Frequency Coding & Localization 1 Sound and Hearing Everything is vibration The universe is made of waves db = 2log(P1/Po) P1 = amplitude of the sound wave Po = reference pressure =.2 dynes/cm 2 Decibels Decibels compress the large range of possible pressures into more manageable units Sound SPL (db) Barely audible sound (threshold) Leaves rustling 2 Quiet residential community 4 Average speaking voice 6 Loud music from radio (or heavy traffic) 8 Subway 1 The Rolling Stones (or @ a RAVE) 12 Jet engine at takeoff 14 Converts a range of.2 (threshold pressure) to 2, (jet at take off) to a range of to 14 db
Frequency Coding & Localization 2 Sine Wave 9 out of phase 9 18 36 27 18 out of phase 1. Frequency: 1-Hz Sine Wave Amplitude Spectrum Amplitude against frequency -1..5 Time (s) Waveform Amplitude against time 1 amp 1 Hz frequency Frequency: 5-Hz Sine Wave Amplitude: 5-Hz Sine Wave 1. Amplitude Spectrum Amplitude against frequency Amplitude Spectrum Amplitude against frequency -1. Time.5 (s) 1 1 Waveform amp Time.5 (s) amp Amplitude against time frequency 1 5 5 frequency frequency 1 5 5 frequency Time (s).5
Frequency Coding & Localization 3 Phase: 5-Hz Sine Wave.9899 sine The amplitude spectrum does not show phase.99.1 Time (s).99 1 cosine amp frequency 1 5 5 frequency.99.1 Time (s) Three primary dimensions of sensations associated with sounds with periodic waveforms Pitch, loudness and timbre. Pitch - sensation of "altitude" or "height" of a tone. Loudness - sensation of "intensity" of a tone. Timbre - quality of a sound. Psychological Dimension Pitch Loudness Timbre Associated Physical Dimension Frequency Amplitude Spectrum
Frequency Coding & Localization 4 The auditory system Middle Ear Fig 1.9 The tympanic membrane has an area of about.6 cm 2, whereas the stapes footplate has an area of about.32 cm 2 - a ratio of about 17 to 1. The ossicles act according to the level principle. They are hinged so that they increase the vibration by a factor of about 1.3 by means of this lever principle. These two mechanisms (size difference and lever principle) increase the strength of vibrations by a factor of at least 22 (17 x 1.3 = 22.1). QuickTime and a Animation decompressor are needed to see this picture. Without the amplifying role of the middle ear it would be much more difficult for us to hear. 1
Frequency Coding & Localization 5 Inner Ear Organ of Corti Outer hair cell bundles Inner hair cell bundles Electron micrograph of the organ of Corti from the top with the tectorial membrane removed to expose hair cells QuickTime and a Animation decompressor are needed to see this picture. When sound causes the basilar membrane to deflect upward, the reticular membrane moves up and inward causing the stereocilla to bend outward 2 cochlmtn.mov
Frequency Coding & Localization 6 QuickTime and a Animation decompressor are needed to see this picture. QuickTime and a Animation decompressor are needed to see this picture. 3 4 Inner vs Outer Hair Cells Inn er H air C ells O ut e r H air C ells QuickTime and a Animation decompressor are needed to see this picture. Sensory Motor 5 Afferent nerves Si ngl e row Efferent nerves c.3 row s The Auditory System Auditory cortex Inner hair cells give rise to the electrical signals in the auditory nerve Auditory thalamus Outer hair cells exert a mechanical amplifier action on the basilar membrane and sharpen the response of the basilar membrane Superior colliculus Inferior colliculus cochlea Cochlear nucleus Superior olive
Frequency Coding & Localization 7 The Auditory System (cortical route) cortex The Auditory System (sub-cortical route) thalamus Inferior colliculus Superior colliculus Inferior colliculus Cochlear nucleus Cochlear nucleus Superior olive The Auditory System
Frequency Coding & Localization 8 a. Primary and secondary auditory cortex b. Tonontopic organization - numbers are characteristic frequencies Frequency Coding Three primary dimensions of sensations associated with sounds with periodic waveforms Pitch, loudness and timbre. Pitch - sensation of "altitude" or "height" of a tone. Loudness - sensation of "intensity" of a tone. Timbre - quality of a sound. Psychological Dimension Pitch Loudness Timbre Associated Physical Dimension Frequency Amplitude Spectrum Historically, there have been two major approaches to frequency coding. Temporal theory (or timing) and Place theory - both focused on the basilar membrane
Frequency Coding & Localization 9 Volley Principle The response of three fibers (a,b, & c) that are phase-locked to the stimulus. Note that the fibers don t fire on every cycle of the stimulus, but when they do fire, they fire only when the stimulus is at its peak. 6-midleear.mov
Frequency Coding & Localization 1 Response of basilar membrane to sine waves Each point on the membrane acts like bandpass filter tuned to a different frequency: high freq at base, low at apex. Each point vibrates at frequency of pure tone (-> phase locking) 5 Hz 4 khz 1 khz 7-allkhalf.mov 8 khz 2 khz 16 khz
Frequency Coding & Localization 11 Excitation pattern of a complex tone on the basilar membrane 12 1 Frequency Mechanisms 2 Hz 4, Hz SPL (db) 8 6 Audibilty Curve (Threshold of hearing) Place Mechanisms 15 Hz 2, Hz 4 2 2 1 5 1 5 1, Frequency (Hz) 5 Hz 1 Hz 3 Hz 5 Hz Hearing Loss 1 Hz 3 Hz 5 Hz 75 Hz 1 Hz 125 Hz 15 Hz 18 Hz An estimated 28 million Americans suffer some loss of hearing, making this the most common of all physical disabilities 12 SPL (db) 1 8 6 Audibilty Curve (Threshold of hearing) 4 2 2 1 5 1 5 1, Frequency (Hz)
Frequency Coding & Localization 12 Five main causes of hearing loss 1. Heredity. At least 1 hereditary syndromes can result in hearing loss. 2. Infections, such as bacterial meningitis and rubella (German measles). 3. Acoustic trauma produced by acute or chronic exposure to loud sounds. 4. Prescription drugs, such as streptomycin and tobramycin. 5. Presbycusis, the hearing loss of old age, is thought to result from repeated acoustic trauma and hardening of microscopic blood vessels in the inner ear with aging. Sensori-neural Hearing Loss (SNHL) Breakdown of cochlear frequency analysis Outer-hair cell damage from cumulative exposure to loud sound Only partial help from hearing aids Symptoms of SNHL Raised thresholds: helped by amplification Wider bandwidths: no help possible Often accompanied by tinnitus Loud music and headphones seem to be contributors to the high incidence of hearing loss among people in their late teens and early twenties - maybe 6%. One manufacturer has gone so far as to install a warning light on its portable cassette players to indicate when the volume exceeds safe listening levels. The loud, amplified music often encountered at rock concerts and clubs also can cause permanent hearing loss. In one study it was found that pure-tone thresholds were greater in a group of college students who regularly attended rock concerts than in those who attended less often. Leisure vs Work Work levels should be < 85 dba Rock concerts generally 97-11 dba 11 db is more than 3 times the energy of 85 db (ie the difference between a night-light and car headlight) 8-Herdmans.mov
Frequency Coding & Localization 13 9-Herdmans1k4.mov 1-herd_4_5.mov Be kind to your ears!!! Hearing Loss is cumulative Hearing Loss is NOT reversible Impulsive sounds (eg gunshot or percussive) are the most dangerous Tinnitus ( ringing in the ears) often accompanies hearing loss and can be VERY nasty Auditory Localization A B Sound waves coming from the right side reach the right ear first, and there will be a large interaural delay before the sound propagates to the left ear.
Frequency Coding & Localization 14 Interaural delays for three different sound directions With high frequency sound the head will cast a sound shadow Slight asymmetries in arrival times between the two ears can be used to locate the lateral position of a stimulus. Delay lines and coincidence detectors may perform this computation.