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How We See and Hear

How We See and Hear. IENG 321 – Ergonomics and Human Factors Engineering Spring 2009. 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. Straight or Curved. Lighter or Darker.

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How We See and Hear

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  1. How We See and Hear IENG 321 – Ergonomics and Human Factors Engineering Spring 2009

  2. 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.

  3. Straight or Curved

  4. Lighter or Darker

  5. Lighter or Darker

  6. 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

  7. 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

  8. Line of Sight

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

  10. Stroke width-to-height Sufficient illumination 1:6 to 1:8, white background 1:8 to 1:10, black background Printed characters Low illumination/contrast, 1:5 Width-to-height 3:5 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 Text Characteristics

  11. 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

  12. 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

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

  14. 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

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

  16. 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

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

  18. 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

  19. 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

  20. Loudness • Intensity of audible vibration in Pascals (Pa) • 1 Pa = 1 N/m2 • Thresholds • Minimum = 20 mPa (20 dB) from 1-5 kHz • Pain > 2000 Pa (140 dB) • Doubling sound pressure causes increase of 6 dB

  21. Loudness • Intensity of audible vibration in dB

  22. Loudness

  23. Phon Curves

  24. Simultaneous Sound

  25. 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

  26. 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

  27. 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 > 10 dB ~ 80% understanding with normal hearing • Lower S/N, lower intelligibility • 1000 – 3000 kHz most important

  28. 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

  29. Reductions with age 10 dB @ 50 25 dB @ 60 35 dB @ 70 Age and Hearing Loss

  30. 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

  31. NIHL Prevention

  32. 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

  33. Warning Signals • Low frequencies < 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

  34. The End

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