Chapter 10: Sound And The Ears Flashcards
Sources of Sound
In order:
- Sound is initiated by movement that disturbs air molecules
- Molecules collide with other air molecules resulting in air pressure that propagates outward from source
- As the sound wave travels outward form the source in all directions, the wave front resembles a sphere that grows continuously larger
- Sound energy at any given point on the wave from decreases with distance from the source (inverse square law)
Sound Waves
Waves of pressure changes in air caused by vibrations of a source
Cycle
In sound wave, a repeating segment of air pressure changes
Inverse square law
Energy of sound decreases in proportion to square of distance from source
3 physical dimensions of sound:
- Frequency- pitch
- Amplitude- loudness
- Waveform- timbre
Periodic Sound Waves
Waves in which the cycles of compression and rarefaction repeat in regular (periodic) fashion
Pure Tone
Sound wave in which air pressure changes over time according to sine wave (sinusoid)
Frequency
Physical dimension of sound that is related to perceptual dimension of pitch
- expressed in hertz (number of cycles/ second of periodic sound wave)
Pitch
Perceptual dimension of sound that corresponds to physical dimension of frequency
- perceived highness/ lowness of sound
Hertz (Hz)
- physical unit used to measure frequency
- cycles per second
Amplitude
Difference between maximum and minimum sound pressure in sound wave
- physical dimension of sound that is related to perceptual dimension of loudness
Loudness
Perceptual dimension of sound that corresponds to physical dimension of amplitude
Decibels (dB)
- physical unit used to measure sound amplitude
- logarithmically related to sound pressure measured in micropascal
Waveform
Physical property for perceptual correlate for timbre
Periodic sound waves
Waves in which the cycles of compression and rarefaction repeat in a regular, or periodic, fashion
Compression
Region in a longitudinal wave where the particles are closest together
Rarefaction
Region in a longitudinal wave where the particles are furthest apart
- follows period of compression
Frequency and pitch
- the frequency of a pure tone is the physical dimension related to the perceptual dimension of pitch and expressed in hertz (Hz)
- Young human adult sound detection range: 20- 20000 Hz
- The loudest sounds a human can hear are approximately 1 million times the amplitude of the softest sounds that can be heard
Amplitude and Loudness
- Amplitude: expressed as dB SPL= 20 log (p/po)
- Loudness: perceptual dimension of sound that is related to physical dimension of amplitude
- how intense or quiet a sound seems
Audibility Curve
- shows minimum amplitude at which sounds can be detected at each frequency
- horizontal axis of this graph uses a logarithmic scale to allow the clear presentation of a wide range of frequencies
Equal Loudness Contours
Curve showing amplitude of tones at different frequencies that sound about equally loud
Phon
Unit of loudness
- loudness of tones is numerically equal to amplitude of 1000 Hz tones that sound equally loud
Fourier
Provided that waveforms of most periodic sounds have a more complex shape than a sine wave
- Fourier analysis
- Fourier spectrum
- Fundamental frequency
Fourier Analysis
Mathematical procedures for decomposing a complex waveform into collection of sine waves with various frequencies and amplitudes
Fourier Spectrum
Depiction of amplitudes at all frequencies that make complex waveform
Fundamental frequency
Frequency of lowest- frequency component of complex waveform
- determines perceived pitch of sound
Harmonic
Component frequency of complex waveform that is an integer multiple of the fundamental frequency
- First harmonic- fundamental frequency
- what we’re most likely to hear - Second harmonic- fundamental frequency x2
Timbre
Difference in sound quality between two sounds with same pitch and loudness
- for complex periodic sounds, timbre is mainly due to differences in relative amplitudes of sounds’ overtones
- perceptual dimension of waveform
- illusion of the missing fundamental
- manner of onset and offset
Illusion of the missing fundamental
Shows that the auditory system uses patterns of frequencies in a sound’s harmonics as part of pitch perception
Manner of onset and offset
Manner of onset (attack) and offset (decay) also affect timbre perception
Complex waveforms
- do not sound like pure tone
- have sound quality that depends on frequency and amplitude of their fundamental frequency and each overtone
Sounds that differ in timbre
Two complex sounds that have the same pitch and loudness but do not sound the same due to differences in relative amplitudes of various overtones
Anatomy of the ear
Outer ear
Middle ear
Inner ear
Outer ear
Pinna, auditory canal, outer tympanic membrane
Middle ear
Air-filled chamber with ossicles
Inner ear
Cochlea and semicircular canals
Acoustic Reflex
Allows us to not amplify noise in times of really loud noises using ossicles
Pinna
Outermost portion of the ear; shape can modify incoming sound and contribute to sound localization
Auditory canal
Narrow channel that funnels sound waves gathered by the pinna onto the tympanic membrane and that amplifies certain frequencies (2000- 5000 Hz) in those waves and contributes to high sensitivity to those frequencies
Tympanic Membrane (or Eardrum)
Thin, elastic diaphragm at the inner end of the auditory canal that vibrates in response to the sound waves that strike it
-it forms an airtight seal between the outer ear and the middle ear
Ossicles and Sound Amplication
- Ossicles transmit sound energy from the tympanic membrane to the inner ear
- tympanic membrane pushes on the malleus
- Malleus pushes on the incus
- Which in turn displaces the stapes
- Which then pushes in on the oval window at the base of the cochlea
Oval Window
Membrane-covered opening at the base of the cochlea
- vibrations of membrane transmit sound energy from ossicles into cochlea
Two characteristics of ear anatomy that help compensate for loss of sound energy
- Larger size of tympanic membrane concentrates sound energy in much smaller area and effectively amplifies its effect
- Physical arrangement of ossicles produces a lever action that magnifies vibrations of tympanic membrane
Eustachian Tube
includes tube connecting the middle ear and top part of the throat
- normally closed but can be briefly opened (eg. by swallowing or yawning) to equalize air pressure outside
Cochlea
Coiled, tapered tube within temporal bone of head, partitioned along its length into three chambers
- contains structures involved in auditory transduction
Three chambers of cochlea
- Vestibular canal
- Cochlear duct
- Tympanic canal
Vestibular Canal
Separated from cochlear duct by Reissner’s membrane
- filled with perilymph
Cochlear duct
Separated from tympanic canal by basilar membrane
- contains organ of Corti
- filled with endolymphs
Tympanic Canal
Separated from cochlear duct by basilar membrane
- filled with perilymph
Round Window
Membrane- covered opening at the base of tympanic canal in the cochlea
- relief valve for pressure waves traveling through perilymph
Helicotrema
Opening at base of tympanic canal in cochlea
- provides open pathway for perilymph to carry vibrations through cochlea
Basilar Membrane
Tapered membrane suspended between walls of cochlea
- thicker, narrower, and stiffer at base than apex
Characteristic Frequency
Frequency to which each location on basilar membrane responds most readily
Characteristic frequencies in basilar membrane
- stiff base= high frequencies
- floppy apex= low frequencies
Organ of Corti
Structure in cochlea situated on basilar membrane
Neurons in organ of Corti
- Inner hair cells- auditory transduction
2. Outer hair cells- amplify and sharpen resources of inner hair cells
Tectorial membrane
Membrane that lies above hair cells in organ of Corti
Stereocilia
Small hairlike projections on top of inner and outer hair cells
Movement of tectorial membrane
Process this as neural signals
Auditory Nerve
Nerve that conveys signals from hair cells in organ of Corti to brain
- made up of Type I and II auditory nerve fibers bundled together
Type 1 Auditory Nerve Fibers
Thick and myelinated promotes rapid AP conduction
Type II Auditory Nerve Fibers
Thinner and unmyelinated
Slow AP
Inner hair cells
- Pear- shaped
- tips of stereocilia float free in endolymph
- responsible for transduction sound into neural signals
- connected to Type I auditory nerve fibers
Outer hair cells
- cylindrical
- tips of stereocilia attached to tectorial membrane
- serve to amplify and sharpen responses of inner hair cells
- connected to Type II auditory nerve fibers
Tip Links
Tiny fibers connecting tips of adjacent stereocilia on hair cells
- increased tension pull open ion channels in membranes of stereocilia
[…] open ion channels
K+ and Ca2+ ion enter channels —> depolarization
Motile Response
Response by outer hair cells that magnifies movements of basilar membrane, amplifying sounds and sharpening response to particular frequencies
Outer Hair Cells and Auditory Transduction
Tuning curves for an auditory nerve fiber with a characteristic frequency of 8000 Hz before and after destruction of the outer hair cells by chemical injection
- Type II response only to very intense sounds
Neural Representation of Frequency and Amplitude
Auditory systems mechanisms are used to encode frequency in the neural signals sent to the brain
Frequency is represented by:
Place code
Temporal code
Place code
Frequency representation based on displacement of basilar membrane at different locations
Temporal Code
Frequency representation based on match between frequencies in incoming sound waves and firing rates of auditory nerve fibers
Frequency theory (temporal code)
- suggests that the neurons’ firing rate matches the cycles per second (Hz)
- works only for lower frequency (20-4000/5000 Hz) due to limitations in cell firing rates and their ability to work collectively
Place code for frequency
- von Helmholtz: established physical basis for place code
- von Bekesy: made measurements of basilar membrane movement using microscope and strobe light
- stiffness of basilar membrane at each location determines response and different sound frequencies
Physiological frequency tuning curves
- Frequency tuning of Type I auditory nerve fibers can be almost entirely accounted for by the frequency tuning of the basilar membrane, a purely mechanical factor
- place code provides relatively better frequency representation of high-frequency sounds than of low-frequency sounds
Psychophysical frequency tuning curves
- when a noise masker has a center frequency at or near the frequency of the target tone, the masker activates the Type I auditory nerve fibers in the region of the basilar membrane with a characteristic frequency corresponding to the frequency of the target tone
- Psychophysical frequency tuning curves provide supporting evidence for the place code for frequency
Temporal Code for Frequency
- is based on a match between the frequencies in incoming sound waves and the firing rates of Type I auditory nerve fiber
- movements of inner hair stereocilia are caused by and time locked to displacements of basilar membrane (time locked to changes in air pressure of incoming sound waves)
Temporal code- with phase locking
Can precisely represent frequencies up to about 5000 Hz, while the place code described in the previous section provides the sharpest representations for frequencies about about 5000 Hz
Narrowband white noise
Sound waves with equal amplitudes at all frequencies within narrow band of frequencies
Noise masker
Narrowband and white noise and target tone
Volley Principle
- demonstrates that each nerve fiber in a population of auditory nerve fibers produces action potentials in phase with the peaks in the incoming sound wave, even if not at every peak
- explains how a temporal code could represent frequencies much higher than the maximum firing rate of any individual fiber
Amplitude Representation
- rate of action potentials produced by auditory nerve fibers increases with the amplitude of the incoming sound wave
- the relationship between loudness and neural firing is not linear because the dynamic range of hearing is much greater than the range of firing rates of any auditory nerve fiber
- Dynamic range
Dynamic Range
Range of amplitudes that can be heard and discriminated
- when applied to individual auditory nerve fiber, range of amplitudes over which firing rate of fiber changes
Patterns of Auditory Nerve Fiber Responses to Tones with Different Amplitudes
- different auditory nerve fibers have different thresholds and different dynamic ranges
- just as the auditory system can use the patterns of response of nerve fibers with different characteristic frequencies to gauge the amplitude of an incoming sound
Audiogram
Graphical depiction of auditory sensitivity to specific frequencies, compared to sensitivity of standard
- used to characterize possible hearing loss
Audiometer
Instrument that presents pure tones with known frequency and amplitude to right or left ear
- used in estimating listener’s absolute threshold for specific frequencies and to construct an audiogram
Hearing Impairments
Decrease in person’s ability to detect of discriminate sounds, compared to ability of health young adult
Conductive hearing impairments
Hearing impairments characterized by a loss of sound condition to the cochlea, as a result of problems in the outer or middle ear
- Otitis media- inflammatory of middle ear
- Cerumen- blocking of auditory canal
Sensorineural hearing impairments
Hearing impairments caused by damage to the cochlea, the auditory nerve, or the auditory areas or pathways of the brain
Tinnitus: Causes
Persistent perception of sound, such as ringing/ buzzing, not caused by any actual sound
- Are variable and not well understood
- damage to cochlea
- irritation of or pressure on auditory nerve by blood vessel or tumor
- change in neural circuits within auditory cortex
Age-Related Hearing Impairment: Causes
- Mechanical damage due to very high amplitude pressure waves pulsing through the cochlea
- Hair cell death
- Excitotoxicity
- Reduced cochlear blood flow
- Production of oxygen-based free radicals
Excitotoxicity
Too much Glu is released —> swelling in and damage to auditory nerve fibers —> death of hair cells
Reduced cochlear blood flow
Due to mechanical damage to wall of cochlea, which can kill hair cells
Production of oxygen-based free radicals
Molecules that can destabilize other molecules, damaging tissues and causing hair cells to die
Age-Related Hearing Impairment: Prevention
- using hearing protection
- keeping volume at safe level when listening to recorded music
Tinnitus: Treatments
- drugs to reduce neural activity in auditory nerve or cortex
- hearing aids
- electrical stimulation of auditory nerves
- magnetic stimulation of auditory cortex
Cochlear Implants
Devices designed mainly to enable deaf individuals to hear spoken language
External components: consist of a microphone, sound processor, and transmitter
Internal components: consist of receiver-stimulator and an electrode system that spirals around the cochlea and stimulates auditory nerve fibers, using both place coding and temporal coding
- each electrode stimulated auditory nerve fibers at different location along cochlea
