Sound Level Meter. A precision sound level meter meeting all the requirements of American National Standards Specification for Sound Level Meters ANSI S1.4—1971, or later revision, for a type 1 or a type S-1A instrument shall be used. The sound level meter shall be calibrated and certified annually with a method traceable to the U.S. National Bureau of Standards to verify its performance within design tolerances. Acoustical Calibrator. An acoustical calibrator of the microphone coupler-type shall be used for calibration of the sound level meter. The frequency of the calibration signal shall be 1000 Hz, ±5 percent. The calibrator shall be adjusted and certified annually with a method traceable to the U.S. National Bureau of Standards to verify its correct performance within ±0.5 dB. Microphone Windscreen. A windscreen of the type recommended by noise meter manufacturers shall be used for all measurements to protect from wind noise. Installation of the windscreen may not cause a change in the sensitivity of more than ±.05 dB in the frequency range from 50 Hz to 5KHz or more then ±2 dB in the frequency range from 5 KHz. It is sufficient that this compliance be demonstrated only at the annual system calibration and certification. Anemometer. An anemometer, which has a steady-state accuracy within ±10 percent of any reading above 10 mpg (16 km/h), may be used to determine the wind velocity prevalent at the time of measurement.