Sound Audition Perception of sound works the same way as light. Have receptors to convert a physical stimulus to action potentials Action potentials are organized in brain structures You apply some meaning to the organization. Physics of Sound Is a form of mechanical pressure Is a pattern of pressure disturbances occurring in some molecular medium Medium can be 1. gaseous 2. liquid 3. solid Generally, Molecules collide with other molecules and can be transmitted great distances. Not completely efficient - thus you lose some volume as the distance increases Must have some vibration of a medium. Thus, cannot travel through a vacuum Speed of sound depends on the medium Travels faster in a denser or more elastic medium where molecules are close together Thus speed of sound in water = 1360 m/sec air = 345 m/sec also changes with temperature Sound travels much slower than electromagnetic waves Properties of sound Sound is conceptualized in waves Has several physical and psychological properties 1
Amplitude Refers to the extent of displacement of vibrating particles in either direction from the position of rest. Psychological property is loudness Measured in Decibels Frequency The number of cycles or pressure changes/sec one peak to the next Unit measured = Hertz (Hz) 1 Hz = 1 Cycle per second Psychological Perception = Sound Pitch Pitch Range = 20-20000 Hz Below 20 = vibration Above = heard in kids (May be painful) and in critters such as dogs, bats, fish 2
Structures of the Ear The ear is divided into three areas Outer Ear Middle Ear Inner Ear Outer Ear Structures Pinna Channels or directs sound waves into the ear Has ridges to help focus different frequencies into the ear In humans does not move much In some animals can move a lot Auditory Canal Is basically a tube Has a dip near the eardrum Can fill with water when swimming Can also fill with wax and other things Never put anything in your ear smaller than your thumb Middle Ear Has several structures Tympanic Membrane Ossicles Malleus Hammer Incus Anvil Stapes Stirrup Eustachian Tube Ear Drum 3
Tympanic Membrane Is a thin clear membrane Vibrates in synchrony with the sound pressure and sound waves. Ossicles Are the smallest bones in the body Are designed to amplify the pressure of the tympanic membrane to the oval window of the cochlea. Are attached by small muscles With loud noises, ossicles can break or tear muscles. Eustachian Tube Connects the middle ear to the throat Is designed to keep air pressure on each side of the eardrum the same Does not do this fast Changes in altitude will cause differences in pressure Can be painful especially in babies or when you have a cold Chew gum on takeoff Pop ears when landing Key is to open the tube Inner Ear Structures Has several structures Cochlea Like a snail Auditory Nerve Semi Circular Canals 4
Cochlea Has two membranes oval and round window Contains three tubes Two are filled with fluid is a very complex structure Oval Window Is where the Stapes attaches Is a membrane Passes on vibrations from the eardrum Vibration causes motion in the fluid within the cochlea Vestibular Canal Starts at the oval window and connects with the lower canal Tympanic canal on the lower side has the round window At the tip small opening called the helicotrema Both are filled with fluid On both sides of the cochlear duct are membranes Top Reissner s membrane Bottom Basilar membrane Appears to differentially displace in response to frequencies of sound. Organ of Corti Extends and rests on the Basilar Membrane Contains hair cells Inner hair cells 3500 Outer hair cells 20,000 Each hair cell contains about 100 cilia Hair cells are attached to tectorial membrane 5
Tectorial Membrane is attached at one end and partially crosses the cochlear duct When basilar membrane is moved hair cells are bent Causes action potentials Theoriesof Audition Place Theory Frequency Theory Volley Theory 6