Handbook for Sound Engineers

Test and Measurement 1607

46.1 Test and Measurement

Technological advancements in the last two decades
have given us a variety of useful measurement tools,
and most manufacturers of these instruments provide
specialized training on their use. This chapter will
examine some principles of test and measurement that
are common to virtually all measurement systems. If the
measurer understands the principles of measurement,
then most any of the mainstream measurement tools
will suffice for the collection and evaluation of data.
The most important prerequisite to performing mean-
ingful sound system measurements is that the measurer
has a solid understanding of the basics of audio and
acoustics. The question “How do I perform a measure-
ment?” can be answered much more easily than “What
should I measure?” This chapter will touch on both, but
readers will find their measurements skills will relate
directly to their understanding of the basic physics of
sound and the factors that produce good sound quality.
The whole of this book will provide much of the
required information.

46.1.1 Why Test?

Sound systems must be tested to assure that all compo-
nents are functioning properly. The test and measure-
ment process can be subdivided into two major
categories—electrical tests and acoustical tests. Electri-
cal testing mainly involves voltage and impedance mea-
surements made at component interfaces. Current can
also be measured, but since the setup is inherently more
complex it is usually calculated from knowledge of the
voltage and impedance using Ohm’s Law. Acoustical
tests are more complex by nature, but share the same
fundamentals as electrical tests in that some time vary-
ing quantity (usually pressure) is being measured. The
main difference between electrical and acoustical test-
ing is that the interpretation of the latter must deal with
the complexities of 3D space, not just amplitude versus
time at one point in a circuit. In this chapter we will
define a loudspeaker system as a number of components
intentionally combined to produce a system that may
then be referred to as a loudspeaker. For example, a
woofer, dome tweeter, and crossover network are indi-
vidual components, but can be combined to form a
loudspeaker system. Testing usually involves the mea-
surement of systems, although a system might have to
be dissected to fully characterize the response of each
component.

46.2 Electrical Testing

There are numerous electrical tests that can be per-

formed on sound system components in the laboratory.

The measurement system must have specifications that

exceed the equipment being measured. Field testing

need not be as comprehensive and the tests can be per-

formed with less sophisticated instrumentation. The

purpose for electrical field testing includes:

1. To determine if all system components are oper-
   ating properly.


2. To diagnose electrical problems in the system,
   which are usually manifested by some form of
   distortion.


3. To establish a proper gain structure.

Electrical measurements can aid greatly in estab-

lishing the proper gain structure of the sound system.

Electrical test instruments that the author feels are

essential to the audio technician include:

• ac voltmeter.


• ac millivoltmeter.


• Oscilloscope.


• Impedance meter.


• Signal generator.


• Polarity test set.

It is important to note that most audio products have

on-board metering and/or indicators that may suffice for

setting levels, making measurements with stand-alone

meters unnecessary. Voltmeters and impedance meters

are often only necessary for troubleshooting a

nonworking system, or checking the accuracy and cali-

bration of the on-board metering.

There are a number of currently available instru-

ments designed specifically for audio professionals that

perform all of the functions listed. These instruments

need to have bandwidths that cover the audible spec-

trum. Many general purpose meters are designed

primarily for ac power circuits and do not fit the wide

bandwidth requirement.

More information on electrical testing is included in

the chapter on gain structure. The remainder of this

chapter will be devoted to the acoustical tests that are

required to characterize loudspeakers and rooms.

46.3 Acoustical Testing

The bulk of acoustical measurement and analysis today

is being performed by instrumentation that includes or

is controlled by a personal computer. Many excellent

systems are available, and the would-be measurer