The worker signals when he hears the sound. This must be performed by a state licensed audiologist in a sound proofed booth on a regularly calibrated audiometer. Air Conduction is done by placing ear phones over the ears. This measures how the person hears this sound. Low tone such as 250 and 500 Hertz like a rumble or roar are tested.
Hertz refers to the pitch of the sound.
Hertz are high pitched sounds like a tea kettle. Decibel refers to volume. Zero volume is ideal. 30 decibel indicates a problem with hearing. At 70 db the hearing loss is severe.