Download Auditory System

HUMAN FACTORS ENGINEERING – IEE437/IEE547
Reading Guide for Chapter 5 (Part a)
Auditory System
Readings: Textbook pp. 91 - 119
1. The stimulus for hearing is sound, a vibration (actually compression and rarefaction) of the air
molecules. The acoustic stimulus can be represented by a sound wave.
Physical Entity (sensation)
Frequency
Amplitude (intensity)
Auditory Experience (perception)
Pitch
Loudness (intensity), dB pressure
2. Sound waves are characterized by the number of cycles or pressure changes completed in a
second (frequency). The number of cycles per second is usually denoted by the term, Hertz (Hz).
The range of human hearing is 20 to 20,000 Hz. We hear best at around 1,000 to 3,000 Hz.
3. A decibel is the measure of sound pressure level. The formula for sound pressure level is sound
intensity (in decibels) = 20 log(P1/P2) , where P1 is the sound pressure amplitude we want to
express in dB and P2 is the standard reference level. P2 is fixed at the threshold of hearing under
optimal conditions (a pure tone of 1000 Hz at 20 micro Newtons/square meter).
4. The Ear: The sensory Transducer (How we hear): The hearing experience: traveling wave along
the basilar membrane moves the hair cells sensitive to that frequency. The movement of the
hair cells sends an electrical signal to the brain. What is a transducer?
5. Typical sound levels from different sources are:
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Loudest rock band on record: 160 dB
Pain threshold: 140 dB (approximate)
Heavy auto traffic: 100 dB
Conversation: 60 dB
Soft whisper: 20 dB
Threshold of hearing: 0 dB
6. Sound sustained (8 hours) at a level of 85 – 90 dB has the potential to damage hearing.
7. Loudness doubles with each 10 dB increase in sound intensity.
8. Frequency influences the experience of loudness.
9. Perceived location has to due with the phase shift of the sound wave between the right and left
ears. Does this remind you of binocular disparity in the visual sensory system?
10. Sounds can be masked by other sounds.
 The minimum intensity difference must be at least 15 dB.
*Notes (C) 2005 by Rebecca W. Boren from Wickens, C.D., Lee, J., Liu, Y., and Gordon-Becker, S.E. (2003) An
Introduction to Human Factors Engineering ( 2nd ed.) Upper Saddle Hill, N.J.: Prentice Hall.
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Sounds tend to be masked most by sounds in a critical frequency band surrounding the
sound that is masked.
Low-pitch sounds mask high-pitch sounds more than the converse.
Why is this important to human factors specialists?
11. Implications for auditory alarms.
 Alarm should be at least 15 dB above the background noise, preferably 30 dB to
guarantee it will be heard.
 Must not be above the noise level that can damage hearing.
 Alarm should not be startling. Alarm should rise in intensity.
 Should not interfere with critical speech communication.
 Alarm should be informative (e.g. voice telling you what to do)
 Alarms should not be confusable. No more than 5-6 alarms that must be
distinguished (absolute judgment).
12. Sound localization
 Not quite as good as visual
 Better in the right/left position than up/down or front/back
 Especially helpful when coupled with a visual display (e.g. cockpit)
13. Speech transmission.
 Top-down and bottom-up.
 Masking
 S/N ratio
 Vowels versus consonants
14. Hearing loss
 Aging
 Exposure to noise
 Temporary threshold shift (TTS) – defined as the loss in hearing (threshold shift in dB)
present 2 minutes after the noise has terminated.
 Permanent threshold shift (PTS)
 OSHA standards
15. Enhancing the speech signal
 Redundancy (face to face)
 Choice of vocabulary
 Use of phonetic alphabet (alpha, bravo, Charlie …)
16. Reducing noise in the work place:
 Ear protection (NNR or noise reduction ratio)
 Choice of equipment or tools
 Noise dose meter can be worn by individual workers.
 Focus on the source - environment
*Notes (C) 2005 by Rebecca W. Boren from Wickens, C.D., Lee, J., Liu, Y., and Gordon-Becker, S.E. (2003) An
Introduction to Human Factors Engineering ( 2nd ed.) Upper Saddle Hill, N.J.: Prentice Hall.
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17. When is noise good?
18. Modalities: strengths & weaknesses. When is auditory preferable to visual? When is visual
preferable to auditory?
*Notes (C) 2005 by Rebecca W. Boren from Wickens, C.D., Lee, J., Liu, Y., and Gordon-Becker, S.E. (2003) An
Introduction to Human Factors Engineering ( 2nd ed.) Upper Saddle Hill, N.J.: Prentice Hall.
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