Calculate TWA from Noise Dose: A Guide for Hearing Conservation

Extended exposure to loud noise can degrade a person's hearing over time. For employees in loud working environments exposure to occupational hearing loss is a real risk. The Occupational Safety and Health Administration has mandated hearing conservation programs administered by employers who subject employees to a time-weighted average of 90 decibels or more in an 8-hour shift. Calculation of occupational noise exposure is done by taking direct measurements using a sound-level meter and using a mathematical equation to determine noise dosage. The dosage can be used to determine an employee's time-weighted average to occupational noise exposure through the use of another mathematical formula.

Things You'll Need

• Sound level meter or dosimeter
• Calculator with logarithmic function capabilities
• 29 CFR 1910.95 Appendix A Noise Exposure Computation Table G-16A

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Instructions

• 1

  Take direct readings of sound levels in the workplace using a sound-level meter or dosimeter. Ensure your equipment is properly calibrated and programmed before use. It is important to use the slow response setting in determining sound-level measurements for OSHA compliance. Follow all other set-up guidelines of the equipment manufacturer. Some modern sound-level meters and dosimeters have functions that will automatically calculate sound-level dose and time-weighted average. If the equipment used does not have this capability, readings must be used for mathematical calculations of dose and TWA.

• 2

  Calculate the noise dosage. Noise dosage can be estimated using a noise-dose table as illustrated by the Michigan Mine safety and Health Training Program's website entitled "Noise Basics." For an accurate accounting of noise dosage, the OSHA noise-dosage formula must be used. Noise Dosage is calculated by the formula D=100(C/T) where D equals total dosage, C equals the measured or estimated length of time spent at a certain noise-level reading and T is a set value given by OSHA table G-16A found in the Code of Federal Regulations in Appendix A of the standard for Occupational Health and Environmental Control.

• 3

  Calculate the Time Weighted Average. The TWA is calculated by 16.61 times the base 10 logarithm of the dosage divided by 100 plus 90, as seen in this formula: TWA= 16.61 Log10(D/100) + 90. An estimate of the Time Weighted Average can be found using Table A-1 in the the Code of Federal Regulations in Appendix A of the standard of Occupational Health and Environmental Control as noted above.