Noise and Hearing Loss Basics

Noise Basics

• Noise is considered unwanted sound
• Sound is created by vibrating objects that introduce over and under pressures
• These vibrations travel through the air as pressure waves until they reach the ear
• The ear amplifies sound intensity, producing pressure changes with a wide sensitivity and range
• Noise is subjective and depends on the person and the circumstance
• Hazardous to hearing with excessive volume or extended exposure

The Ear

• Sound waves enter the ear canal and vibrate the eardrum
• This vibration is transmitted through three small bones in the middle ear to the inner ear
• Hairs (stereocilia) in the inner ear transform vibrations into electrical signals
• Auditory nerves then transmit these signals to the brain

Hearing Loss

• A decrease in the rate of hearing loss occurred in the early 90s because of the awareness of the issue and corrective action
• Hearing loss is still an issue today
• There are three methods of hearing loss

Sensorineural Hearing Loss

• This occurs when the cochlea or auditory nerve do not properly transmit signals
• Most pronounced effects are from 4000 Hz
• Individuals have different susceptibility

Conductive Hearing Loss

• This is due to issues in the outer or middle ear
• Conductive hearing loss can be treated but results in deafness if ignored
• Otosclerosis (the overgrowth of bones in the middle ear, affecting vibration) is an example
• Otosclerosis becomes severe within 15 years

Acoustic Trauma

• This is conducive loss is caused by a single loud noise

Units of Sound Pressure Level (SPL)

• Sound pressure level (SPL) is measured in decibels [dB]
• It is converted from sound pressure [Pa] using the formula SPL = 10 log (p/pr)^2 = 20 log (p/pr)
• p = sound pressure [Pa]
• pᵣ = reference pressure, root mean squared (RMS) lowest audible/detectable sound by the ear [pᵣ = 2 x 10^-5 Pa @ 1 kHz]
• 0 dB corresponds to the lowest audible sound measurement

Examples of SPL

• Quite Office is approx. 0.02 Pa or 60 dB
• Jackhammer is approx. 2 Pa or 100 dB
• Pain threshold is approx. 200 Pa or 140 dB
• SPL can also be calculated from sound wave intensity [W/m²]: SPL = 10 log (I/Iᵣ),where Iᵣ = 10^-12 W/m^2
• Sound pressure (p) can be converted to sound wave intensity (I) using: I = p^2/ρv
• ρ = density of the medium
• v = the wave propagation speed [344 m/s in air]

Units of Sound Watt Level (SWL)

• SWL is another referenced measurement
• The formula is SWL = 10 log (W/Wᵣ)
• W = acoustic energy produced per unit time [W] (W = IA, intensity times available area)
• Wᵣ = reference acoustic wattage [Wᵣ = 10^-12 W]

SPL VS Distance

• The distance from the source is important when measuring sound pressure level
• RMS sound pressure varies inversely with distance
• Doubling the distance reduces the sound pressure level by 6 dB
• The inverse square law is applied to the intensity

Simplified Formula for Adding SPL

• It is not as simple as adding the decibel levels
• Intensities add up, so the logarithmic nature of SPLs can complicate the addition

Steps

• Arrange all measurements in order from highest to lowest
• Take the highest two values and get the difference between the two numbers

Calculation Based on Difference

• Subtract measurements using the following relationship
• 0-1 dB; Higher value + 3
• 2-3 dB; Higher value + 2
• 4-7 dB; Higher value + 1
• 8 or more dB; Higher value (+ 0)
• Replace the two values with the new value and repeat for the difference between the new value and the next value in the arranged measurements
• Repeat until there is a single (final) value
• This method gives values of values ± 1 dB
• Error margin is generally acceptable for occupational noise measurement applications

Method 2: Equation or Graph

• More accuracy can be achieved through equation or graph

Formula

• B = 10log10 [(Σn i=1) 10(βi/10)]
• Where n measurements are added, with the sum of each measurement (β₁) giving β

Measurement Systems

• There are 3 weighting systems (A, B, and C)
• Designed to mimic ear response at low, medium and high frequencies

"A" Weighting

• "A" deweights low frequencies
• Primarily focuses on the 1000 – 5000 Hz range, which is the main contributor to hearing damage
• "A" weighting is the most suitable for routine noise surveys

"A" Weighting Scale

• The "A" scale gives an approximate indication of the noise level as it affects the human ear

Sound Level Meters

• Fast response (125 ms) registers rapid changes in sound level
• Slow response (1 s) averages rapid fluctuations to give a steadier reading
• Impulse mode (35 ms, decay time 1.5s) to measure impulse noises

Dosiometers

• Personal dosimeters integrate the sound level meter that is worn for a full shift (placed near the ear)
• Dosiometers are A-weighted and measure average exposure and peak exposure
• Measuring sound frequency helps determine engineering controls as high frequency sound is more easily absorbed

Indicators of a Sound Problem

• Any "yes" answers to the below questions may indicate a sound problem in the workplace
• Do people need to raise their voices?
• Do workers have a ringing in their ears after a shift?
• Do workers need to adjust car radios louder after a shift than before?
• Do long time workers have trouble hearing conversations in noisy environments (with competing noises)?

Noise Thresholds

• The threshold limit (T, minutes) for different SPL – dB (A) can be calculated using: T = 480/ 2^((SPL-85)/3)
• The threshold protects the median population from a loss of < 2 dB at the frequencies of 500, 1000, 2000, and 3000 Hz

Combined Effect of Exposure

• If exposed to multiple decibel levels, the combined effect of exposure can be calculated: D = C1/T1 + C2/T2 +…
• D = the combined effect of exposure, if D > 1 then overexposure has happened
• C₁ = total exposure time at a given noise level
• T₁ = threshold limit exposure permitted at that noise level

Noise Overexposure

• Overexposure to noise can destroy sensory hair cells in the cochlea
• Destruction can progress to nerve fibers, this damage is cumulative and irreversible
• A pneumatic drill at >120 dB can cause damage in minutes

Impulse Noise

• Impulse noise can be damaging through high instant pressures (ex. shotgun, striking a hammer)
• Vibrating or impulse noises can be more damaging than steady noises
• Hearing loss is often gradual and may go unnoticed at first High frequency hearing is the first to go
• Hearing loss is naturally progressed through aging and begins in the 30s and 40s with high frequencies
• Working lower as time passes
• Overexposure can also lead to hypertension and sleeplessness in adults
• Overexposure can lead to learning difficulties and behavioral issues in children

Health Effects

• Overexposure can increase blood pressure
• A perceived doubling in sound energy is a 10-fold increase in intensity
• The smallest noticeable increase (3 dB) actually doubles the energy

Hearing Impairment

• Defined as a loss of 25 dB at 500, 1000, 2000, 3000, 4000 Hz

Shifts in Hearing

• Hearing loss can cause either temporary or permanent shifts in hearing
• Temporary Threshold shift is recovered after the noise is removed - Think about leaving a concert
• Repeated exposure results in a permanent shift and is a function of noise intensity and time

Legislation

• Ontario regulation 381/15 uses a different method of calculation than comparing different SPL to their respective summary limits
• Ontario regulation calculates a single value: An equivalent sound exposure level is the steady sound level in dBA which, if present in a workplace for eight hours a day, would contain the same total energy as that generated by actual and varying sound levels

Formula for Equivalent Sound Exposure Level

• Lex,8 = 10 log10 [ Σn i=1 (ti x 10(0.1 SPLi))/8 ]
• Lₑₓ,₈ is the equivalent sound exposure level in 8 hours
• Σ is the sum of the values in the enclosed expression for all activities from i = 1 to i =n
• I is a discrete activity of a worker exposed to a sound level
• tᵢ is the duration in hours of i
• SPLᵢ is the sound level of i in dBA
• n is the total number of discrete activities in the worker’s total work day

Duty to Protect Workers

• Every employer shall take all measures reasonably necessary to protect workers from exposure to hazardous sound levels
• Protective measures should include engineering controls, work practices, and hearing protection devices
• Sound levels should be measured in the workplace to determine what protective measures are needed without regard to hearing protection devices
• Employers must ensure no worker is exposed to a sound level greater than an equivalent sound exposure level of 85 dBA, Lₑₓ,₈

Circumstances for Hearing Protection

• Employers must protect workers from sound levels greater than the limit without requiring hearing protection devices - Except when:
• Controls are not in existence or obtainable -Controls are not reasonable or practical because of frequency/duration of exposures or nature of process/operation/work
• Controls are ineffective because of temp breakdown
• Controls are ineffective to prevent, control, or limit exposure because of emergency
• A clearly visible warning sign is necessary near areas regularly exceeds 85 dBA if practicable

Training and Instruction

• An employer who provides a hearing protection device must provide adequate training
• Training must include worker instruction in the care and use of the device, limitations, proper fit, inspection, maintenance, cleaning, and disinfection

Hearing Protection Devices

• A hearing protection device must be selected with regard to
• sound level to which worker exposed
• attenuation provided by device
• manufacturer’s info about use and device limitations
• Hearing protection devices shall be used and maintained in accordance with manufacturer’s instructions

Control Program - Common Elements

• Recognize the hazard
• Evaluate the hazard
• Control the hazard
• Monitor the controls
• Evaluate/maintain controls
• Worker training

Hierarchy of Controls

• Elimination – physically remove the hazard, most effective, always attempt first
• Substitution – replace the hazard with machinery or processes
• Engineering controls - isolate people from the hazard
• Administrative controls – change the way people work
• PPE – protect the worker with personal protective equipment

Engineering Controls

• Maintenance of equipment, lubricate equipment, oil bearings
• Isolating processes, place barrier between source and worker, enclose noise source
• Isolate the Operator, enclose worker, move to remote operation
• Use sound absorptive materials

Administrative Controls

• Schedule unit operating time to decrease impact
• Reassign workers to less noisy areas
• Shift scheduling
• Equipment procurement standards
• Training and education
• Policy and procedure for PPE
• Signage

Personal Protective Equipment

• Education and training are required
• the use of PPE
• the need for PPE
• Enforcement of the control measures Controls can be placed at the source, path, or worker, with decreasing effectiveness

Source Controls

• Relocation
• Isolation
• Dampening
• Enclosures
• Muffling
• Substitution
• Improvement of Maintenance

Path Controls

• Absorption
• Barriers
• Enclosures

Worker Controls

• PPE

Absorption

• When sound hits a physical barrier it is reflected at an angle (angle of incidence)
• A perfect reflector is ridged and impervious to air, reduces noise intensity very little
• Sound absorption = frequency(material, frequency, angle of incidence)

Resonant Mufflers

• Utilize a specially designed chamber to induce resonance
• Waves in the exhaust are reflected to cancel out noise
• Must be designed for a specific frequency