Do Reference Mics Drift with Age?

Here’s a 8-year-old Behringer ECM8000 compared to a brand new, out of the box ECM8000, both measuring the same nearfield studio monitor.

The slight phase deviation at HF can be attributed to a slight position error of about a tenth of an inch.

Differences this small are absolutely dwarfed by the change you would see by moving the mic one inch.