Hearing: Physiology and Psychoacoustics

Transcription

1 9 Hearing: Physiology and Psychoacoustics 9 The Function of Hearing The Basics: Nature of sound Anatomy and physiology of the auditory system How we perceive loudness and pitch Impairments of hearing 9 What Is Sound? 9 Sound Wave and Air Pressure Sounds are created when objects vibrate Vibrations of object cause molecules in object s surrounding medium to vibrate as well, which causes pressure changes in medium Waves pressure changes Compression increased pressure Rarefaction decreased pressure 1

2 9 What Is Sound? (cont d) Sound waves travel at a particular speed Depends on medium Example: Speed of sound through air is about 340 meters/second Speed of sound through water is 1500 meters/second 9 What Is Sound? (cont d) Basic qualities of sound waves Frequency: For sound, the number of times per second that a pattern of pressure change repeats Amplitude: Magnitude of displacement of a sound pressure wave Waveform: The shape of the soundwave 9 What Is Sound? (cont d) 9 Frequency and Amplitude Frequency is associated with pitch Low-frequency sounds correspond to low pitches, (e.g., low notes played by a tuba) High-frequency sounds correspond to high pitches, (e.g., high notes from a piccolo) 2

3 9 Frequency One cycle: rarefaction and compression Frequency: # of cycles per unit time Distinction: Physical stimulus -- frequency Psychological experience -- pitch Unit: Herts (hz) #cycles / second 500 Hz cycles/second 2000 Hz cycles/second Human range: 20-20,000 hz 9 What Is Sound? (cont d) Human hearing uses a limited range of frequencies: From about 20 to 20,000 Hz 9 What Is Sound? (cont d) Humans can hear across a wide range of sound intensities Ratio of pressure changes between faintest and loudest sounds is more than one to one million Faintest:.0002 dynes/cm 2 High Risk: >200 dynes/cm 2 In order to describe differences in amplitude, sound levels are measured on a logarithmic scale, in units called decibels (db) Relatively small decibel changes can correspond to large physical changes (e.g., increase of 6 db corresponds to a doubling of the amount of pressure) 9 Decibels db = 20 log (p1/p0) P1 = pressure of interest P0 = standard pressure (threshold =.0002 dynes/cm 2 ) Absolute threshold example db = 20 log (.0002 /.0002) db = 20 log (1) db = 0 High Risk example db = 20 log (200 /.0002) db = 20 log (1,000,000) db = 120 3

4 9 Intensity of Environmental Sounds 9 What Is Sound? (cont d) One of simplest kinds of sounds: Sine wave, or pure tone Sine wave: Waveform for which variation as a function of time is a sine function 9 What Is Sound? (cont d) 9 Sine Sine waves: Not common everyday sounds because not many vibrations in the world are so pure Most sounds in world: Complex sounds, (e.g., human voices, birds, cars, etc.) 4

5 9 Example waveforms 9 Human speech (long e) 9 Complex Waveforms 9 Complex Sound Waves Sound waves -- pressure changes Will summate Point by point addition of pressure fluctuations Example Consequently Can think about assembling complex wave Can think about disassembling complex wave 5

6 9 What Is Sound? (cont d) 9 Wave Form and Spectrum (Part 1) Complex sounds can be described by Fourier analysis A mathematical theorem by which any sound can be divided into a set of sine waves. Combining these sine waves will reproduce the original sound Results can be summarized by a spectrum 9 What Is Sound? (cont d) 9 Harmonic Sounds with the Same Fundamental Harmonic spectra: Typically caused by simple vibrating source, (e.g., string of guitar, or reed of saxophone) Relative intensities of different frequency components Waveform of sound Timbre: Psychological sensation by which listener can judge that two sounds that have same loudness and pitch are dissimilar 6

7 9 Basic Structure of the Mammalian Auditory System (cont d) 9 Outer Ear Outer ear: Sounds are first collected from environment by the pinnae Sound waves are funneled by the pinnae into ear canal Length and shape of ear canal enhance sound frequencies Main purpose of canal is to insulate structure at its end: Tympanic membrane 9 Mammalian Pinnae 9 Basic Structure of the Mammalian Auditory System (cont d) Tympanic membrane: Eardrum; a thin sheet of skin at end of outer ear canal; it vibrates in response to sound Increased pressure moves in Decreased pressure moves out 7

8 9 Basic Structure of the Mammalian Auditory System (cont d) 9 Structure of the Human Ear (Part 1) Middle ear: Air Filled pocket behind tympanic membrane Three tiny bones: Ossicles Malleus, Incus, Stapes (aka: Hammer, Anvil, Stirrup) Role: Amplify sounds Stapes transmits vibrations of sound waves to oval window, another membrane which represents border between middle ear and inner ear 9 Basic Structure of the Mammalian Auditory System (cont d) Amplification provided by ossicles is essential to ability to hear faint sounds Inner ear is made up of collection of fluid-filled chambers Need to amplify pressure to create pressure waves in cochlear fluid Amplification (magnify pressure 30x) Lever principle Funnel energy from larger tympanic membrane to smaller foot plate of stapes 9 Basic Structure of the Mammalian Auditory System (cont d) Ossicles also important for loud sounds Middle ear: Two muscles-tensor tympani and stapedius Purpose: To tense when sounds are very loud, muffling pressure changes However, acoustic reflex follows onset of loud sounds by about one-fifth of second, so cannot protect against abrupt sounds, (e.g., gun shot) 8

9 9 Basic Structure of the Mammalian Auditory System (cont d) Inner ear: Changes in sound pressure are translated into neural signals Function is roughly analogous to that of retina 9 Basic Structure of the Mammalian Auditory System (cont d) Cochlear canals and membranes Cochlea: Spiral structure of the inner ear containing the organ of Corti Cochlea is filled with watery fluids in three parallel canals 9 The Cochlea (Part 1) 9 The Cochlea (Part 2) 9

10 9 The Cochlea (Part 3) 9 The Cochlea (Part 4) 9 Basic Structure of the Mammalian Auditory System (cont d) The three canals of the cochlea Tympanic canal Vestibular canal Middle canal 9 Basic Structure of the Mammalian Auditory System (cont d) Vibrations transmitted through tympanic membranes and middle-ear bones cause stapes to push and pull flexible oval window in and out of vestibular canal at base of cochlea If sounds are extremely intense, any remaining pressure is transmitted through helicotrema and back to cochlear base through tympanic canal, where it is absorbed by another membrane: Round window 10

11 9 Basic Structure of the Mammalian Auditory System (cont d) Organ of Corti Movements of cochlear partition are translated into neural signals by structures in the organ of Corti; extends along top of basilar membrane Made up of specialized neurons called hair cells, dendrites of auditory nerve fibers that terminate at base of hair cells, and scaffold of supporting cells 9 Basic Structure of the Mammalian Auditory System (cont d) Hair cells in each human ear: Arranged in four rows that run down length of basilar membrane 9 Vibration and the Tectorial Membrane Tectorial membrane: Extends atop organ of Corti ; gelatinous structure 9 Review of Neural Functioning - charge inside / + charge outside Negative membrane potential Ion channels -- change membrane potential Transduction -- modify ion channels Modify potential of neuron Neural signal 11

12 9 Hair cells -- resting potential (-50 to -70 mv) Movement of BM - movement of cilia (back and forth) Cilia movement One direction Opens ion channels Depolarize hair cell (more +) Increase release of NT to auditory nerve fibers Other direction Transduction in Audition Closes ion channels Hyperpolarize hair cell (more -) Decrease release of NT to auditory nerve fibers 9 Transduction 9 Auditory Nerve Fibers 12