VU Meter   

A VU or volume unit meter is designed to provide a reasonable way to estimate the loudness of a signal.
It is roughly the average signal level though not precisely.  A VU meter is not recommended as the sole metering method for digital systems - it is too slow to catch transients which may overload the system. A VU meter registers zero level on its scale when the voltage = 0.775 at 1 milliwatt & 600 ohms impedance.  This is usually modified so that 0 (zero) corresponds to 1.23 volts or + 4 dbm.
The ballistic characteristics of the device do not permit it to accurately register transient signals shorter than .33 second duration.  Current practice is to include a peak meter in the design to overcome this.  The sluggish nature of the VU can be a benefit as a peak recording may cause the engineer to be too cautious in the level set as some transients cause very little distortion.  When both peak and Vu are calibrated to the same sine wave signal, the peak meter will read about 6 - 7 db higher with most musical material.
Different procedures call for different monitoring devices: in critical transfer processes such as mastering a peak meter is essential while in public address systems, the ballistics of the VU match our perception of loudness more accurately.