Outline: Sound and the Ear




Sound
Pressure change over time
Sound waves

Pure tones and complex tones
Frequency = cycles/second = Hertz = Hz
Spatial Period = meters/cycle
Temporal Period = seconds/cycle
Speed = meters/second
Waveform versus spectrum
Amplitude relates to loudness
Frequency relates to pitch
Spectral envelope relates to timbre
Musical sounds are a mixture of integral multiples of a fundamental frequency
Noise sounds are broadband

Dynamic range is huge (1014, even more than for vision)

Amplitude Units
1 Bel = factor of 10 increase in sound energy
1 decibel = 1 dB = .1 Bel, so 10 dB = factor of 10 in energy
Since it's a ratio, a unit requires a standard energy
dB SPL (Sound Pressure Level) is relative to a particular near threshold sound at 1 kHz
Thus, sound intensity in dB SPL = 10 log(I / Iref) = 20 log(P / Pref)
where I is energy (intensity) and P is pressure
Examples:
0 dB = threshold
20 dB = whisper
30 dB = quiet room
60 dB = conversation
100 dB = subway
120 dB = rock concert
130 dB = threshold of pain

The ear
Consists of outer, middle and inner ear
Outer ear
Pinna
Sound gathering (movable in some species)
Directional filter
Ear canal = auditory meatus
Bandpass filtering
Responsible for much of auditory frequency sensitivity curve
Peak is a gain of 10 dB around 3 kHz (useful for speech recognition)
Middle ear
Ear drum = tympanic membrane
Ossicles (malleus/hammer, incus/anvil, stapes/stirrups)
Oval window
Impedance matcher (air to fluid), without it 99.9% of the energy reflects (lose 30 dB)
Ear drum area 17 times greater than oval window area
Ossicles act like a lever with 30% mechanical advantage
Also a bandpass filter
Acoustic reflex
Tensor tympani tightens eardrum
Stapedius muscle pulls stapes sideways
Drops gain for sounds more intense than 100 dB
Like the pupil, is a binaural reflex
Especially effective for low frequencies (e.g. to unmask by self-utterances)
Inner ear
Cochlea
Regions
Top: Scala vestibuli (to the oval window)
Middle: Scala media/cochlear duct
Bottom: Scala tympani (to the round window)
Top and bottom communicate at the inner end (helicotrema)
Basilar membrane at border of middle and bottom
All filled with fluid (perilymph top & bottom, endolymph in the middle
Oval window receives stimulation from stapes
Round window provides pressure release
Basilar membrane supports Organ of Corti which in turn supports inner & outer hair cells
Hearing loss
Conductive, possibility of bone conduction
Neural
Cochlear implants
Hair cells
Mechanical to voltage transduction
Incredibly sensitive (threshold as low as .001 nm, perhaps)
Poor analogy to rods (outer) and cones (inner)
Fewer inner
More outer, but responses are pooled
More auditory nerve fibers than inner hair cells
Hair cells don't spike
Are depolarized by compression
Feedback (outer hair cells as muscles/loudspeakers)