Obtaining accurate results using a microphone calibrator requires a controlled interface between the measurement microphone tip and calibrator cavity. Professional and laboratory equipment have precise specifications for measurement microphone dimensions ensuring this interface is maintained properly. Less expensive mics do not adhere to the dimensional precision. Examples inches - outside diameter of microphone tip: 0.520 - ACO Pacific 7046 0.525 - Josephson C550H 0.422 - Dayton emm6 0.236 - typical 6 mm omni capsule from digikey Without a proper fit into the calibrator cavity resulting sound pressure experienced by the microphone may easily be 3 dB or more in error. From the examples above Dayton's emm6 is 0.10 inch small and a typical 6 mm mic capsule is going to need an adapter. ACO Pacific 7046 properly inserted into ND9 calibration cavity: Data is accurate as expected. Dayton emm6 has a loose fit to ND9 calibration cavity: Measurement is 97.12 dB when it should be 93.98: A shim will be improvised to fit the Dayton emm6 properly into the ND9. This strip of gaffer tape is approximately 6 to 7 cm in length. A post-it note could be used as an alternative with only a minor degradation in result obtained. Be very careful to keep adhesives from contaminating the microphone calibrator cavity. Note the snug fit now achieved between emm6 and ND9 emm6 properly sealed into ND9 cavity. A result that is within 0.02 dB of the expected measurement.