User Guide

Sound System Design Reference Manual
In other voltage measurements, dBV refers to
levels relative to 1 volt.
Rarely encountered by the sound contractor will
be acoustical power levels. These are designated
dB-PWL, and the reference power is 10
-12
watts. This
is a very small power indeed. It is used in acoustical
measurements because such small amounts of
power are normally encountered in acoustics.
Peak, Average, and rms Signal Values
Most measurements of voltage, current, or
sound pressure in acoustical engineering work are
given as rms (root mean square) values of the
waveforms. The rms value of a repetitive waveform
equals its equivalent DC value in power
transmission. Referring to Figure 2-8A for a sine
wave with a peak value of one volt, the rms value is
.707 volt, a 3 dB difference. The average value of the
waveform is .637 volt.
For more complex waveforms, such as are
found in speech and music, the peak values will be
considerably higher than the average or rms values.
The waveform shown at Figure 2-8B is that of a
trumpet at about 400 Hz, and the spread between
peak and average values is 13 dB.
In this chapter, we have in effect been using
rms values of voltage, current, and pressure for all
calculations. However, in all audio engineering
applications, the time-varying nature of music and
speech demands that we consider as well the
instantaneous values of waveforms likely to be
encountered. The term headroom refers to the extra
margin in dB designed into a signal transmission
system over its normal operating level. The
importance of headroom will become more evident
as our course develops.
2-8
Figure 2-8. Peak, average, and rms values.
Sinewave (A); complex waveform (B).