Audio Engineering

Measurement 63

VU. The dBm and the VU are almost identical; the only difference is their usage. The
instrument used to measure VU is called the volume indicator (VI) instrument. (Some
users ignore this and incorrectly call it a VU meter.) Both dBm meters and volume
indicator instruments are specially calibrated voltmeters. Consequently, the VU and dBm
scales on these meters give correct readings only when the measurement is being made
across the impedance for which they are calibrated (usually 150 or 600 Ω ). Readings
taken across the design impedance are referred to as true levels, whereas readings taken
across other impedances are called apparent levels.

Apparent levels can be useful for relative frequency response measurements, for example.
When the impedance is not 600 Ω , the correction factor of 10 log (600/ new impedance )
can be added to the formula containing the reference level as in the following equation:

True VU Apparent VU

Z measured

 10.

600

log (2.40)

The result is the true level.

2.19.1 The VU Impedance Correction

When a VI instrument is connected across 600 Ω and is indicating 0 VU on a sine wave
signal, the true level is 4 dB higher, or  4 dBm, instead of 0 dBm or zero level. The
reason this is so is shown in Figure 2.13. The VI instrument uses a 50- μ A D’Arsonval

rms

Peak

Peak

to

peak

Max

neg

One

alteration

One cycle

Max

pos

90 °

0 ° 180 °

270 °

360 °

rms  0.707 peak voltage

rms  0.3535 peak to peak voltage

peak  1.414 rms voltage

peak-to-peak  2.828 rms voltage

Figure 2.12 : Sine wave voltage values. The average voltage of a sine wave is zero.