1244 Chapter 34
may be obtained by using a directional microphone (i.e.,
a cardioid pattern microphone) and/or by using a direc-
tional loudspeaker (a horn-type loudspeaker or line
array). This only occurs if the D 1 is less than the critical
distance Dc. (Dc is discussed in Section 34.2.3.2.4 and is
only important in an indoor environment.)
A cardioid microphone could provide as much as
6 dB of additional gain before feedback if the rear of the
microphone were pointed directly at the loudspeaker, as
often happens with foldback stage monitor loud-
speakers. The more typical case of a microphone at a
podium and an overhead loudspeaker is a much less
favorable arrangement since the side, not the rear, of the
microphone will be pointed at the loudspeaker and D 1 is
at or near Dc, providing 1 or 2 dB of additional gain
before feedback at best. Because the microphone’s
cardioid pattern varies with frequency (it is nearly
omnidirectional at low frequencies), even this 1 or 2 dB
of feedback reduction may be optimistic. Thus, while a
directional microphone may provide some additional
gain before feedback, it’s best to plan the system as if an
omnidirectional microphone were to be used and take
any additional gain provided by a cardioid microphone
as a welcome bonus.
A directional loudspeaker may also provide some
additional gain before feedback. For a horn-type loud-
speaker pointed at the farthest listener, for example, the
microphone will be off-axis of the horn, and the sound
level at that off-axis angle may be 6dB or more
(down) compared to the on-axis level. This could theo-
retically provide an additional 6 dB gain before feed-
back. By using a highly directional horn, this 6 dB
might be increased to 10 dB or even more. The fault
with this theory is that there will almost always be a
near-fill loudspeaker aimed at listeners near the micro-
phone. This loudspeaker then becomes the limiting
factor in the feedback loop. Even when the nearest
listeners are far enough away from the microphone that
the near-fill loudspeaker can be aimed well away from
the microphone, the system woofer remains a potential
feedback problem-causer. A highly directional
low-frequency horn is physically very large and, thus, is
seldom used in a cluster. A smaller low-frequency horn
or vented, box-type, low-frequency component will be
almost omnidirectional at low frequencies. Thus, the
use of directional horns usually cannot improve gain
before feedback because the feedback problems simply
shift to below the crossover frequency.
A line array may also provide some additional gain
before feedback. A properly designed and installed line
array has a narrow vertical dispersion over a wide
frequency range. This can keep the sound aimed at the
audience and away from the system microphone. At
some low frequency, however, the line array’s ability to
control its vertical dispersion is degraded and feedback
may become an issue below this frequency. Also, it is not
always possible to position the line array in such a way
to avoid all feedback issues. For example, column-style
line arrays are commonly placed to the left and right of
the system microphone where their vertical directional
control offers little advantage in controlling feedback.34.2.2.13 How These Equations Answer the Four
QuestionsThere are many other things to consider for even a sim-
plified system. Outdoors, for example, there are the
effects of wind, humidity, and temperature layers. (See
Chapter 2 and Section 34.6.3.7.) However, the answers
to the four questions supply a great deal of information
about whether or not a system will actually reinforce
sound in a satisfactory manner.34.2.2.13.1 Question 1: Is It Loud Enough?Eq. 34-6 helps answer Question 1, “Is it loud enough?.”
This equation doesn’t take gain or feedback into
account, however. Those concerns are covered in
Question 4.34.2.2.13.2 Question 2: Can Everybody Hear?This question “Can everybody hear?” involves the cov-
erage patterns of the loudspeakers and the way they are
aimed into the audience. This topic is covered in detail
in Section 34.3.2 and in Chapters 18 and 35.34.2.2.13.3 Question 3: Can Everybody Understand?For the simplified (outdoor) system, the answer to this
question is yes if the system is loud enough (Question
1) and if it avoids problems like very poor frequency
response and excessive distortion. Indoors, this question
also involves the effects of reverberation.34.2.2.13.4 Question 4: Will It Feed Back?The answer to the question “Will it feed back?” is no
(the desired answer) if PAG is equal to or greater than
NAG, Eqs. 34-5 and 34-8.
Again, there’s a lot more to sound system design
than answering just four questions, but these particular