620 Chapter 17
cient to be recognized by a listener as comprising
distinct multiple events, but it is enough to have audible
and undesirable effects on a loudspeaker’s amplitude
response, as well as on its ability to reproduce transient
signals. Even when an axis or plane exists in which
signal synchronization has been achieved, positions off
the axis or outside of the plane will not receive the
benefits of such synchronization.
The subject of complex addition of time-varying
signals is beyond the scope of this chapter, but it is dealt
with in many introductory circuit analysis texts. Addi-
tional effects of signal synchronization, and the lack
thereof, on loudspeaker response are illustrated in the
section on crossovers.
There are a number of ways in which the problems
caused by noncoincident transducer locations may be
addressed by a loudspeaker designer. One common
approach is simply to assert that the response anoma-
lies caused by this configuration are not audibly signifi-
cant and to accept (or avoid acknowledging) their
presence. Another is to employ crossover filters with
very steep slopes so as to minimize the frequency range
over which anomalies due to path length differences
will be present. As will be shown in the crossover
section, the latter technique has its own set of draw-
backs and may in some cases create more serious prob-
lems than it solves.
One means of addressing the synchronization issue
is with a coaxial loudspeaker. This type of loudspeaker
is most often two-way, although it is possible to design
a three- or four-way coaxial system. There are benefits
in making the midrange and high-frequency compo-
nents of a three-way system coaxial, while leaving the
low-frequency portion displaced in the more conven-
tional manner.
A coaxial loudspeaker will always possess symmet-
rical response behavior, Fig. 17-43. That is, the
response at a given angle from its axis will be mirrored
at the same angle in the opposite direction. Additionally,
if the acoustic path lengths from transducer to listener
are equal on the system’s axis, it is possible to preserve
this synchronization at all listening positions with a
coaxial design. Even though it is possible to achieve
signal synchronization over a wide angular range with a
coaxial loudspeaker, this possibility is not always real-
ized in practice. When a coaxial loudspeaker fails to
achieve coincident performance (i.e., it fails to behave
as a single full range radiator), its sole distinction as
compared to more conventional configurations is that
frequency-dependent anomalies related to crossover
interactions will be symmetrically located about the
loudspeaker’s axis.
Fig. 17-44 illustrates some of the effects caused by
displaced transducers. The frequency selected for
display is the closest -octave band center to the
crossover frequency of the speaker. Figure 17-45 shows
the improved polar behavior that can be produced with
a coaxial loudspeaker, again with the -octave band
center chosen so as to most closely match the crossover
frequency. Note that the polar pattern of this speaker is
still not perfectly symmetrical, even though the trans-
ducers are coaxial. This is due to asymmetric place-
ment of the coaxial woofer/tweeter assembly within the
enclosure. The effects of enclosure design on loud-
speaker performance will be covered in more detail in
the section on acoustic boundaries.17.8.4 Line ArraysAnother type of loudspeaker system is a line array.
Although line arrays have much in common with other
types of loudspeaker systems, they have some attri-
butes that are unique enough to justify their separate
treatment. A line array may form a complete full-range
loudspeaker or one or more bands thereof. In a line
array, individual radiators are arranged in a straight line
or an arc segment. It is also possible for a number of
complete loudspeaker systems to be configured as a line
array. It is this configuration that has come into fashion
in recent years. In the simplest form of line array, each
of the elements—usually a small cone transducer—is
supplied an identical full-range signal. This type of
array, also called a sound column, was popular in thisFigure 17-43. Small two-way coaxial loudspeaker.
Courtesy Frazier Loudspeakers.(^1) » 3
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