Test fixtures meeting the same standard still differ in headphone measurements.

Evidence

In summary, there are significant differences in headphone measurements made on different test fixtures even those that meet the same standard (Fixtures 2 and 4). The flat plate fixtures produce less leakage at low frequencies than measurements made on HATS. The Type 4.3 fixture produced measurements with elevated high-frequency levels compared to Type 3.3 fixtures. Headphones designed to a target curve on Fixture 1 (our reference) will measure differently on another test fixture, and therefore the same target cannot be applied. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1734