User Guide

Sound System Design Reference Manual
6-18
At A , we see the on-axis (solid curve) and
power (dotted curve) response of a 2-way system
making use of a ported LF horn unit and an older
type HF radial horn. When such a system is
equalized for smooth power response, as in the case
of the standard mid-house equalization procedure,
then the on-axis, or direct field response of the
system will have a couple of “bumps” in its response.
This will have the effect of making both speech and
music sound unnatural.
Now let us examine the case at B . Here, the LF
part of the system consists of a single 380 mm (15”)
LF driver in a ported enclosure, and the HF horn is a
JBL 2360 Bi-Radial. Note that the power response
and on-axis response very nearly lie over each other.
Thus, the adjustment of the system out in the house
will result in both reverberant field response (power
response) and direct field response (on-axis
response) tracking each other closely. Such a system
can often be broad-band-equalized merely through
the proper choice of components, dividing network
and transducer drive levels, requiring little, if any,
added electronic equalization.
The graph shown in Figure 6-19 shows this
clearly. Here, we have plotted the variation in R over
the frequency range for a large auditorium. The room
we have chosen has the following characteristics:
V = 13,500 m
3
S = 3538 m
2
RT
125 Hz
= 1.5 sec R
125 Hz
= 1774 m
2
RT
1 kHz
= 1.2 sec R
1 kHz
= 2358 m
2
RT
4 kHz
= 0.8 sec R
4 kHz
= 3989 m
2
This spread between reverberation times at
low, mid, and high frequencies is typical of a good
auditorium. When we calculate the room constant as
a function of frequency and plot it, along with the
sound level that would be produced by one acoustic
watt in the room, we see that the total variation in
SPL is only about 3 dB. The importance of this
observation is that, if we had a loudspeaker system
exhibiting flat power response, then it would produce
a reverberant SPL in this auditorium that would vary
no more than the inverse of the curve shown in
Figure 6-19. Obviously, the smoother the power
response of a loudspeaker, the less equalization it
will require and the more natural it will sound on all
types of program.
Another use of equalization is in controlling
feedback. As we have stated many times, a sound
reinforcement system should be operated at least 6
dB below the point of feedback if it is to be stable.
Through careful and selective use of narrow-band
notch filters, the first several ring modes of a sound
system can be minimized, and the overall system
gain can be increased perhaps 3 or 4 dB. The
practice of narrow-band equalization is complex, and
it is best left to those who have been trained in it.
Figure 6-19. Variation in R and reverberant level with frequency