Question for everyone. Usually I use both the delta mass and delta compliance methods unless the driver is unusually large and heavy ( subs), then delta compliance is the best bet as the 25% shift would take a brick. The most recent drivers I have been playing with happen to be small units with phase plugs. On the Dayton rs150 series, my measurements were within 7% between methods. For vas, not bad. Thought nothing of it. Measuring the Zaph units I seem to be having more difficulty with repeatable measurements. Could the leakage around the voice coil out the gap of the phase plug be an issue?