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Chapter 11

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Gautam Ullal

Psych 2E03: Sensory Processes Chapter 11: Sound, The Auditory System, And Pitch Perception Pressure Waves and Perceptual Experience - Sometime sound refers to a physical stimulus (sound is pressure changes in the air or other medium), and sometimes it refers to a perceptual response (sound is the experience we have when we hear) - The sound stimulus produced by a loudspeaker:  Sound stimulus occurs when the movements or vibrations of an object cause pressure changes in air, water, or any other elastic medium that surrounds the object  Sound waves: pattern of alternating high- and low-pressure regions in the air as neighbouring air molecules affect each other, traveling at 340m/s  Air pressure changes move outward from the speaker, the air at each location remains in approximately the sample place  Sound waves are invisible to our eyes, but they can sometimes be felt as tactile perceptions through the skin  Hearing is the predominant experience caused by sound waves  Specific perceptual experiences caused by sound waves are determined by the nature of the pressure changes  Pure tone: pressure changes occur in a pattern described by a mathematical function called a sine wave  Amplitude: size of the pressure change  Frequency: number of times per second that the pressure changes repeat - Amplitude and loudness:  Larger amplitudes are associated with an increase in the perceptual experience of loudness – the magnitude of auditory sensation  Increases in amplitude are associated with increases in loudness and the sound waves created by events in the environment have an extremely large range of amplitudes  Decibel: unit of sound  dB scale is useful because it compresses the large range of sound pressures into a more manageable range - Frequency and pitch:  Frequency is indicated in units called Hertz, 1Hertz is 1 cycle/s  Pitch: psychological quality of a tone that we describe as high or low  Tone height: perceptual experience of increasing pitch that accompanies increases in a tone’s frequency  Notes with the same letter sound similar and have the same tone chroma  Tones separated by octaves have the same tone chroma  Notes with the same chroma have frequencies that are multiples of one another - The range of hearing:  We hear only within a certain range of frequencies (range of hearing)  20Hz-20,000Hz  Audibility curve: indicates how sensitivity to sound changes across the range of hearing  We are most sensitive (the threshold for hearing is lowest) at frequencies between 2,000 and 4,000Hz  Auditory response area: can hear tones that fall within this area  Threshold for feeling  Audibility curve and auditory response area indicate that the loudness of pure tones depends not only on sound pressure but also on frequency  Equal loudness curves: indicate the number of decibels that create the same perception of loudness at different frequencies - Sound quality – timbre:  Two tones can have the same loudness and pitch an sound different  Complex stimuli contain many frequencies, and it is those additional frequencies that help create different timbres  Additive synthesis: used to produce complex sounds by adding simple components  Fundamental frequency: first harmonic of the complex tone  Adding harmonics to the fundamental creates a sound that can resemble a musical instrument because the sounds produced by most musical instruments have many harmonics, with the amplitudes of the various harmonics giving each instrument its distinctive sound  Frequency spectrum: way to indicate the harmonic components of a complex wave that has been created by additive synthesis  Timbre also depends on the time course of the tone’s attack (the buildup of sound at the beginning of the tone) and on the time course of the tone’s decay (the decrease in sound at the end of the tone)  Timbre depends both n the tone’s steady-state harmonic structure and on the time course of the attack and decay of the tone’s harmonics The Ear - The outer ear:  Pinnae: structure that stick out from the sides of the head help determine the location of sounds  Auditory canal: tubelike structure that protects the delicate structures of the middle ear from the hazards of the outside world and helps keep the membrane and the structures at a constant temperature  Enhance the intensities of some sounds by means of the physical principle of resonance  Resonance occurs when sound waves that are reflected back from the closed end of the auditory canal interact with sound waves that are entering the auditory canal  Resonant frequency: the frequency reinforced the most - The middle ear:  When airborne sound waves reach the tympatic membrane at the end of the auditory canal, they set it into vibration, and this vibration is transmitted to structures in the middle ear, on the other side of the tympatic membrane  Middle ear: is a small cavity, about 2 cubic centimeters in volume, which separates the outer and inner ear  Ossicles: three smallest bones in the body  Malleus: set into vibration by the tympatic membrane, to which it is attached  Incus: transmits its vibrations to the stapes  Stapes: transmits its vibrations to the inner ear by pushing on the membrane covering the oval window  Outer and inner ear are filled with air, inner ear contains a watery liquid that is much denser than the air  Pressure changes in the air are transmitted poorly to the much denser liquid  Ossicles help by amplifying the vibration in two ways:  Concentrating the vibration of the large tympanic membrane onto the much smaller stapes  Being hinged to create a lever action similar to what enables a small weight on the long end of a board balanced on a fulcrum to overcome a large weight on the short end of the board  Middle ear also contains the middle-ear muscles, these muscles are attached to the ossicles, and at very high sound intensities they contract to dampen the ossicle’s vibration, thereby protecting the structures of the inner ear against potential and damaging stimuli - Inner ear:  Liquid-filled cochlea: liquid is set into vibration by the movement of the stapes against the oval window  Upper half of cochlea, the scala vestibule, and the lower half, the scala tympani, are separated by a structure called the cochlear partition  Partition extends from the base of the cochlea near the stapes to its apex at the far end  Organ of Corti:  The hair cells are the receptors for hearing. The cilia, which protrude from the tops of the cells, are the sound acts to produce electrical signals. There are two types of hair cells, the inner hair cells an the outer hair cells  The basilar membrane supports the Organ of Corti and vibrates in response to sound  The tectorial membrane extends over the hair cells  Bending of cilia transduces the vibrations caused by the sound stimulus into electrical signals  Cilia is bent because the in-an-out movement of the stapes creates pressure changes in the liquid inside the cochlea that sets the cochlear partition into an up-and-down motion  Up-and-down motion of the cochlear partition causes the organ of Corti into and up-down vibration set and causes the tectorial membrane to move back-and-forth  Bending of the cilia of the inner hair cells generates the electrical signal that is transmitted to fibres in the auditory nerve  Cilia movements as small as 100 trillionths of a meter can generate a response in the hair cell The Cochlea - Two ways these fibres can signal the frequency of a tone are by: which of the fibres are firing and how these fibres are firing - Bekesy’s place theory of hearing:  Frequency of a sound is indicated by the place along the organ of Corti at which nerve firing is highest  By observing the vibration of the basilar membrane and by building a model of the cochlea that took into account the physical properties of the basilar membrane we could approach the problem of determining the code for frequency by determining how the basilar membrane vibrates in response to different frequencies  Vibrating motion of the basilar membrane is similar to the motion that occurs when one person holds the end of a rope and snaps it, sending a wave traveling down the rope (traveling wave)  Base of the basilar membrane Is three or four times narrower than the apex of the basilar membrane  Base of the membrane is about 100 times stiffer than the apex  Pressure changes in the cochlea cause the basilar membrane to vibrate in a traveling wave  Most of the membrane vibrates, but some vibrates more than others  The envelope of the traveling wave, which is indicated by the dashed line, indicates the maximum displacement that the wave causes at each pint along the membrane telling is which hair cells and nerve fibres along the basilar membrane will be affected the most by the membrane’s vibration  The amount that the cilia move depends on the amount that the basilar membrane is displaced  The envelope has a peak amplitude at one point on the basilar membrane  The position of this peak on the basilar membrane is a function of the frequency of the sound. High frequencies cause less of the membrane to vibrate, and the maximum vibration is near the base. - Evidence for place theory:  Bekesy’s linking of the place on the cochlea with the frequency of the tone has been confirmed by measuring the electrical response of the cochlea and of individual hair cells and auditory nerve fibres  Tonotopic map: orderly map of frequencies along the length of the cochlea  Apex of the cochlea responds best to low frequencies and the base responds best to high frequencies  Frequency tuning curve determined by presenting pure tones of different frequencies, and measuring how many decibels are necessary to cause the neuron to fire  Characteristic frequency of the particular auditory nerve fibre is the frequency to which the neuron is most sensitive  Idea that the frequency of a t
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