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3.7 - Audition.pdf

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McMaster University
Joe Kim

Arnav Agarwal 2011 Audition Module 1: Introduction If A Tree Falls In The Forest… - What is sound? o Product of an external stimulus (eg: fallen tree)?  Tree falling in forest makes a sound regardless of who is around to hear it o Result of sensory processing  Tree falling in forest doesn’t make a sound unless someone hears it Sound Waves and Auditory Systems - Just as visual system can interpret light waves to collect info about environment’s stimuli, auditory system translates sound waves from vibrating objects into psychological experience known as audition Arnav Agarwal 2011 - Psychological point of view: o fallen tree -> sound waves o sound waves -> no sound if no auditory system present to translate them into auditory perceptual experience - Sound is the result of our interpretation of waves (result of sensory processing) Module 2: The Auditory Mechanisms of Different Species Introduction to Auditory Mechanisms - Cross-species variation based on specific needs (communication distances, mediums, what frequency needs to be received, etc.) - Different species develop different auditory mechanisms through process of evolution Sound Frequency - Hearing abilities among species differs in the range of frequencies detected o Eg: dog whistle -> doesn’t produce audible sound to human ear but does to dog’s attention  High frequency: beyond human ear range but within dog’s auditory system range Sound Frequency Perception in Vertebrates - Human auditory perception: 20 Hz – 20,000 Hz is detectable - Whales, dolphins, dogs: wider hearing range - Frogs, birds: much narrower range - Fish: lower frequency detection extreme - Bats, rodents: higher frequency detection extreme Arnav Agarwal 2011 o Therefore, to communicate with fish, convo would have to be based on very narrow frequency range overlap available to both: approx.. 2000 Hz.  Would sound very high-pitched to the fish and very low-pitched to the bat - Auditory frequency range determined partly by evolution of auditory system structures - Basilar membrane –> key structure containing hearing receptors o Sounds of different frequencies are processed along different parts of it Arnav Agarwal 2011 Basilar Membrane - Varies in length across species - Longer basilar membranes allow processing of wider frequency range - Shortest: amphibians and reptiles - Longest: mammals o Can therefore discriminate widest frequency range while most others can’t discriminate frequencies over 10,000 Hz Arnav Agarwal 2011 Concept Check 1) Dogs can detect whistle sounds humans cannot because: a. They can detect higher frequency sounds than humans. b. They can detect lower frequency sound than humans. c. They can detect higher amplitude sound than humans. d. They can detect lower amplitude sound than humans. 2) A longer basilar membrane is useful for: a. Hearing a wider range of sounds b. Hearing a smaller range of sounds c. Hearing fainter sounds d. Hearing louder sounds Module 3: The Stimulus: Sound Waves Introduction to Sound Waves - Stimulus: sound wave - Sound travels much slower than light waves and needs a medium to travel through - Initiated by either: o A vibrating object (eg: vocal chords, guitar strings) Arnav Agarwal 2011 o Sudden burst of air (eg: clap) o Forcing air past a small vacity (eg: pipe organ) - Result: air molecules surrounding sound source move -> chain reaction of moving air particles Responding to Changes in Air Pressure - Chain reaction: similar to ripples in pond when stone is dropped o Source: point where stone hits pond o Waves produced travel away in all directions in bands  Similar to alternating bands of more and less condensed air particles, travelling away from sound source The Eardrum Responds to Air Pressure Changes - Alternating bands of more and less compressed air molecules -> eardrum - > auditory processing - Band of compressed air causes eardrum to get pushed slightly inward - Band of less dense air causes eardrum to move outward Sine Waves - Changes in air pressure over time that make up a sound wave can be graphed as a sine wave - Amplitude: loudness - Wavelength: pitch - Purity: timbre Arnav Agarwal 2011 Amplitude: Measure of Loudness - Amplitude: height of a sound wave - Variations affection perception of loudness - Greater amplitude -> waves of greater intensity -> louder sounds - Humans sensitive to a very wide range of different sound amplitudes o Loudness is measured on a logarithmic scale as a result: decibels (dB)  Perceived loudness doubles for every 10 dB increase on this scale Arnav Agarwal 2011 - Examples: o Normal conversation: 60 dB o Whisper: 27 dB o Rock concert, front row: 120 dB o Cranked-up music-listening device: 120 dB - Even brief exposure to sound this loud can cause physical pain and permanent damage Frequency: Measure of Pitch - Variation in sound wave wavelengths: distance between successive peaks - Wavelength related to frequency of a given wave o “Sound waves also vary in the distance between successive peaks; this is called the wavelength or frequency of the sound” - Frequency affects perception of pitch - Pitch (measured in Hz): number of cycles per second/times in a second a sound wave makes a full cycle from one peak to the next - Eg: high frequency -> many wave peaks condensed into one second -> high-pitched sound perceived Arnav Agarwal 2011 - Similar to visible spectrum being small portion of total light wave spectrum, audible zone of frequencies that humans can detect is only a portion of the possible frequencies that can be produced - Low pitch sound: low frequency, long wavelength - High pitch sound: high frequency, small wavelength Arnav Agarwal 2011 Timbre: Measure of Complexity/Purity - Physical property: purity - Affects perception of timbre: complexity of a sound - Most of the sounds we hear are composed of multiple sound waves of varying frequencies - Eg: plucked guitar string -> vibrates as a whole: fundamental tone o Also vibrates at shorter segments along the string: overtones  This combination is timbre - Different instruments might play same note, but each produces a unique combination of fundamental frequencies and overtones, sounding different to us despite producing the same frequency and amplitude as a result Concept Check 1) Loudness, or amplitude, of a sound is measured in: a. dB (decibels) 2) Sound pitch is measured in: a. Hz (hertz -> cycles per second) 3) A low pitch sound is: a. A long amplitude b. A short wavelength c. A high frequency d. A low frequency e. A long wavelength Arnav Agarwal 2011 4) The reason a saxophone and a flute sound different while playing the same note is due to their different: a. Timbre Module 4: The Ear Introduction to the Ear - Instrument that is used to detect the sound waves and convert them into something the brain can interpret The Structure of the Ear - Ear divided into areas, each one conducting sound in a different way: o External area  Incoming changes in air pressure channeled through here o Middle area  Amplifies changes in air pressure o Inner area  Detects changes in fluid pressure - Changes in fluid pressure finally converted to auditory neural impulses Arnav Agarwal 2011 The External Ear - Made up of the pinna, ear canal and eardrum - Pinna: folded cone that collects sound waves in environment and directs them along the ear canal o What one thinks of when referring to an ear; the visible part - Ear canal: amplifies incoming sound waves o Narrows as it moves towards the eardrum to do so  Works like a horn - Eardrum: thin membrane vibrating at the frequency of the incoming sound wave o Forms the back wall of the ear canal Arnav Agarwal 2011 The Middle Ear - Begins on other side of the eardrum - Eardrum connects to ossicles (named on appearance): three smallest bones in the body o Ossicles consist of the hammer, anvil and stirrup Arnav Agarwal 2011 Ossicles Amplify Signal Sent to the Oval Window - Amplification of vibrating waves continues here in the middle ear - Vibrating ossicles: 20 times larger than area of oval window to which they connect o Sound goes like this from ossicles to oval window: > - Lever system created -> amplifies the vibrations even more - Additional amplification necessary: o Changes in air pressure originally detected by external ear are about to be converted to waves in the fluid-filled inner ear Arnav Agarwal 2011 The Inner Ear - Vibrating oval window (middle ear) connects to cochlea (inner ear) - Cochlea: fluid-filled 35 mm-long tube, coiled like a snail shell o Contains neural tissue necessary to transfer changes in fluid to neural impulses of audition Arnav Agarwal 2011 The Cochlea - Oval window is actually a small opening in the side of the cochlea - When oval window vibrates, fluid inside cochlea becomes displaced - Round window: located at other end of cochlea o Accommodates for fluid movement by bulging in and out accordingly Arnav Agarwal 2011 Basilar Membrane - Flexible membrane that runs the length of the cochlea; like a carpet - When pushed downwards: fluid inside the cochlea causes round window to bulge out - When pushed upwards: round window bulges inward Arnav Agarwal 2011 - Although the cochlea gets narrower towards the end, basilar membrane gets wider towards the end - Basilar membrane varies in flexibility and width across its length o Result: sounds of different frequencies cause different regions of membrane to vibrate  Higher frequency sounds: end nearest oval window vibrates  Lower frequency sounds: end nearest round window vibrates Arnav Agarwal 2011 Hair Cells - Basilar membrane houses the auditory receptors (ie: “hair cells”) - As the membrane moves in response to fluid waves, hair cells also move - Movement is finally converted to neural impulses understood by the brain - Basilar and tectorial membranes bend -> cilia of outer hair cells (embedded In tectorial membrane) bend -> hair cell neural activity generated Concept Check o This ear structure protrudes from the head:  Semicircular canal  Eardrum  Pinna  Ear canal
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