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McMaster University
Joe Kim

Audition Introductin Sound = the product of an external stimulus making a sound, but no one would hear it. On the other hand, sound may be the result of our own sensory processing, thus if no one there to hear the tree fall, the tree did not make the sound. Sound Waves and Auditory System The auditory system can translate sound waves from vibrating objects into the psychological experience of audition. The sound waves themselves dont make sound unless an auditory system is present to translate those sound waves into to perceptual experience of audition. The Auditory Mechanisms of Different Species Auditory mechanisms vary across different species according to specific needs Sound Frequency - Dog whistle produces a sound at a high frequency that is beyond the range of human ears but well within the range of the dog's auditory system. Humans can perceive sounds that lie anywhere between 20-20,00 Hz. Wider hearing range: Whales, dolphins, dogs Narrower hearing range: frogs, birds Lower hearing range: fish Higer hearing range: bats and rodents Sound Frequency Perception in Vertebrates: Basilar Membrane Audible frequency range is determined in part by the evolution of the structures of the auditory system. One key structure is the Basilar membrane - contains the hearing receptors; sounds of different frequencies are processed along different areas of the basilar membrane. Characteristics of Basilar Membrane: Varies in length across species; shortest in amphbians and reptiles, longer in birds, and longest in mammals. *Long basilar membrane*: Allows processing of a wider range of frequencies. So mammals can discriminate the widest range of frequencies while most other species cannot discriminate frequencies over 10,1000Hz. The stimulus: Sound Waves Sound travels in waves, (although sound waves travel much slower and require some medium to travel). Sound waves are initiated by either a vibrating object (Ex. our vocal cords or a guitar string, a sudden burst of air, like a clap) or by forcing air past a small cavity like a pipe organ. This causes the air molecules surrounding the source of the sound to move, causing a chain reaction of moving air particles. (Ie. the chain reaction is much like the ripples you observe when you throw a stone in the pond) How Eardrum works: These altering bands of more and less compressed air molecules interact with the eardrum to begin the auditory processing. A bnd of compressed air moleculess causes your eardrum to get pushed slightly inwards, whereas a band of less dense air particles causes the eardrum to move outwards. Sine waves: The changes in air pressure over time that make up a sound wave can be graphed as a sine wave. 3 Phsyical characteristics of the wave = 1. Amplitude - Loudness 2. Wavelength - Pitch 3. Purity - Timbre 1. Amplitude: Measure of Loudness (Intensity?) Variations of amplitude (or height of the sound wave) affect the perception of loudness. Waves of greater amplitude correspond to vibrations of greater intensity -> higher waves correspond to louder sounds. Measured: using a logrithmic scale decibels (dB). The perceieved loudness of a sound doubles for every 10dB increase. Examples: Whisper - 27 db Normal convo - 60 db Concert - 120 db 2. Frequency: Measure of Pitch Variations of frequency/wavelength (or distance between successive peaks) affects pitch. Measured: in Hertz (Hz) which represents the number of cycles per second that a sound wave makes on full cycle from one peak to the next. Example: Low pitch sound has: A low frequency, and long wavelength The audible zone of frequencies that humans can detect represents only a portion of the possible frequencies that can be produced. 3. Timbre: Measure of complexity/purity Purity affects perception of timbre. Simple sounds - consist of only a single frequency of vibration. HOWEVER, Most of the sounds we hear everyday are complex sounds that are composed of multiple sound waves that vary in frequency. Example: Guitar string, vibrates as a whole, its the fundamental tone BUT it also vibrates at shorter segments along the string, called the overtones. Final sound you hear = mix/combination of fundamental tone and all the overtones (aka timbre!!) Piccolo and Bassoon may both play the same note, but bc each instrument produces a unique combination of the fundamental frequency and overtones, they still sound diff to us even though each instrument is producing the same frequency and amplitude. The Ear Structure of the Ear Outer/External ear - Incoming changes in air preassure channelled here Middle ear - Amplified so that it can be detected as changes in fluid pressure by the inner ear Inner ear - These changes in fluid preassure are then finally convered to auditory neural impulses Outer/External Ear: Pinna = folded cone that collects sound of waves in the environment and directs them along the ear canal. Ear canal = Since its narrow as the sound waves move towards the eardrum, itfunctions to amplify the incoming sound waves (like a horn). Ear drum = a thin membrane vibrating at the frequency of the incoming sound wave and forms the back wall of the ear canal. The Middle Ear: Begins on the other side of the eardrum, which connects to the ossicles (ie the 3 smallest bones in the body!). These ossicles consist of: Hammer Anvil Stirrup Ossicles amplify signal sent to the oval window The amplication of the vibrating waves CONTINUES here in the middle ear. The vibrating osiscles are 20x larger than the area of the oval window to which they connect to create a lever system that amplifies the vibrations even more. Needed because the changes in air preassure originally detected by the external ear are about to be convered to sound waves in the fluid inner membrane. The Inner ear: Vibrating oval window connects to the cochlea Cochlea = fluid-filled tube, about 35mm long, coiled like a snail shell. Contains neural tissue that is necessary to transfer the changes in fluid to neural impulses of audition Oval window = small openning in the side of the cochlea, and when the oval window is made to vibrate it causes the fluid inside the cochlea to become displaced. Round window = located at the other end of cochlea, and accomodates for the movement of the fluid by buldging in and out according ~Inside the Cochlea~ Basilar membrane = Flexible, runs the length of the cochlea like a carpet. When its pushed downwards, the fluid inside the cochlea causes the round window to bulge out and when the basilar membrane is forced upwards, round window buildges inwards. Although cochlea itself gets narrower towards the end, the basilar membrane actually gets wider towards the end. Bc the length of the bassilar membrane varies in both flexibility and width, sound of different frequencies cause different region of the membrane to vibrate. Higher frequency sounds cause the end nearest the oval window to vibrate whereas lower frequency sounds cause the end nearest the round window to vibrate. High frequency sounds are decoded on the basilar membrane at a location closest to the oval/round window, while low frequency sounds are coded on the basilar membrane at a location farthest from the oval/round window. ~Inside the Basilar Membrane~ Hair Cells = Auditor
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