How We See and Hear

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How We See and Hear

• IENG 321 Ergonomics and Human Factors
  Engineering
• Spring 2009

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How We See

• Eyes sense energy in form of light rays which are
  converted to nerve impulses.
• Perceived picture a subjective modification of
  what eye reports.

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Straight or Curved
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Lighter or Darker
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Lighter or Darker
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The Eye
Focuses light on retina by changing shape through
ciliary muscle action thin distance,
Thin lining covering about ¾ of inner surface,
high conc of arteries and veins 130 million
receptors Rods 120 million, most
important visual information white/black/shades
of gray Cones 10 million, mainly in
fovea, respond to color if enough light 3 types
w/single pigment blue, green, red, 150 color
hues
Fibrous tissue surrounding eye
Black area in middle of Iris
2.5 cm Dia
Area directly behind lens,
Light enters, transparent dome Protective
covering, weak lens
Colored area with changes by pupillary dilator
and sphincter muscles
Exits rear of eye, 15o medially
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Visual Acuity

• Ability to discriminate fine detail based on
  accommodation of the eye
• Accommodation Ability of lens to focus light on
  retina
• Snellen Acuity
• 20/20
• Minimum Separable Acuity
• Smallest feature or space between features able
  to detect

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Line of Sight
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Subtended Visual Angle

• Angle formed by the pupil
• Minimum 1 minute of arc
• Diopter
• Optical refraction needed to optimal focus, 1/D

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Text Characteristics

• Stroke width-to-height
• Sufficient illumination
• 16 to 18, white background
• 18 to 110, black background
• Printed characters
• Low illumination/contrast, 15
• Width-to-height
• 35 commonly used
• Styles
• Roman - serfis
• Gothic uniform stroke width, sans serif
• Script mimic handwriting
• Block Letter resemble German manuscript
  handwriting of 15th century

• Size
• Points 1 pt 1/72 in. (.35 mm), better est. by
  .25 mm

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Reading Distance

• Books and similar printed material
• 12 16 in., 14 in. considered normal
• 9 11 pt. (22-27 mm visual angle)
• Computer
• 24 36 in., mean 30 in.
• 9 13 pt. (20 22 mm visual angle)
• Distance Reading

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Light Measures

• SI Units
• Luminous flux lumen (lm)
• Rate of energy emitted from a source
• Luminous intensity candela (cd)
• Lumens emitted from source per unit solid angle
• Steradians (sr), 12.57 sr in a sphere
• 1 cd 12.57 lm
• Illuminance
• Amount of light striking inside of a sphere
• Lm/area
• 1 lm per ft2 footcandle (fc)
• 1 lm per m2 lux (lx)
• Amount of light stricking a surface follows the
  Inverse-square law

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Lighting

• Minimum lighting required to stimulate rods .01
  lux
• gt.1 lux rods and cones
• Darkness events
• Autokinetic phenomenon
• Oxygen deficiencies
• Additional risk factors with smoking
• Night myopia

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Light Accommodation

• Dark adaptation
• 30 minutes
• Cones most sensitive, rods follow more slowly
• Light Adaptation
• A few minutes
• Yellow wavelengths easiest
• Illuminate instruments with yellow or red light

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Designing Illumination

• Make sure sufficient lighting
• Reflected of emitted
• Quantity and direction of lighting
• Age related concerns
• Sufficient lighting for color

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Eye Fatigue

• Excessive demands on muscles of the eye
• Age Lenses become thicker and more difficult to
  move neck
• Poor placement of monitors, documents, visual
  targets
• Poor lighting (See ANSI documents)
• Arrange visual targets properly
• Front, distance, below horizontal

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How We Hear

• Energy through pressure waves converted into
  nerve impulses
• Psycho-acoustical perception
• Subjective modification of what the ear reports

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The Hearing Process

• Outer Ear
• Ear drum vibrates according to the frequency
• Sound amplified 10-15 dB

• Inner Ear
• Sound waves propagate along cochlea from
  oval/round window actions
• Basilar membrane deformations, cilia and organs
  of Corti (structure and location determine
  frequency)
• Sound amplified 10-15 dB

• Middle Ear
• 3 vibrating bones (ossicles) transmit vibration
  from ear drum to oval window
• Vibrations 22 times larger than at the ear drum

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Frequency

• Measured in Hertz (Hz)
• Pure tone
• Sound
• 16 Hz 20 kHz
• May feel infrasonic vibrations
• Most sensitive from 2-5 kHz
• 10 kHz or less with age

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Loudness

• Intensity of audible vibration in Pascals (Pa)
• 1 Pa 1 N/m2
• Thresholds
• Minimum 20 mPa (20 dB) from 1-5 kHz
• Pain gt 2000 Pa (140 dB)

• Doubling sound pressure causes increase of 6 dB

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Loudness

• Intensity of audible vibration in dB

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Loudness
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Phon Curves
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Simultaneous Sound
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Acoustic Events

• Directional Hearing
• Distinguish where sound is coming from phase
  differences or intensities
• Distance Hearing
• Determine distance as result sound energy
  (proportional to square of distance) and
  frequency
• Doppler Effect
• Changes in distance changes frequency
• Common Difference Tone
• When common interval (100 Hz) separates several
  tones, perceive additional frequency
• Concurrent Tones
• In-phase, out-of-phase

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Noise

• Unwanted/objectionable sound
• Create changes in mood
• Disturb sleep/rest
• Difficulties in hearing desirable sounds
• Alterations in body chemistry
• Interfere with sensory/perceptual capabilities
• Temporary or permanent change to hearing
  capability

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Intelligibility

• Intensity of speech relative to ambient noise is
  fundamental to speech intelligibility
• Signal-to-Noise Ratio (S/N)
• Difference of competing sounds
• S/N gt 10 dB 80 understanding with normal
  hearing
• Lower S/N, lower intelligibility
• 1000 3000 kHz most important

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Noise Induced Hearing Loss

• Changes in hearing as a result of
• Frequency, intensity, duration,
  continuous/intermittent
• Temporary Threshold Shift (TTS)
• Less acute exposures where normal hearing returns
• Permanent Threshold Shift (PTS)
• Irrecoverable loss of hearing from frequency of
  noise (typically 4000 Hz), then spreads
• Damage to occicles, cilia, organs of Corti,
  auditory nerve

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Age and Hearing Loss

• Reductions with age
• 10 dB _at_ 50
• 25 dB _at_ 60
• 35 dB _at_ 70

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NIHL Prevention

• Avoid Generation
• Most successful strategy
• Design machine parts, rotational velocities, air
  flow, etc to minimize noise
• Leave the Area
• Remove people from noisy areas, or at least for
  noisy parts of shifts (when possible)
• Impede Transmission
• Mufflers, enclose source (keep within operating
  parameters), increase distance, sound absorbing
  material

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NIHL Prevention
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NIHL Prevention

• Hearing protection
• Passive
• Proper fit important
• Ear plugs
• Useful NRR (NRR-7)/2
• Ear muffs
• Particularly good between 500-200 Hz
• Active
• Destructive interference, 180o out of phase

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Warning Signals

• Low frequencies lt 500 Hz
• Diffract easily around barriers
• 1000 4000 Hz
• Not if noise in these frequencies (Harmonic
  frequencies)
• Contrast to noise
• Intensity, frequency, warbling
• 15 dB over masking noise
• Can combine with visual and tactile indicators

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• The End