two theories of hearing Place theory- different frequencies of sound waves are said to vibrate different places on the cochlea. These places are wired to different parts of the auditory cortex in the brain so that the sound can be processed correctly Frequency theory- the entire cochlea is believed to vibrate at a particular frequency, thus sending the signal of the quality of sound to the brain 100 Hz

Amplitude: The physical strength of a wave-the “volume” of the sound Frequency & Amplitude Frequency: The number of cycles completed by a wave in a given amount of time-determines pitch Amplitude: The physical strength of a wave-the “volume” of the sound

Hearing: The Structure of the Auditory System

Divisions of the Ear Ear’s structure can be divided into: Outer Ear: Pinna. Collects sounds Middle Ear: Chamber between eardrum and cochlea containing three tiny bones (hammer, anvil, stirrup) that concentrate the vibrations of the eardrum on the cochlea’s oval window Inner Ear: Innermost part of the ear, containing the cochlea, semicircular canals, and vestibular sacs

Cochlea A hearing organ where sound waves are changed into neural impulses The major organ of hearing Filled with fluid; a snail shaped body tube

Auditory Canal When your best friend shouts your name across campus, what happens? First, obviously, the vibrations that constitute the sound must travel through the air from your friend’s mouth to your ear. When the sound waves reach your ear, they travel down your auditory canal. The opening through which sound waves travel as they move into the ear for processing Ends at the tympanic membrane (eardrum)

Tympanic Membrane (eardrum) The tissue barrier that transfers sound vibration from the air to the tine bones of the middle ear Can be damaged by objects in the ear or exceptionally loud noises

Ossicles Three tiny bones that transfer sound waves from the eardrum to the cochlea Hammer, anvil and stirrup

Oval Window The point on the surface of the cochlea which receives the sound vibration from the ossicles As the oval window vibrates, the fluid in the cochlea vibrates

Similar to the rods and cones within the eye Hair Cells The receptor cells for hearing in the cochlea that change sound vibrations into neural impulses Similar to the rods and cones within the eye

Auditory Nerve The nerve that carries sound information from the ears to the temporal lobes of the brain

Semicircular Canals Organs in the inner ear used in sensing body orientation and balance (vestibular sense) Relies on fluid in the canals Spinning in circles disrupts the fluid

The Process of Hearing 1.) The middle ear transmits the eardrum’s vibrations through a “piston” made of 3 small bones (the hammer, anvil and stirrup) to the cochlea (snail shaped tube) 2.) The incoming vibrations cause the cochlea’s membrane (oval window) to vibrate, moving the fluid that fills the tube. This motion causes ripples in the basilar membrane (hair cells) 3.) The movement of cells triggers impulses in the adjacent nerve fibers which from the auditory nerve that connects via the thalamus to the temporal lobe

Hearing: Sound Localization

Localization of Sound Locating where sound is originating from Done through two cues: Which ear hears the sound first? (sound travels through the air at about 750 mph) Which ear hears the louder sound? By the time a sound gets around to the ear farther from the source, the sound is muted enough for your brain to be aware of the difference.

Hearing Deficits Older people tend to hear low frequencies well but suffer hearing loss when listening for high frequencies

Deafness Conduction deafness- involves a blockage of the sound as it travels through the ear. Earwax, foreign objects, and damage to the eardrum or ossicles are the main causes of this kind of deafness You can replace the bones or get a hearing aid to help Nerve (sensorineural) deafness- involves damage to the auditory nerve. Illness, genetic determination, loudness, or traumatic head injuries NO WAY to replace the hairs Cochlea implant is possible ---Cochlear implants are electronic devices that enable the brain to hear sounds