Handbook for Sound Engineers

36 Chapter 2

must not exceed the peak output capability of any
component in the system. Ironically the peaks have little
to do with the perceived loudness of the signal or the
electrical or acoustic power generated by it. Both of
these parameters are more closely tied to the rms
(root-mean-square) value of the signal. Measurement of
the true rms value of a waveform requires specialized
equipment that integrates energy over a time span, much
like the hearing system does. This integrated data will
better correlate with the perceived loudness of the sound
event. So audio practitioners need to monitor at least
two aspects of the audio signal—its relative loudness
(related to the rms level) and peak levels. Due to the
complexity of true rms monitoring, most meters display
an average value that is an approximation of the rms
value of the program material.

Many audio processors have instrumentation to
monitor either peak or average levels, but few can track
both simultaneously. Most mixers have a VI (volume
indicator) meter that reads in VU (volume units). Such
meters are designed with ballistic properties that
emulate the human hearing system and are useful for
tracking the perceived loudness of the signal. Meters of
this type all but ignore the peaks in the program mate-
rial, making them unable to display the available head-
room in the system or clipping in a component. Signal
processors usually have a peak LED that responds fast
enough to indicate peaks that are at or near the compo-
nent’s clipping point. Many recording systems have
PPM (peak program meters) that track the peaks but
reveal little about the relative loudness of the waveform.

Fig. 2-20 shows an instrument that monitors both
peak and relative loudness of the audio program mate-
rial. Both values are displayed in relative dB, and the
difference between them is the approximate crest factor
of the program material. Meters of this type yield a
more complete picture of the audio event, allowing both
loudness and available headroom to be observed
simultaneously.

2.11 Sound Propagation

Sound waves are emitted from acoustic sources—

devices that move to modulate the ambient atmospheric

pressure. Loudspeakers become intentional acoustic

sources when they are driven with waveforms that cause

them to vibrate at frequencies within the bandwidth of

the human listener. A point source is a device that radi-

ates sound from one point in space. A true point source

is an abstract idea and is not physically realizable, as it

would be of infinitesimal size. This does not prevent the

use of the concept to describe the characteristics of

devices that are physically realizable.

Let us consider the properties of some idealized

acoustic sources—not ideal in that they would be desir-

able for sound reinforcement use, but ideal in respect to

their behavior as predictable radiators of acoustic energy.

2.11.1 The Point Source

A point source with 100% efficiency would produce

1 watt of acoustical power from one watt of applied

electrical power. No heat would result, since all of the

electrical power is converted. The energy radiated from

the source would travel equally in all directions from

the source. Directional energy radiation is accomplished

by interfering with the emerging wave. Since interfer-

ence would require a finite size, a true infinitesimal

point source would be omnidirectional. We will intro-

duce the effects of interference later.

Using 1 pW (picowatt) as a power reference, the

sound power level produced by 1 acoustic watt will be

(2-20)

Note that the sound power is not dependent on the

distance from the source. A sound power level of

LW= 120 dB would represent the highest continuous

sound power level that could result from 1 W of contin-

uous electrical power. All real-world devices will fall

short of this ideal, requiring that they be rated for effi-

ciency and power dissipation.

Let us now select an observation point at a distance

0.282 m from the sound source. As the sound energy

propagates, it forms a spherical wave front. At 0.282 m

this wave front will have a surface area of one square

meter. As such, the one watt of radiated sound power is

passing through a surface area of 1 m^2.

Figure 2-20. A meter that can display both average and
peak levels simultaneously. Courtesy Dorrough Electronics.

Average Peak

LW 10 1W

10 –^12 W

= log-------------------

=120 dB