c. All of the area inside the contour is considered noise hazardous. Hearing
protection must be worn while inside the contour.
3-20. THE SOUND LEVEL CALIBRATOR
The overall accuracy of sound measuring equipment may be checked by using
an acoustical calibrator such as is shown in Figure 3-11. It consists of a small, stable
sound source that fits over the microphone and generates a predetermined sound level
within a fraction of a decibel. If the meter reading is found to vary from the known
calibration level, the meter may be adjusted to eliminate this error. The acoustical
calibration procedure supplements the electrical calibration incorporated in some meters
to check the gain of all electronic components following the microphone. Sound level
calibrators should be used only with the microphones for which they are intended in
order to avoid errors and microphone damage. Appendix G illustrates representative
procedures for calibrating sound level meters.