A VU meter/PPM meter in a professional environment has to be tested for response time - there are tone generators to do this.
A VU Ballistics check is a sine wave at -12dBFS (or other reference level if it's different in your studio) gated on and off at 300ms intervals. For a VU meter to be passed, it's got to stay within 1dB of 0 on the scale.
A PPM ballistics check I think is on for 10ms and off for 1000ms (1 sec), I don't remember the exact "pass" criteria (I think it's got to JUST slip into the yellow band and not 0 out before the next pulse, but don't quote me on that).
VU meters register perceived loudness, PPM meters register peak performance.
So, if we all had these meters, we'd all be testing them on a daily basis to make sure they were accurate.
Bren R.
