Handbook for Sound Engineers

1608 Chapter 46

should select the one that best fits their specific needs.
As with loudspeakers, there is no clear-cut best choice
or one size fits all instrument. Fortunately an under-
standing of the principles of operating one analyzer can
usually be applied to another after a short indoctrination
period. Measurement systems are like rental cars—you
know what features are there; you just need to find
them. In this chapter I will attempt to provide a suffi-
cient overview of the various approaches to allow the
reader to investigate and select a tool to meet his or her
measurement needs and budget. The acoustical field
testing of sound reinforcement systems mainly involves
measurements of the sound pressure fluctuations pro-
duced by a loudspeaker(s) at various locations in the
space. Microphone positions are selected based on the
information that is needed. This could be the on-axis
position of a loudspeaker for system alignment pur-
poses, or a listener seat for measuring the clarity or
intelligibility of the system. Measurements must be
made to properly calibrate the system, which can
include loudspeaker crossover settings, equalization,
and the setting of signal delays. Acoustic waveforms are
complex by nature, making them difficult to describe
with one number readings for anything other than
broadband level.

46.3.1 Sound Level Measurements

Sound level measurements are fundamental to all types
of audio work. Unfortunately, the question “How loud is
it?” does not have a simple answer. Instruments can eas-
ily measure sound pressures, but there are many ways to
describe the results in ways relevant to human percep-
tion. Sound pressures are usually measured at a discrete
listener position. The sound pressure level may be dis-
played as is, integrated over a time interval, or fre-
quency weighted by an appropriate filter. Fast meter
response times produce information about peaks and
transients in the program material, while slow response
times yield data that correlates better with the per-
ceived loudness and energy content of the sound.
A sound level meter consists of a pressure sensitive
microphone, meter movement (or digital display), and
some supporting circuitry, Fig. 46-1. It is used to
observe the sound pressure on a moment-by-moment
basis, with the pressure displayed as a level in decibels.
Few sounds will measure the same from one instant to
the next. Complex sounds such as speech and music
will vary dramatically, making their level difficult to
describe without a graph of level versus time, Fig. 46-2.
A sound level meter is basically a voltmeter that oper-
ates in the acoustic domain.

Sound pressure measurements are converted into

decibels ref. 0.00002 pascals. See Chapter 2, Funda-

mentals of Audio and Acoustics, for information about

the decibel. Twenty micropascals are used as the refer-

ence because it is the threshold of pressure sensitivity

for humans at midrange frequencies. Such measure-

ments are referred to as sound pressure level or LP (level

of sound pressure) measurements, with LP gaining

acceptance among audio professionals because it is

easily distinguished from LW (sound power level) and LI

(sound intensity level) and a number of other LX metrics

used to describe sound levels. Sound pressure level is

Figure 46-1. A sound level meter is basically a voltmeter

that operates in the acoustic domain. Courtesy Galaxy

Audio.

Figure 46-2. A plot of sound level versus time is the most

complete way to record the level of an event. Courtesy

Gold Line.

L10 = 75.1 dB L50 = 72.4 dB L90 = 71.1 dB LMean = 72.9 dB

LMin = 70.4 dB LMax = 95.7 dB Leq = 74.4 dB

09:30 TIME (HH:MM) 15 s/divDuration - 00:01 09:31

4/26/94

Leq

0.1

seconds

130

110

90

70

50

30

Channel A

Preamp

Gain: 24 dB

Weight: A

Output: Off