A calibration is only as precise as the standard against which it is compared—and the coupling between reference and device under test. The incorruptible reference in our test bench is a G.R.A.S. 40AE, a long-term stable 1/2-inch precision condenser microphone of Class 1 (according to IEC 61094-4) with an exactly calibrated sensitivity of 54.94 mV/Pa.
The crucial difference in our setup: Many basic calibration services use the substitution method—they measure the reference microphone first, remove it, insert the customer's microphone, and hope that neither temperature, contact pressure, nor speaker level changed in the meantime.
By contrast, we use a true simultaneous two-channel comparison method: Our symmetrical measurement chamber features two identical 1/2" mounts. The G.R.A.S. 40AE and your ECM8000 (DUT) sit inside the same chamber at the same time and experience the exact same sound field. The Audio Precision APx555 captures both channels phase-synchronously in a single measurement sweep.
| Advantage of Simultaneous Measurement | Physical Benefit for Your .cal File |
|---|---|
| No Time or Temperature Drift | Because reference and DUT are measured in the exact same second, room temperature fluctuations or amplifier warm-up have zero influence. |
| Elimination of Source Non-Linearities | Driver non-linearities inside the pressure chamber cancel out completely via real-time differential calculation (Channel A vs. Channel B) inside the APx555. |
| Exact Sensitivity (mV/Pa) | Thanks to the fixed 54.94 mV/Pa reference, you receive not just a relative frequency curve, but the true absolute transfer factor of your unit for accurate SPL measurements. |