acoustics and noise. physics of sound sound is a response to pressure waves = c = 344 m/s @ 20° c...
TRANSCRIPT
Acoustics and Noise
Physics of Sound
• 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?– 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
• 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
⎟⎠
⎞⎜⎝
⎛= ∑
=
N
i
Ltotal
iSPL1
10/10log10
Noise:UnwantedSound
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 90 dB(A)
6 92
4 95
3 97
2 100
1.5 102
1 105
0.5 110
0.25 115
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:– 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:
n
nm T
C
T
C
T
CE +++= ...
2
2
1
1
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– 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
• Then25.1
50.0
25.0
4
2
8
20 =+++=mE
• So the worker was over exposed
Transient Noise
• Report % of time an SL is exceedede.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
• 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)