In order to establish the effectiveness of a hearing protector, the following method (with certain modifications) is used by all current standards. Ten subjects are exposed to sounds at 9 different frequencies (tones) and the lowest sound level that can be heard is established. The subjects are tested three times for each frequency, resulting in 27 individual sets of data. Thereafter each subject uses a hearing protector, and the procedure is repeated. The difference between the hearing threshold with and without hearing protector is the attenuation (noise reduction) achieved by a particular hearing protector. The average for all subjects become the mean attenuation, and the variance establishes the standard deviation. The example below shows the data compiled into a chart.