1. Copyright Catherine M. Burns
SOUND and NOISE
Chapter 5
Copyright Catherine M. Burns

2. Copyright Catherine M. Burns
Anatomy of the Ear
White: ear drum (tympanic membrane)
Beige: hammer and anvil
Yellow: cochlea
Blue/Yellow/white: semi-circular canals
Teal: auditory nerve
Source: internet “earspin”
Copyright Catherine M. Burns

3. Copyright Catherine M. Burns
Tech Details!
Ear:
pressure/force to electrical wave transducer
interpretation in the brain
Sound
basically air pressure waves
frequency determines tone, vibrations per second (Hz), "pitch"
amplitude of pressure variation is intensity
tone
loudness
Copyright Catherine M. Burns

4. Copyright Catherine M. Burns
Intensity/Volume
really measured in pressure units (Pa)
human range is from 20mPa to about 20, 000 Pa (one million times more)
max idea is jet engine
large range so use a log scale, decibel scale
Copyright Catherine M. Burns

5. Copyright Catherine M. Burns
The Decibel Scale
20mPa is reference.
every increase of x10, is addition of 20dB
Sound pressure level (dB) = 20 log (P/Pref) for absolute measures where Pref = 20mPa
So absolute sound intensity (dB) of P = 20 log (P/20mPa)
See Table 5.1 for example sounds and their volume
Copyright Catherine M. Burns

6. Copyright Catherine M. Burns
Pitch or Tone
Young people typically 16Hz to 20, 000 Hz
about 9 octaves
below 16Hz you feel as vibrations
above Hz is "ultrasonic", we can't hear
Copyright Catherine M. Burns

7. Loudness (versus intensity)
Loudness is the psychological experience of sound volume
Differs from intensity
Figure 5.3
Key idea: Very loud sounds seem even louder
Loudness
Intensity (of 1000HZ tone)
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8. Copyright Catherine M. Burns
Loudness and Pitch
generally high pitched sounds sound louder
most sensitive range is about 4000Hz
dB(A) weights sounds by pitch to reflect psychological loudness
Human speech: vowels below 1000Hz, consonants higher frequency
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9. Copyright Catherine M. Burns
Equal Loudness Curves
Fletcher.H. and Munson.W., ``Loudness, its definition, measurement and calculation,'' J. Acoust. Soc. Am., vol. 5,
pp , 1933.
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10. Copyright Catherine M. Burns
Masking
Sounds can be masked by other sounds
Minimum intensity difference to ensure a sound is heard is 15dB above the masking sound
Sounds in the same frequency band are masked
Low pitch sounds mask high pitched more than the reverse.
Copyright Catherine M. Burns

11. Copyright Catherine M. Burns
Alarm Design
Auditory signals are used for alarms because they don’t require orientation to be heard
People can’t “close their ears”
Auditory alarms should be reserved for highly critical events, affecting multiple personnel (fire alarms)
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12. Criteria for Alarm Design
must be heard over the background (15dB more minimum, usually 30dB is suggested)
cover different frequencies to avoid masking (chord alarms)
shouldn’t exceed 85-90dB (dangerous levels)
avoid startling people
not interfere with communications, other alarms
be informative
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Other Alarms
Voice Alarms
can be confused with speech
can be clearer in meaning
False Alarms
people will ignore and distrust the alarm
maybe even turn it off
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14. Copyright Catherine M. Burns
Sound localization
Demo
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15. Copyright Catherine M. Burns
Noise
unwanted sound
generally “too loud”
Copyright Catherine M. Burns

16. Noise Induced Hearing Loss
slow progressive degeneration of cells in the inner ear
increases with intensity and repetition
high frequency and intermittent is worse
usually starts at 4000 Hz and moves to lower frequencies
How measured - "pure tone audiometry" - progressively trying tones and adjust volume level
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17. Basic Pure Tone Audiometry
500Hz
1000Hz
2000Hz
3000Hz
Calibrate to a test tone (about 1000) at lowest level person can here
Can you hear the tone
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18. Temporary Hearing Loss
hearing returns to normal, temporary threshold shift
begins at 80-90dB, causes 8-10dB shift
also affected by duration
100dB sound for 10min shifts 16dB, 100 minutes, 60 dB
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19. Permanent Hearing Loss
Permanent Threshold Shift
Extensive exposure to noise
Often high frequencies (e.g Hz)
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20. Age Related Hearing Loss
worse men than women
50 years 10dB
60 years 25dB
70 years 35dB
High frequency losses
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21. What life is like if you have hearing loss
Normal Speech
Loss at 2000 Hz (2000Hz_9)
Typical of noise exposure
Loss at 4000 Hz (4000Hz_9)
General loss due to middle ear infection (mild_hl)
Masking effects of noise
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22. Noise Level Guidelines
ISO standards - considers anything above 90db(A) to be damaging
90dB must reduce noise, 85dB must provide ear protection
Equivalence over duration
Hours dB(A)
8 90
6 92
3 97
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23. Physiological and Psychological Effects of Noise
impaired alertness
disturbed sleep
annoyance
loss of communication
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24. Physiological Effects
increased blood pressure
accelerated heart rate
contracted blood vessels on the skin
slowed digestion
increased muscular tension
waking people from sleep - connect with circadian rhythms
research is unsure whether people adapt to noise or become increasingly sensitive about it
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25. Designing to reduce noise
recommending ear protection
designing quieter equipment
designing buildings and surfaces that don't propagate noise
sound absorption
enclosing the noise source
acoustic tiles
Designing to avoid masking
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26. Ear Protection Solutions
Ear plugs - can reduce about 30dB
Ear muffs - about 40dB
Problems though - workers can't hear other workers
don't like wearing them
sound reduction is somewhat isolating
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27. Copyright Catherine M. Burns
Touch/Haptic
Alternative form of information (force feedback mouse)
Identification of shape, texture
Alerting when sounds can’t be used (cell phones that vibrate)
Braille
Could be used more powerfully
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28. Copyright Catherine M. Burns
Kinesthetic Senses
Knowledge of where your limbs are
Critical when doing tasks without looking (e.g. touch typing, driving)
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29. Copyright Catherine M. Burns
Vestibular Senses
Sense of acceleration (in the ear)
Sense of turning and motion
Key role in motion sickness, vertigo, simulator sickness
Copyright Catherine M. Burns