NOISE HAZARD NOISE HAZARD EVALUATIONEVALUATION

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Purpose of Noise Hazard Evaluation

Measurement of noise levels to determine if they are hazardous to hearing

Hazardous noise is defined as:– > 85 dBA steady state noise over an 8 hour

period– >140 dBP impulse/impact noise

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Purpose of Noise Hazard Evaluation (cont.)

To determine if noise hazards can be eliminated or reduced through engineering controls:– Engineering controls should be the primary

means of protecting personnel from hazardous noise

– All practical approaches to engineering noise out of the work place or reducing noise levels to below hazard criteria should be explored

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Purpose of Noise Hazard Evaluation (cont.)

To determine if noise hazards can be eliminated or reduced through engineering controls:– Engineering controls will be applied to military

unique workplaces within the constraints of maintaining combat readiness

– New equipment being considered for purchase should have the lowest sound emission levels that are technologically and economically possible

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Purpose of Noise Hazard Evaluation (cont.)

To identify and label noise-hazardous areas and equipment

Signs and labels inform workers when it is necessary to wear HPD’s

Exception: labels are not to be placed on combat equipment and tactical vehicles

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Purpose of Noise Hazard Evaluation (cont.)

To enroll noise-exposed personnel in the Hearing Conservation Program – Noise level data is a vital element of the HCP– Absence of noise level data does not preclude

enrolling personnel in the HCP

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Noise Hazard Evaluations may be performed by:

Industrial Hygienists – (primary responsibility)

Audiologists Trained technicians

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Equipment UsedEquipment Used

Sound Level Meter (SLM) – used to screen for noise hazards– if the screening detects

noise levels above 84 dB, a dosimeter is then used to determine individual noise dose

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Equipment Used (cont.)Equipment Used (cont.)

Dosimeter – measures the average decibel exposure level over an 8 hour day

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Types of Sound Level MetersTypes of Sound Level Meters

A. Type 1 – Precision Laboratory Meter Very expensive Allowable variance is +/- 1 dB accuracy

B. Type 2 – General Purpose Allowable variance is +/- 2 dB accuracy

C. Type 1 or Type 2 may be used for hearing conservation purposes

D. Impulse noise measurements require a special meter with peak holding capability

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Weighting Networks in Sound Weighting Networks in Sound Level MetersLevel Meters

“A” Scale– Filters out low frequencies– Response curve is similar to sensitivity of human ear

“C” Scale– Filters out very little (only the extreme low

frequencies)– If a measurement is higher on the C scale than the A

scale, the noise has a low frequency component– Used to estimate the effectiveness of ear protectors

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Operation of a Sound Level Meter

A. Control Switches On /off switch Battery check switch Network selector Fast / slow meter response

selector Attenuator – dB range

selector Display meter

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Operating InstructionsOperating Instructions Check calibration – demonstrate use of calibrator Set weighting switch to dBA Set meter response to slow Adjust meter range switch until response is seen on display Hold the SLM at ear level, close to the worker’s most

exposed ear Read dB level on display meter Record results on NEHC 5100/17214 (Sound Level Survey

Form) or other form containing all the information on the NEHC form

Re-check calibration

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Factors Affecting Validity of Factors Affecting Validity of Sound Level ReadingsSound Level Readings

Weak battery Body baffle effect – microphone held too close to your

body may cause absorption or reflection of sound Shielding effect – occurs when measurer’s body or other

object is located between the sound source and the microphone

Wind noise – wind velocities above 5 mph may affect measurements; microphone windscreen helps reduce wind noise

Humidity / moisture on microphone or inside meter Microphone size – larger microphone (one inch) is best for

measuring a broad frequency range Operator errors

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Inverse Square Law

Doubling the distance from a sound source reduces noise level by 6 dB

This principle is used to define the noise hazard radius

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Care of the Sound Level MeterCare of the Sound Level Meter

Must be electroacoustically calibrated annually

Calibration must be checked both before and after measurements are taken

Kept in a dry, safe place

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Time-Weighted Average

Time-weighted average sound level

– That sound level, which if constant over an 8-hour exposure, would result in the same noise dose as is measured

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Dosimetry: Measures Dosimetry: Measures Time-Weighted AveragesTime-Weighted Averages

Time-Weighted Average – average noise level measured over an 8 hour time period

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Strategies for acquiring TWA’s Strategies for acquiring TWA’s on noise-exposed personnelon noise-exposed personnel

Personal dosimetry Conservative estimate for group based on sample

TWA measurements Area monitoring Calculation based on SLM data (possible but not

practical)

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QUESTIONS???QUESTIONS???