Extech 407744 94dB Sound Calibrator for 0.5" and 1" Microphones Calibrates and verifies Sound Level Meter operation For use on Sound Level Meters with 0.5" or 1.0" microphones Total Harmonic Distortion (THD) &lt;2% at 94dB Meets IEC standard 60942 1997-11 Battery test position with battery status LED indicates if battery is good