Audio Engineering

66 Chapter 2

o r

True VU level Instrument indication

Zact

 10

600

log

⎛

⎝

⎜⎜

⎜⎜

⎞

⎠

⎟⎟

⎟⎟⎟

where, apparent level  instrument indication  attenuator or sensitivity indicator.

Thus we can have the following.

1. A direct reading from the face of the instrument (zero preferred).


2. The reading from the face of the instrument plus the reading from the
   attenuator or other sensitivity adjustment—normally a minimum of  4 dB or
   higher. When the instrument indicates zero, the apparent level is the attenuator
   setting.


3. The correction factor for impedance other than the reference impedance. 600 Ω is
   the normal impedance chosen for a reference, but any value can be used so long
   as the voltage across it results in 0.001 W ( Figure 2.16 ).

Example
We have an indication on the instrument of  4 VU. The sensitivity control is at  4. We
are across 50 Ω (a 100-W amplifi er with a 70.7-V output). Using Figure 2.16 , our true VU
would be  4 V U  (  4 VU)  10.8 correction factor  10.8 VU.

Figure 2.15 : Examples of commercial-type VI instrument panels.