human factors

Tomm.

HFchapter5.ppt

Home >Science homework help >human factors

AUDITORY, TACTILE, AND VESTIBULAR SYSTEMS

AHF

PROPERTIES OF SOUND

Sound is the vibration of air molecules

Amplitude - sound pressure perceived as loudness

Frequency - Cycles per second(Hertz) perceived as pitch

Timbre - quality of sound

Which sound has the greatest amplitude?

Which has the highest frequency?

Pressure

Time

DECIBEL SCALE

Sound intensity (dB) = 20 log (P1/P2); where P2 is the threshold of hearing

Source	Intensity	# Times > TOH
Jet at take-off; ear damage likely	140 dB	1014
Threshold of pain	130 dB	1013
Front row of a rock concert	110 dB	1011
Walkman at maximum volume	100 dB	1010
Vacuum cleaner	80 dB	108
Busy street	70 dB	107
Normal conversation	60 dB	106
Quiet office	40 dB	104
Whisper	20 dB	102
Normal breathing	10 dB	101
Threshold of hearing	0 dB	100

OSHA sets legal limits on noise exposure in the workplace. These limits are based on a worker's time weighted average over an 8 hour day. With noise, OSHA's permissible exposure limit (PEL) is 90 dBA for all workers for an 8 hour day. The OSHA standard uses a 5 dBA exchange rate. This means that when the noise level is increased by 5 dBA, the amount of time a person can be exposed to a certain noise level to receive the same dose is cut in half. NIOSH also recommends a 3 dBA exchange rate so that every increase by 3 dBA doubles the amount of the noise and halves the recommended amount of exposure time. Here's an example: OSHA allows 8 hours of exposure to 90 dBA but only 2 hours of exposure to 100 dBA sound levels. NIOSH would recommend limiting the 8 hour exposure to less than 85 dBA. At 100 dBA, NIOSH recommends less than 15 minutes of exposure per day.

PSYCHOPHYSICAL SCALING OF SOUND

1 sone = 40 dB tone of 1,000 Hz; Loudness doubles with each 10 dB increase

The perceived loudness is a function of sound duration and frequency The perception of loudness is related to both the sound pressure level (SPL) and duration of a sound. The human auditory system averages the effects of SPL over a 600–1000 ms interval.[citation needed] A sound of constant SPL will be perceived to increase in loudness as samples of duration 20, 50, 100, 200 ms are heard, up to a duration of about 1 second at which point the perception of loudness will stabilize. For sounds of duration greater than 1 second, the moment-by-moment perception of loudness will be related to the average loudness during the preceding 600–1000 ms. For sounds having a duration longer than 1 second, the relationship between SPL and loudness can be approximated by a power function in which SPL has an exponent of 0.6, while that between SPL and intensity can be approximated by a power function with an exponent of 0.3 (Stevens' power law). More precise measurements indicate that loudness increases with a higher exponent at low and high levels and with a lower exponent at moderate levels.

EQUAL LOUDNESS CURVES

Loudness is affected by sound frequency. Humans are sensitive to sounds between 20 Hz and 20,000 Hz, but most sensitive to 1,000 - 4,000 Hz range. All tones along a contour are equally loud. 1 phon = perceived loudness of a 1 dB, 1000 Hz tone

100 Hz

500 Hz

1000 Hz

2000 Hz

5000 Hz

10000 Hz

20000 Hz

If all of these tones are played at the same volume, then the 2000 Hz tone should appear to be the loudest. The 20kHz tone will not be heard by some people.

The sensitivity of the human ear changes as a function of frequency, as shown in the equal-loudness graph. Each line on this graph shows the SPL required for frequencies to be perceived as equally loud. It also shows that humans with good hearing are most sensitive to sounds around 2–4 kHz, with sensitivity declining to either side of this region. A complete model of the perception of loudness will include the integration of SPL by frequency and the duration of each.[2]

ANATOMY OF THE EAR

Converts sound energy (outer ear) to mechanical energy (middle ear) to electrical nerve energy (inner ear), then sends signal to the brain

OUTER EAR

Pinna
Provides protection to inner parts of ear
Collects sound
Provide information regarding vertical direction
Ear Canal
Collects sound and directs it towards the ear drum

Mechanical energy of sound waves travels down canal – hits ear drum in middle ear and translated into mechanical vibration of ear drum

MIDDLE EAR

Eardrum (tympanic membrane)
Cavity (also called the tympanic cavity)
Ossicles (3 tiny bones that are attached)
Malleus (or hammer) – long handle attached to the eardrum
Incus (or anvil) – the bridge bone between the malleus and the stapes
Stapes (or stirrup) – the footplate; the smallest bone in the body

Conducts and amplifies sound

Sounds waves put ear drum/tympanic membrane in motion. This motion is transferred to the ossicles, putting them in motion. The ossicles transfers the energy to the fluid in the inner ear’s cochlea.

INNER EAR

Oval window – connects the middle ear with the inner ear
Semicircular ducts – filled with fluid; attached to cochlea and nerves; send information on balance and head position to the brain
Vestibular Component
Cochlea – spiral-shaped organ of hearing; transforms sound into signals that get sent to the brain
Hearing Component
Auditory tube – drains fluid from the middle ear into the throat behind the nose

COCHLEA

High frequencies ------------------------------------------------- Low frequencies

PARTS OF THE EAR REVIEW

Pinna – collects sound, helps localization (Holds up glasses)

Tympanic Membrane (ear drum)– at end of ear canal, vibrates to sound pressure (like a drum head)

Ossicles – bones of middle ear that convert sound to mechanical energy.

Malleus (hammer) is the largest bone and receives vibration from ear drum, which then strikes the Incus (anvil), which is hinged to the smallest bone, the Stapes (stirrups), which presses on the Oval Window of the cochlea.

Cochlea – “snail-like organ” where mechanical energy is transduced to electrical nerve energy, by way Hair Cells along the waving Basilar Membrane that “fire” when they are bent against the rigid Tectorial Membrane of the Organ of Corti, which sends a signal along the Auditory Nerve to the brain.

ALARMS

Criteria for good alarms:

Must be heard above background noise (approx 30 dB above)

Avoid excessive intensity
Should not be above the danger level for hearing (85-90 dB)
using a very different frequency may help (esp if conflicts with crit #1)

Should not be too startling

Should not disrupt processing of other signals
Do not want alarm to mask speech or other important signals

Should be informative, not confusing
Should communicate the appropriate actions

PROPERTIES OF GOOD ALARMS

Increases likelihood of Detection

Decreases likelihood of Distraction

ALARM DESIGN

Conduct environment/task analysis – must understand what sounds/noises (and their qualities) are associated with the job
Make sure alarms are within human’s capability of discrimination by varying on different dimensions:
Pitch (low to high), Envelope (rising/falling pitch), Timbre (quality), and Rhythm (synchronous vs. asynchronous)
Design specific qualities of sound
For example: Use pulses to create unique sound and to give perception of an approaching, then receding sound to create sense of urgency
Establish repeating sequence
After initial alert, may be less intense

VOICE ALARMS

Advantages

Meaning is obvious, minimized confusion regarding meaning

Disadvantages

Less discriminable from background of voice communications
More susceptible to masking
Care must be taken in multilingual environment so not misunderstood

Best when used redundantly with other non speech alarm sound features

“Fly to” vs. “fly through” – impact of confusion could be huge

FALSE ALARMS

Cry Wolf Syndrome – Human operator fails to respond to alarm due to the large number of false alarms in the past.

To avoid “Cry Wolf Syndrome”:

Set the alarm criterion to be sensitive enough to minimize misses, without increasing false alarms.
May use more complex algorithms to determine true threshold.
may use more than one signal measure
Train operators on the tradeoffs of false alarms/misses
understand actual false alarm rates
Use multiple alert levels (denote different urgency states)

SPEECH PERCEPTION

McGurk Effect – demonstrates top down processing of speech and the importance of redundant visual information for perception

Speech communication measures:

Articulation Index (bottom up) – signal to noise ratio
(speech dB – background noise dB)
Higher frequencies are more vulnerable to being masked by noise
Speech Intelligibility Index (top down) – percentage of items correctly heard

OCCUPATIONAL NOISE

Dangers of excessive noise:

Hearing loss – caused by exposure to loud noises. Some hearing loss is expected with age (higher freqs)
Loss of sensitivity while noise is present
Temporary Threshold Shift (TTS) – Loss of hearing that lingers after noise is terminated (post-rock concert)

- Tinnitus or ringing in the ears

- 100 dB for 100 min causes a 60 dB TTS

Permanent Threshold Shift (PTS) – Occupational Deafness caused by long term exposure (esp high freqs)

NOISE REMEDIATION

Signal Enhancement – increase the signal to noise ratio (make signal louder relative to background)
Noise Exposure Regulations – OSHA standards based on Time Weighted Average (calculated with dosemeter)
if TWA > 85 dB (action level) employer must provide hearing protection
if TWA > 90 dB (permissible exposure level) employer must take noise reduction measures
The Source – Select equipment and tools that have built in sound dampening
The Environment – Use sound attenuating or sound absorbing materials to reduce transmission and reverberation
White Noise – Humming noise used to mask distracting sounds
The Listener – Ear protection such as earplugs (internal) or earmuffs (external)

VESTIBULAR SYSTEM

Vestibular System – detects acceleration forces, maintains upright posture/balance and controls eye position relative to head

Semicircular Canals – detect angular acceleration (rotation) in 3 axes

- a crista embedded in a jelly-like material (cupola) is supported by hair cells that bend and fire when the crista moves in response to head rotation.

Vestibular Sacs (Utricle & Saccule) – detect linear acceleration

- hair cells embedded in jelly-like substance lag behind when the head moves. When motion becomes steady, otoliths catch up and hairs no longer bent.

MOTION DISTURBANCES

Spatial Disorientation – vestibular illusion which tricks the brain into thinking body is a different position than it actually is.

Vection – the illusion of self-motion induced my visual cues

Motion Sickness – nausea, disorientation and fatigue attributed to disturbance of vestibular system caused when vision and inner ear send conflicting (decoupled) signals

To experience seasickness without leaving home click on this picture:

Treatments –

Medications – Antihistamines (Dramamine), Dopamine blockers or anti-psychotics (Thorazine), anti-nausea (serotonin) and Scopolamine (anticholinergic)
Behavioral strategies – sit facing front with front window view, eat bland foods such as bread, bananas, rice. If on a boat, stay in middle (less rocking) and look forward at the horizon, not at the waves.

SOPITE SYNDROME

Sopite Syndrome – motion induced drowsiness

Subset of motion sickness symptoms, but sometimes the sole manifestation
Dangerous because victims often not aware of its onset or the likelihood of onset
Found to affect passengers and operators of cars, trucks, ships, helicopters, planes, and simulators
No known prevention techniques (many motion sickness medications increase drowsiness)
May be a major cause of accidents and military pilot pilot training washout

SENSE OF TOUCH:
TACTILE AND HAPTIC

Tactile – Cutaneous or somatosensory sense provided by receptors just under the skin.

Types of Receptors:

Thermoreceptors – detect heat/cold

Mechanoreceptors – detect pressure

Nociceptors – detect noxious stimuli (caustic substances)

Haptic – Shape information provided through manipulation of fingers

Human factors application of haptic research

This device provides haptic information to aid in performing a tracking task. The user feels the button pop out and must move the stick in the same direction to maintain course.

HAPTIC RESPONDING EXPERIMENT

VISION SUBSTITUTION SYSTEM

White, Saunders, Scadden, Bach-y-Rita, & Collins’ (1970) Vision substitution system converts camera image to pattern of vibration on user’s back. Subjects are able to discriminate a wide variety of different stimulus patterns and perceive relative distance.

Human factors application of tactile research

TACTILE SITUATION AWARENESS SYSTEM

Link to Tactile Research Laboratory: http://www.princeton.edu/~rcholewi/TRLindex.html

Tactile stimulation used to prevent spatial disorientation

Human factors application of tactile research

PROPRIOCEPTION & KINETHESIS

Proprioception – Receptors in the limbs provide information of limb position in space.

Kinesthesis – Receptors in the muscles provide information about limb motion.

This subject’s proprioception and vision are providing conflicting information about his limb position. This not only makes this stacking task difficult, but could lead to motion sickness symptoms.

304050607080

Intensity of 1000 Hz tone (dB)

Loudness (sones)