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Chapter 11

Sensation and Perception Psych 367 Chapter 11.docx

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Douglas Wylie

Chapter 11 Sound, the auditory system and pitch perception The sound stimulus - Sound o Aphysical stimulus: pressure changes in the air or other medium o Perceptual response: experience we have when we hear Sound as pressure changes - Sound stimulus occurs when the movements or vibrations of an object cause pressure changes in the air, water or any other elastic medium that surrounds the object - When the diaphragm of the speaker moves out, it ouches thee surrounding air molecules together, a process called condensation, which cause a slight increase in the density of molecules near the diaphragm - This increased density result in a local increase in the air pressure that is superimposed on the atmospheric pressure - When the speakers diaphragm moves back in, air molecules spread out to fill the increased space, a process called rarefaction - The decreased density of air molecules caused by rarefaction causes a slight decrease in air pressure - Speaker creates a pattern of alternating high and low pressure regions in the air as neighbouring air molecules affect each other - sound wave: This pattern of air pressure changes, which travels through air at 340 msp a - Air pressure changes move outward from the speaker, the air molecules at each location move back and forth, but stay in about the same place Pressure changes: pure tones - Pure tone: occurs when pressure changes in the air occur in a pattern described by a mathematical function called a sine wave - This vibration can be described by noting its amplitude: the size of the pressure change - Frequency: the number of times per second that the pressure changes repeat Amplitude - one way to specify a sounds amplitude would be to indicate the difference in pressure between the high and low peaks of the sound wave - The physical property of amplitude is associated with our experience of loudness, with higher amplitudes associated with louder sounds - Decibel: which converts the large range of sound pressure into a more manageable scale - dB = 20 x logarithm (p/p) o dB stand for decibles, p is the sound pressure of the stimulus o p is a standard sound pressure o 20 micro pascals is a pressure near the threshold for human hearing - Adding the notation SPL (sound pressure level), indicates that we have used the standard pressure or 20 micro pascals - Multiple the pressure by 10 adds 20 dBs - Alarge increase in amplitude causes a much smaller increase in dB Frequency - Frequency, the number of cycles per second the change in pressure repeats, is the physical measures associated with our perception of pitch, with higher frequency is higher pitches - Frequency is indicated in units called Hertz o 1 Hz is one cycle per second - Human can perceive from 20-20 000 Pressure changes: complex tones - This property of repetition means that this complex tone is a periodic tone - Fundamental frequency: The repetition rate of a complex tone - Property of periodic complex tones is that they consist of a number of pure tones - We can build a complex tone by using a technique called additive synthesis in which a number of sine waves components are added together to create the complex tone - These additional tones are higher harmonics of the tone - Adding the fundamental (also called first harmonic) and the higher harmonics results in the wave form of the complex tone - Another way to represent the harmonic components of a complex tone is by frequency spectra - The position of each line on the horizontal axis indicates the harmonics frequency and the height of the line indicates the harmonics amplitude - Note that removing harmonic changes the tones waveform, but that the repetition rate remains the same - Even though the fundamental is no longer present, the repetition rate indicates the frequency of the harmonic - The distance between harmonics equals the fundamental frequency - When the fundamental is removed, this spacing remains, so there is still information in the waveform indicating the frequency of the fundamental - Since a tones pitch is related to the repetition rate, removing the fundamental does not change the tones pitch Perceiving sound Loudness - Loudness: is the quality most closely related to the amplitude or sound pressure, which is also called the level of an auditory stimulus - Asound with zero dB is just barely detectible and 120 dB is extremely loud - S.S. stevens magnitude estimation procedure o Loudness was judged relative to a standard of 100 hz tone at 40 dB which was assigned a value of 1 o Increasing the sound level by 10dB doubles the loudness - dB are a physical measure, whereas loudness is psychological pitch - pitch: the perceptual quality we describe as high or low is defined as the attribute of auditory sensation in terms of which sounds may be ordered on a musical scale - pitch is most closely related to the physical property of frequency - low fundamental frequencies are associated with low pitches - tone height: is the perceptual experience of increasing pitch that accompanies increase in a tones fundamental frequency - because of this similarity, we say that notes with the same letter have the same tone chroma - every time we pass the same letter on the keyboard, we have gone up an interval called an octave - increasingly, notes with the same chroma have fundamental frequencies that are multiples of one another - somehow this doubling of frequency for each octave results in similar perceptual experiences - removing the first harmonic changes a tones waveform but not its repetition rate and that because the tones repetition rate remains the same, the tone pitch remains the same - the pitch therefore is determined not by the presence of the fundamental frequency, but by information such as the spacing of the harmonics and the repetition rate of the waveform that indicates the fundamental frequency - the constancy of pitch even when the fundamental or other harmonics are removed, is called the effect of the missing fundamental - periodicity pitch: the pitch that we perceive in tones and that has had harmonics removed - removing the fundamental does not affect a tones pitch it does cause a tone to sound different the range of hearing - the range of hearing: we hear sound only within a specific range of frequencies the audibility curve - it indicates the threshold for hearing determined by free field presentation - the range of hearing is 20 Hz and 20 000 Hz o the range that is most important for understanding speech - auditory response area: because we can hear tones that fall within this area - at intensities below the audible curve, we cant hear a tone - the upper boundary of the auditory response area is the curve marked threshold of feelings o over this its painful loudness depends on sound pressure and frequency - loudness of pure tones depends not only on sound pressure but also on frequency - we can tell that these two tones would have very different loudness’s by considering their location relative to the audibility curve - the 100Hz tone is located just above the audibility curve so it is just above the threshold and would be barely heard - the 1000 Hz tone is far above threshold., well into the auditory response area, so it would be much louder then 100Hz - to determine the loudness of any tone, you need the dB level and the frequency - equal loudness curves: these curve indicate the number of decibels that create the same perception of loudness determined by presenting a standard tone of one frequency - equal loudness curve is determined by presenting a standard tone of one frequency and dB level and having a listener adjust the level of tones with frequencies across the range of hearing to match the loudness of the standard timbre - timbre: the quality that distinguishes between two tones that have the same loudness, pitch and duration, but still sound different - timbre is closely related to the harmonic structure of a tone - the relative strengths of the harmonics and the number of harmonics are different in these instruments - the difference in the harmonics of different instruments is one factor that causes musical instruments to have different timbres - timbre also depends on the time course of the tones attack (build up of sound) and tones decay (decrease in sound at the end of the tone) - difficult to distinguish between the same instruments when the tones attack and decay are eliminated by erasing the first and last half second - difficult to distinguish one instrument from another is to play an instruments tone backwards - timbre depends both on the tones steady state harmonic structure and on the time course of the attack and decay of the tones harmonics - aperiodic sounds, which have sound waves that do not repeat the ear - auditory system must accomplish three basic tasks o it must deliver the sound stimulus to the receptors o it must transducer this stimulus from pressure change into electrical signals o it must process these electrical signals so they can indicate qualities of the sound source the outer ear - pinnae, the structure that sticks out from the sides of the head o important in helping use determine the location of sound - sound waves first pass through the outer ear, which consists of the pinna and the auditory canal o tube like structure - the wax protects the tympanic membrane or eardrum at the end of the canal and help keep constant temperature - outer ear also enhances the intensities of some sounds by means of the physical principle of resonance - resonance: occurs in the auditory canal when sound waves that are reflected back from the closed end of the auditory canal interact with sound waves that are entering the canal o reinforces some of the sound frequencies - the frequency reinforced the most is called the resonant frequency - resonance that occurs in the auditory canal has a slight amplifying effect on frequencies between 1000 and 5000 Hz the middle ear - set the tympanic membrane into vibration and this vibration is transmitted to structures - the middle ear is a small cavity which separates the outer and inner ear - ossicles: three small bones in the body - malleus: the hammer - incus: anvil - stapes: stirrup - the stapes then transmits its vibrations to the inner ear by pushing on the membrane covering the oval window - outer ear and middle ear are filled with air, but the inner ear contains liquid that is much denser then air - pressure changes in the are transmitted poorly to the much denser liquid - if the vibrations had to pass directly from the air in the middle ear to the liquid in the inner ear, less than 1% would be transmitted - ossicles help solve this problem in two ways o concentrating the vibrations of the large ear drum onto much smaller stapes, which increases the pressure by 20x o being hinged to create a lever action that creates an effect similar to what happens when a fulcrum is placed under a board, so pushing down on the long end of the board makes it possible to lift a heavy weight on the short end - the middle ear also contains the middle ear muscles, the smallest skeletal muscles in the body o they are attached to the ossicles, and at very high sound intensities, they contract to dampen the ossicles vibration thereby protecting the structures of the inner ear the inner ear - the main structure of the inner ear is the liquid filled cochlea - liquid inside the cochlea is set into vibration by the movement of the stapes against the oval window - upper half, called the scala vestibule, and the lower half called the scala tympani, are separated by a structure called cochlear partition - this partition extends almost the entire length of the cochlea, from its base near the stapes, to its apex as the far end - cochlear partition contains a large structure called the organ of corti - important structures in organ or corti o hair cells: receptors for hearing o cilia which protrude from the tops of the cells are where the sound acts to produce electrical signals o inner hair cells: 3500 o outer hair cells: 12 000 - basilar membrane: supports the organ of corti and vibrates in response to sound - tectorial membrane: extends over the hair cells - one of the most important events in the auditory process is the bending of the cilia of the inner hair cells, which are responsible for transduction - major role of the outer hair cells is to increase the vibration of the basilar membrane - this up and down motion of the cochlear partition causes two effects o it sets the organ of corti into an up and down vibration o it causes the tectorial membrane to move back and forth - these two motions cause the cilia of the inner hair cells to bend because of their movement against the surrounding liquid and affects the outer hair cells because some of the cilia are in contact with the tectorial membrane - movement in one directions opens cannels in the membrane and ions flow into the cell - the ion flow in the inner hair cells has the same effect, creating electrical signals that result in the release of neural transmitter from the inner hair cell - when the cilia bend in the other direction, the ion channel close, so the electrical signals are not generated - the amount of cilia of the inner hair cells must bend to cause an electrical signal is very small - at the threshold for hearing, cilia m
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