A reference sample tells you what the design does. It does not tell you what the unit in the box does.
The rig
A coupler, a calibrated microphone and forty seconds per unit. The curve is printed on a card with the serial number on it, and the same file goes into the record against that serial.
What it catches
About one unit in three hundred is outside tolerance, almost always from a pad seated badly at assembly. Without the rig, that unit ships and comes back as «sounds thin on the left» — which is unreproducible on a reference sample and infuriating for everyone.
