What is TWA?
The time-weighted average always averages the sampled sound over an 8-hour period. TWA starts at zero and grows. The TWA is less than the LAVG if the run time is less than eight hours, and grows higher than LAVG after eight hours. TWA represents a constant sound level lasting eight hours that would result in the equivalent sound energy as the noise that was sampled. Example: Think of TWA as having a large 8-hour container that stores sound energy. If you run a dosimeter for 2-hours, your LAVG is the average level for those 2-hours, consider this a smaller 2 hour container filled with sound energy. For TWA, take the smaller 2-hour container and pour that energy into a larger 8-hour TWA container. The TWA level will be lower. Again, TWA is ALWAYS based on the 8-hour container. When measuring using OSHAs guidelines, TWA is the proper number to report provided that the full work shift was measured. Example: If the work shift is 6.5 hours long, then measure for the entire 6.5 hours. TWA is
