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Chapter 11

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PSYC 369
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Chapter11:Sound,theAuditory System, andPitchPerception Overview • If a tree falls in the forest and no one is there to hear it, is there a sound? • What is it that makes sounds high pitched or low pitched? • How do sound vibrations inside the ear lead to the perception of different pitches? • How are sounds represented in the auditory cortex? The Sound Stimulus Pressure waves and perceptual experience • Two definitions of “sound” – Physical definition - sound is pressure changes in the air or other medium. – Perceptual definition - sound is the experience we have when we hear Perceiving Sound • Loud speakers produce sound by: – The diaphragm of the speaker moves out, pushing air molecules together called condensation. – The diaphragm also moves in, pulling the air molecules apart called rarefication. – The cycle of this process creates alternating high- and low-pressure regions that travel through the air. • Pure tone - created by a sine wave – Amplitude - difference in pressure between high and low peaks of wave • Perception of amplitude is loudness • Decibel (dB) is used as the measure of loudness • Number of dB = 20 logarithm(p/p )o • The decibel scale relates the amplitude of the stimulus with the psychological experience of loudness. • p is the sound pressure of the stimulus, and p is the standard sound pressure o which is usually set at 20 micropascals. The standard pressure is close to the pressure of a 1,000 Hz tone at threshold in a free field. Note that adding sound pressure level (SPL) to a dB measure indicates that the standard pressure of 20 micropascals was used. • Frequency - number of cycles within a given time period – Measured in Hertz (Hz) - 1 Hz is one cycle per second – Perception of pitch is related to frequency. – Tone height is the increase in pitch that happens when frequency is increased. • Both pure and some complex tones are periodic tones. Complex Periodic Sounds • Fundamental frequency is the repetition rate and is called the first harmonic. • Periodic complex tones consist of a number of pure tones called harmonics. – Additional harmonics are multiples of the fundamental frequency. • Additive synthesis - process of adding harmonics to create complex sounds • Frequency spectrum - display of harmonics of a complex sound • Attack of tones - buildup of sound at the beginning of a tone • Decay of tones - decrease in sound at end of tone • Aperiodic sounds, which do have repeating sounds waves, are also widely heard in our environment, but have not been investigated as extensively as periodic sounds. • Timbre - all other perceptual aspects of a sound besides loudness, pitch, and duration – It is closely related to the harmonics, attack and decay of a tone. • Effect of missing fundamental frequency – Removal of the first harmonic results in a sound with the same perceived pitch, but with a different timbre. • This is called periodicity pitch. Musical Scale and Frequency • Letters in the musical scale repeat. • Notes with the same letter name (separated by octaves) have fundamental frequencies that are multiples of each other. – These notes have the same tone chroma. – We perceive such notes as similar to one another. Range of hearing • Human hearing range - 20 to 20,000 Hz • Audibility curve - shows the threshold of hearing in relation to frequency – Changes on this curve show that humans are most sensitive to 2,000 to 4,000 Hz. • Auditory response area - falls between the audibility curve and and the threshold for feeling – It shows the range of response for human audition. • Equal loudness curves - determined by using a standard 1,000 Hz tone – Two dB levels are used - 40 and 80 – Participants match the perceived loudness of all other tones to the 1,000 Hz standard. – Resulting curves show that tones sound • Almost equal loudness at 80 dB. • Softer at 40 dB for high and low frequencies than the rest of the tones in the range. The ear • Outer ear - pinna and auditory canal – Pinna helps with sound location. – Auditory canal - tube-like 3 cm long structure • It protects the tympanic membrane at the end of the canal. • The resonant frequency of the canal amplifies frequencies between 1,000 and 5,000 Hz. Middle Ear • Two cubic centimeter cavity separating inner from outer ear • It contains the three ossicles – Malleus - moves due to the vibration of the tympanic membrane – Incus - transmits vibrations of malleus – Stapes - transmit vibrations of incus to the inner ear via the oval window of the cochlea Function of Ossicles • Outer and inner ear are filled with air. • Inner ear is filled with fluid that is much denser than air. • Pressure changes in air transmit poorly into the denser medium. • Ossicles act to amplify the vibration for better transmission to the fluid. • Middle ear muscles dampen the ossicles’ vibrations to protect the inner ear from potentially damaging stimuli. Inner Ear • Main structure is the cochlea – Fluid-filled snail-like structure (35 mm long) set into vibration by the stapes – Divided into the scala vestibuli and scala tympani by the cochlear partition – Cochlear partition extends from the base (stapes end) to the apex (far end) – Organ of Corti contained by the cochlear partition Organ of Corti • Key structures – Basilar membrane vibrates in response to sound and supports the organ of Corti – Inner and outer hair cells are the receptors for hearing – Tectorial membrane extends over the hair cells • Transduction takes place by: – Cilia bend in response to movement of organ of Corti and the tectorial membrane – Movement in one direction opens ion channels – Movement in the other direction closes the channels • This causes bursts of electrical signals. Neural signals for frequency • There are two ways nerve fibers signal frequency: – Which fibers are responding • Specific groups of hair cells on basilar membrane activate a specific set of nerve fibers; – How fibers are firing • Rate or pattern of firing of nerve impulses Bekesys’ Place Theory of Hearing • Frequency of sound is indicated by the place on the organ of Corti that has the highest firing rate. • Békésy determined this in two ways: – Direct observation of the basilar membrane in cadav
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