User Guide

Line Arrays and Very Low Frequencies
Traditional practice with low frequency radiators, or
subwoofers, has been to groundstack the subs. Groundstacking
produces the familiar 3 db doubling of pressure, because of
the conversion in the acoustic load from a 4π steradian to 2π
steradian load. Figure 41, Equation 6 and Figure 42,
Equation 7 show the change from full space to half space
loading and the subsequent pressure doubling. The physical
height requirements of a full band with line array, however,
bring an important performance advantage to flying subs. While
it is completely true that the pressure doubling is lost when
the subs are removed from the floor, there is a substantial gain
associated with a large vertical array of low frequency sources.
Full space pressure
ρ
~
p.c
QK
4πr
Half space pressure
ρ
1
/2 ~ p.c
QK
2πr
Figure 43 shows polar response of a 3 by 3 groundstack.
The vertical control gained via not only the geometry of the
stack itself, but because of the coherent reflection of the floor.
A 12 high flown array is shown in Figure 44. Although the
polar pattern is partly compromised, the Q is substantially
increased. The associated gain in directivity index is a very
valuable tool for a system designer. In Figure 45 shows a
typical groundstack. A 200-foot long room would exhibit the
following performance. A flow line array would generate, if
properly aimed, a +/– 1dB to 2 dB variation front to back
in the venue described in the example. In that same situation,
the groundstacked sub would exhibit a 24-1/2 db variation of
low frequency material from the front to the back. This is an
obvious compromise in the full bandwidth control (or directivity
index versus frequency control) of the system. With proper
aiming, a 12 box high vertical line array of low frequency
material can substantially improve the overall front to back SPL
coverage of very low frequencies. Although this 12 box hang
is nowhere near high enough to control 100 Hz and below, the
improvement in uniformity of front to back is 5 to 10 times
better than that of the groundstack. Because of that improve-
ment in front to back uniformity, flying subs are highly recom-
mended where improved full frequency coverage is required.
Figure 40
Figure 42, Equation 7
Figure 41, Equation 6
Figure 45
Figure 43
Figure 44
12