Transcript SOUND and NOISE

SOUND and NOISE
Chapter 5
Copyright Catherine M.
Burns
1
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
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Tech Details!
 Ear:
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pressure/force to electrical wave transducer
tone
interpretation in the brain
 Sound
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loudness
basically air pressure waves
frequency determines tone, vibrations per second
(Hz), "pitch"
amplitude of pressure variation is intensity
Copyright Catherine M. Burns
3
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
4
The Decibel Scale
 20mPa is reference.
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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
5
Pitch or Tone
 Young people typically 16Hz to 20, 000 Hz
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about 9 octaves
below 16Hz you feel as vibrations
above 20 000 Hz is "ultrasonic", we can't hear
Copyright Catherine M. Burns
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Loudness (versus intensity)
 Loudness is the psychological experience of
sound volume
 Differs from intensity
Figure 5.3
Loudness
Key idea: Very loud
sounds seem even
louder
Intensity (of 1000HZ tone)
Copyright Catherine M. Burns
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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
Copyright Catherine M. Burns
8
Equal Loudness Curves
Fletcher.H. and Munson.W., ``Loudness, its definition,
measurement and calculation,'' J. Acoust. Soc. Am., vol. 5,
pp. 82-108, 1933.
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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
10
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)
Copyright Catherine M. Burns
11
Criteria for Alarm Design
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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
Copyright Catherine M. Burns
12
Other Alarms
 Voice Alarms
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can be confused with speech
can be clearer in meaning
 False Alarms
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people will ignore and distrust the alarm
maybe even turn it off
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13
Sound localization
 Demo
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14
Noise
 unwanted sound
 generally “too loud”
Copyright Catherine M. Burns
15
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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Basic Pure Tone Audiometry
500Hz
1000Hz
2000Hz
3000Hz
1. Calibrate to a test tone (about 1000) at lowest
level person can here
2. Can you hear the tone
Copyright Catherine M. Burns
17
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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18
Permanent Hearing Loss
 Permanent Threshold Shift
 Extensive exposure to noise
 Often high frequencies (e.g. 4000 Hz)
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Age Related Hearing Loss
 worse men than women
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50 years 10dB
60 years 25dB
70 years 35dB
 High frequency losses
Copyright Catherine M. Burns
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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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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)
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8
6
3
1.5
0.5
90
92
97
102
110
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Physiological and
Psychological Effects of Noise
 impaired alertness
 disturbed sleep
 annoyance
 loss of communication
Copyright Catherine M. Burns
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Physiological Effects
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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
Copyright Catherine M. Burns
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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
Copyright Catherine M. Burns
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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
Copyright Catherine M. Burns
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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
Copyright Catherine M. Burns
27
Kinesthetic Senses
 Knowledge of where your limbs are
 Critical when doing tasks without looking
(e.g. touch typing, driving)
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Vestibular Senses
 Sense of acceleration (in the ear)
 Sense of turning and motion
 Key role in motion sickness, vertigo,
simulator sickness
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