Download Hearing

Hearing
and other senses
Sound
•
Sound: sensed variations in air pressure
•
Frequency: number of peaks that pass a
point per second (Hz)
Pitch
•
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Sound Measurement
•
Range: p < 0.001 mbar (normal breathing)
p > 1000 mbar (jet plane)
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Sound Pressure Level (SPL)
SPL = Lp = 20 log10 ( p/pr)
p = RMS sound pressure of target sound
pr = RMS sound pressure of reference sound
(e.g. 0.0002 mbar)
SPL units: decibels (dB)
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Representative SPLs
Sound
approximate
SPL (dB)
125
Jet takeoff (at 200 ft)
Cutoff saw
105
Inside sports car (at 50 mph)
80
Near freeway (auto traffic)
65
Average residence
50
Soft whisper (at 5 ft)
30
Normal breathing
10
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Sound Measurement Devices
The device on the left is a sound level
meter and is primarily used for noise
abatement activities and acoustical work
such as determining noise control criteria
for an occupancy or for ambient noise
analysis and control. The device in the
center is a sound level meter/noise
dosimeter which accumulates, or logs
noise exposure for an entire work shift.
This instrument is primarily used for
OSHA hearing conservation activities. The
device on the right is a previousgeneration sound level meter.
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A, B, and C Scales
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The Ear
Adapted from: (http://www.teleport.com/~veda/gallery.html)
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Hearing Anatomy & Physiology
Pinna
Auditory
Canal
Tympanic
Membrane
Ossicles
Malleus (hammer) Amplitude reduction
Incus (anvil)
Pressure amplification
Stapes (stirrup)
Attenuation reflex
Ligaments
(protection, low
Muscles
frequency masking)
Oval
Window
Cochlea
Auditory
Nerve
Auditory
Cortex
Cochlea
Oval
Window
Stapes
Round
Window
High Frequency
Scala Vestibuli
& Scala Media
Scala
Tympani
Helicotrema
Basilar Membrane
Organ of corti
Hair cells
Low Frequency
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Auditory Experience
• Sound intensity/SPL  Loudness
• Frequency  Pitch
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Psychophysical Scaling
• loudness not directly proportional to intensity
• psychophysical perceived loudness) scales
• Phons
• Equal loudness contours
• phons = dB @ 1000 Hz
• Sones
• Relative subjective loudness
• 1 sone = 1000 Hz @ 40 dB
• 2 sones = sound judged twice as loud as 1 sone sound
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Sensitivity
Range of Hearing:
20 - 20,000 Hz
Highest Sensitivity:
1,000 - 3,000 Hz
Lowest Detectable Intensity: 0 dB
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Limits
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Equal-loudness contour
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equal pitch contours
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Discriminability
•
Ability to distinguish between two simuli (e.g.
sounds)
•
•
•
•
•
Frequency - Pitch
Intensity - Loudness
Spectrum
Phase (?)
Just Noticable Difference (JND)
•
Least change in a stimulus or the least
difference between two stimuli that can be
detected 50 % of time.
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Absolute Discrimination
Dimension (s)
Intensity
# of Levels
4-5
Frequency
4-7
Duration
2-3
Intensity & Frequency
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Masking
• Sound A (masking sound) reduces
sensitivity of ear to Sound B (masked
sound).
• Raised threshold for B (masked
threshold).
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Masking by Pure Tones
Masking by Wideband Noise
Reducing Masking
•
Signal Control
•
•
•
Selection - distinction from noise
Intensity - above masked threshold
Noise Control
•
•
•
Selection - distinct from signal
Intensity - reduce
Filter - alter spectrum to reduce masking
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Alarms
• Rationale for auditory alarms
• sound omnidirectional
• can’t “close” our ears
• Criteria for auditory alarms
•
•
•
•
•
must be above background sound
must not be above danger level
should not be overly startling (longer rise time)
should not interfere with other signals
should be informative
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Designing Auditory Alarms
• do task analysis
• stay within limits of absolute judgement
• capitalize on the dimensions
•
•
•
•
•
pitch
envelope
rhythm
timbre
design sound specifics
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False Alarms
• false alarms 
• loss of trust
• disabling of alarms
• missed signals
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Speech
• Example: Tenerife
• bottom-up issues
• top-down issues
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Masking Effects
• female voice more vulnerable
• consonant sounds (esp. s, ch) more
susceptible to masking than vowels
• “fly to” vs “fly through”
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Measuring Speech
Communication
• Bottom-Up: Articulation Index (AI)
• signal-to-noise ratio
• speech db – noise db
• weighted across frequency bands
• Top-Down: Speech Intelligibility Level (SIL)
• % items correctly heard
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Speech Distortions
• Examples
•
•
•
•
•
clipping (beginnings, ends of words)
reduced bandwidth
echoes
reverberations
low quality synthesized speech
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Temporary Hearing Loss
• Continuous noise leads to hearing loss
• Temporary threshold shift at 2 min (TTS2)
• 70 - 75 dBA : no TTS2
• 80 - 105 dBA: TTS2 proportional to
exposure
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Permanent Hearing Loss
• Continuous noise may lead to permanent
hearing loss
• Begins at ~ 4000 Hz
• Generally restricted to 3000 - 6000 Hz
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Hearing Loss
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Noise - Induced Hearing Loss
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Noncontinuous Noise
• Impact Noise (e.g. drop forge)
• Impulse Noise (e.g. gunfire)
• Noncontinuous noise may lead to
permanent hearing loss.
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OSHA Standards: Continuous
Noise
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OSHA Standards: Impulse Noise
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Noise Dosage
• total (daily) dose = sum of partial doses
• Requirement: total dose < 1.00
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Example
Worker exposed to 90 dBA for 4 hours, 105
dBA for 30 minutes. Within dosage limits?
4 hr @ 90 dBA = 4 / 8 = 0.5
0.5 hr @ 105 dBA = 0.5 / 1 = 0.5
Total dosage = 0.5 + 0.5 = 1.0
Since 1.0 < 1.0, dosage is OK
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Physiological Effects
• Short Term Effects
• Startle response
• Long Term Effects ( > 95 dBA, > 10 yrs)
•
•
•
•
•
•
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Hypertension
Hypotension
Ulcers
Headaches
Irritability
Sleep disorders
etc.
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Performance Effects
• Increase confidence (increased misses)
• Attention funneling (missed info)
• Performance gaps
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Noise Control
Source
Path
Design
Barriers
Maintenance Enclosures
Mountings
Baffles
Mufflers
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Receiver
Ear plugs
Ear muffs
42
Hearing Protectors
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The Other Senses
• Touch
• Tactile/Haptic Sense
• Proprioception
• joint angles
• Kinesthesis
• movement
• The Vestibular Senses
• motion
• acceleration
• illusions of motion
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Tactual (Tactile) Displays
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Stimuli
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Coding
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•
•
mechanical
thermal
chemical
electrical
shape
pattern
magnitude (pressure, vibration, size, displacement)
Examples
•
•
•
braille
reading devices for blind
K-T display
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