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)