Acoustics and Noise

Physics of Sound Sound is a response to pressure waves  = c = ° C in air Amplitude: Pressure [N/m 2 ] Intensity: Amplitude squared I = P 2 [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 = log 10 (I/I ref ), where I ref is least audible (usually use W) But this is too large, so we use decibels (dB): 10 log 10 (I/I ref ) Since I = P 2, Sound Pressure Level in dB is SPL = 20log 10 (P/P ref ) P ref generally N/m 2 (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: = 0, so combination is 83 dB –3 engines: = 3, so combination is 84.8 dB –4 engines: = 4.8 dB, so combo is 86 dB

Computational Sound Addition Previous Example: –SPL = 10 log(10 80/ / / /10 ) –SPL = 10 log(4*10 8 ) –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 hrs90 dB(A)

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: where C n = amount of time exposed at a level, & T n =amount of time allowed at that level If E m > 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 Then So the worker was over exposed

Transient Noise Report % of time an SL is exceeded e.g. L 10 = 70 dB(A) means that 10% of the time the noise exceeded 70 dB on the A scale Noise Pollution Level: NPL  L 50 + (L 10 - L 90 ) + (L 10 + L 90 ) 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)