Download 7a. Noise

Acoustics and Noise
Physics of Sound
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Sound is a response to pressure waves
 = c = 344 m/s @ 20° C in air
Amplitude: Pressure [N/m2]
Intensity: Amplitude squared I = P2 [Watts]
Fourier Theorem
Anatomy of the human ear
• External Ear
– Pinna
– Auditory Canal
• Middle Ear
– Tympanic Membrane (eardrum)
– Hammer (Malleus)
– Anvil (Incus)
– Stirrup (Stapes)
– Round Window Membrane
Anatomy of the human ear
(cont’d)
• Inner Ear
– Semicircular canal
– Cochlea and Basilar Membrane
– Auditory Nerves
Anatomy of the human ear
(cont’d)
Psychophysics of Sound
• We have a non-linear perceptual response to sound
pressure waves
• Intensity Level [Bels]: IL = log10(I/Iref), where Iref
is least audible (usually use 10-12 W)
• But this is too large, so we use decibels (dB): 10
log10(I/Iref)
• Since I = P2, Sound Pressure Level in dB is SPL =
20log10(P/Pref)
• Pref generally 0.00002 N/m2 (threshold of hearing)
Frequency Components
• Sound is made up of a combination of
frequency components (Fourier Theorem)
• Pitch is perceptual response to frequency
• Human hearing is not uniformly responsive
to all frequencies
Adding dBs from Sound Sources
• E.g.: To combine 80 & 75 dB, difference (5 dB)
intercepts curve at 1.2 dB, so sum is 81.2 dB
Sample Problem
• A jet engine produces 80 dB SPL as heard at 50
feet. What is the heard SPL as each engine is
turned on?
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1 engine: 80 dB
2 engines: 80 - 80 = 0, so combination is 83 dB
3 engines: 83 - 80 = 3, so combination is 84.8 dB
4 engines: 84.8 - 80 = 4.8 dB, so combo is 86 dB
Computational Sound Addition
 N Li /10 
SPLtotal  10 log  10
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 i 1
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• Previous Example:
– SPL = 10 log(1080/10 + 1080/10 + 1080/10 + 1080/10)
– SPL = 10 log(4*108)
– SPL = 86 dB
• More precise, and easy to use for arbitrary
numbers and values of sound sources
Noise:
Unwanted
Sound
Loudness
• Noise measured in terms of “equal
loudness”
• Compared to a reference of 1000 Hz tone:
– If 65 dB SPL tone at 50 Hz is equally loud as
40 dB 1000 Hz tone, then it has loudness level
of 40 phons
• Can draw equal loudness contours as f()
Equal Loudness Contours
Sound Levels
• We measure sound levels with reference to
equal loudness contours corresponding to
three scales:
A: from 40 phon contour
B: from 70 phon contour
C: essentially flat response
• Report results in dB(A), dB(B), dB(C)
Effects of noise
• Psychological
– startle, annoy
– disrupt concentration, sleep, relaxation
• Interference
– disrupts speech communication,  safety
• Physiological
– hearing loss
– aural pain
Health Effects
• Acute damage to eardrum through very
loud, sudden noises
• Chronic damage to inner ear (hair cells of
cochlea):
– Threshold shift in impacted frequencies
– Temporary or Permanent
Audiograms
• Any graph of auditory sensitivity as a function of
frequency
• By convention, increasing intensity of threshold
(decreasing sensitivity) is plotted downwards
• Hearing Level (HL): Correcting minimum SPL for
reference as a function of frequency
– e.g. if 0-ref level for 1000 Hz is 7 dB, then 0 dB
HL is 7 dB SPL
• So in HL, a “normal” audiogram is flat
Permanent Threshold Shift in
Textile Workers
OSHA Maximum Permissible
Industrial Noise Levels
• Protection required when sound levels on A
scale of SLM at slow response exceed:
8 hrs
6
4
3
2
1.5
1
0.5
0.25
90 dB(A)
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95
97
100
102
105
110
115
Hearing Conservation Programs
• 8-hour PEL TWA is 90 dBA, but
• If 8-hour TWA exceeds 85 dBA, a Hearing
Conservation Program must be implemented:
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Noise monitoring
Audiometric testing
Hearing protection
Employee training and tracking
Recordkeeping (2 years req’d - more is prudent)
Dose Calculation
• Mixed exposure based on linear equation:
Cn
C1 C2
Em 

 ... 
T1 T2
Tn
where Cn = amount of time exposed at a level,
& Tn=amount of time allowed at that level
• If Em> 1, an over exposure has occurred
Dose Example
• Suppose an employee is exposed to
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85 dB(A) for 3.75 hours
90 dB(A) for 2 hours
95 dB(A) for 2 hours
110 dB(A) for 0.25 hours
• Then
2 2 0.25
Em  0   
 1.25
8 4 0.50
• So the worker was over exposed
Transient Noise
• Report % of time an SL is exceeded
e.g. L10 = 70 dB(A) means that 10% of the time
the noise exceeded 70 dB on the A scale
• Noise Pollution Level:
NPL  L50 + (L10 - L90) + (L10 + L90)2/60
Transient Noise Survey
Sources of Noise
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Vibrating machinery
Combustion processes
Movement of air
Collision of materials
Noise Measurements
• Sound Level Meter
– Area, grab
– Used for noise surveys (typically annual)
• Noise Dosimeter
– Personal, continuous (integrated)
Sound Level Meters
• Mike  Amp  Freq. Filters  Meter
• Measure immediate sound level
• Noise dosimeter: variation to permit sound
measurement over work period
Engineering Controls
• Maintenance of machinery to eliminate
vibration due to worn parts
• Lubrication
• Fluid damping
• Fan blade redesign
• Room design and layout
• Barriers
Administrative Controls
• Limited exposure time
• Training in use of engineering controls
• Regular hearing tests for exposed
individuals
Personal Protective Equipment
• Earplugs
• Earmuffs
• Enormous employee resistance due to
discomfort
• Safety concerns (e.g. inability to hear
approaching forklift)