The difference between free-field and pressure-field measurement microphones is one of the most important and most frequently misunderstood aspects of acoustic measurement. The two microphone types look identical, can have the same nominal sensitivity, and are both described as Class 1 condenser microphones — but using the wrong type in a given measurement introduces a systematic frequency response error that can exceed 3 dB at high frequencies. This error cannot be corrected by calibration after the fact.
The distinction is about acoustic field conditions, not about microphone quality. A free-field microphone is designed to have a flat frequency response when sound arrives from the front (0° incidence) in a progressive plane wave — the condition that exists in free space or outdoors. The microphone body causes acoustic diffraction that modifies the local pressure at the diaphragm. The capsule acoustic design compensates for this diffraction, so the diaphragm pressure matches the free-field pressure at the measurement point. When you place the same microphone in a coupler (closed cavity), the diffraction compensation overcorrects, and the frequency response is no longer flat.
Note that in practice, for measurements below 2 kHz in any field type, the difference between free-field and pressure-field response is negligible — the diffraction correction is only significant at higher frequencies. However, correct type matching is always best practice.
The calibration certificate for a measurement microphone states the IEC 61094-4 field type designation: FF for free-field, PF for pressure-field, or RI for random-incidence. Always confirm this matches the field type of your intended application before deploying the microphone. When requesting ISO/IEC 17025 accredited calibration, specify the field type in your order — calibration is performed in the field type stated on the certificate, and using the microphone in a different field type introduces an error that is not captured in the uncertainty budget.