Audio Engineering

62 Chapter 2

2.18 System Gain Changes ..............................................................................................

Imagine a noise generator driving a power amplifi er and a loudspeaker ( Figure 2.11 ). If
the voltage out of the noise generator is raised by 6 dB, what happens?

Reads twice the

voltage6 dB

Four times the

power6 dB

Loudspeaker

AC

high Z

meter

Noise

generator

Power

amplifier

Twice the

S.P.6 dB

Sound

level

meter

Figure 2.11 : Voltage, electrical power, Pw , and sound pressure compared.

Voltage Electrical power LP LW

Doubled Quadrupled Doubled Quadrupled

 6 d B  6 d B  6 d B  6 d B

This means that, in a linear system, a level change ahead of any components results in
a level change for that same signal in all subsequent components, although it might be
measured as quite different voltages or wattages at differing points. The change in level
at any point would be the same. We will work with this concept a little later when we plot
the gains and losses through a total system.

2.19 The VU and the Volume Indicator Instrument ........................................................

Volts, amperes, and watts can be measured by inserting an appropriate meter into the
circuit. If all audio signals were sine waves, we could insert a dBm meter into the circuit
and get a reading that would correlate with both electrical and acoustical variations.
Unfortunately, audio signals are complex waveforms and their rms value is not 0.707
times peak but can range from as small as 0.04 times peak to as high as 0.99 times
peak ( Figure 2.12 ). To solve this problem, broadcasting and telephone engineers got
together in 1939 and designed a special instrument for measuring speech and music
in communication circuits. They calibrated this new type of instrument in units called