210 Chapter 8
50 in. In order for such a reflector to form a
well-defined beam it is necessary that the diameter of
the reflector be considerably larger than the wavelength.
This is true for this unit except at its low-frequency
limit of 500 Hz. In the vicinity of 500 Hz diffraction
produces some out-of-beam energy that is undesirable.
This is compensated for through the placement of a
12 in diameter cone driver within the enclosure behind
the reflector. This driver radiates through an opening at
the center of the reflector. This driver’s electrical signal
is band-pass limited in the vicinity of 500 Hz and is
phased to cancel the diffracted out-of-beam signal
produced by the compression driver. Separate power
amplifier and processing circuitry is provided for the
two drivers with all of the electronics and associated
power supplies being located in the main enclosure.
Once the system is assembled and aimed it is necessary
only to supply the appropriate ac power and audio
signal. The system is specified to produce a maximum
SPL of 110 dB at 100 m in a pass band of 500 to
15,000 Hz with a coverage angle of 20°.
8.6 Single Source Location
Single source loudspeaker arrays are usually mounted at
one end of an axis of symmetry of the stadium seating
with the long axis being preferred. It is desirable to
place the cluster at such an elevation that the compo-
nents of the array are aimed down on the audience. This
positioning minimizes the spill of sound into the
surrounding community.
8.7 Covered Seating
Many stadiums feature double and occasionally triple
decking such that a portion of the lower seating is
obscured from a line-of-sight view of the single source
point. In this instance the perception of a single source
can be maintained while still providing direct sound to
the covered seats by creating a stepped zone delay
system. In this system, a distributed loudspeaker system
is installed beneath the upper deck and arranged in a
series of coverage zones such that the obscured seats in
a given zone are all approximately the same distance
from the single source point. The electrical signals to the
loudspeakers in a given zone are delayed by an amount
equal to the transit time of sound from the single source
point to the given zone. If the zone loudspeakers radiate
principally in the direction which would have been taken
by the single source system had it not been obscured,
one generates a traveling source of sound from one zone
to the next that is in sync with sound from the single
source system. Zonal boundaries at a linear spacing of
about 20 m (65 ft) have been found to produce very
intelligible apparently echo-free results.8.8 Distributed SystemsDistributed systems are capable of producing full band-
width sound throughout a stadium provided that the
individual loudspeaker systems are installed with suffi-
cient density such that the axial throw of any given unit
is 50 m (165 ft) or less. A throw of 50 m will require
variable equalization for air absorption if proper
high-frequency balance is to be maintained. The unifor-
mity of sound distribution is improved with an
increasing loudspeaker density and hence an increasing
expense.
The design of the individual sources in such a system
is carried out as one would for an interior space that has
a designated seating area. An array of loudspeakers is
formed using conventional arraying techniques with the
view toward providing uniformity of coverage and full
bandwidth. The successive distributed areas are chosen
such that the areas overlap at the 6 dB point of an indi-
vidual area. This will provide quite uniform coverage
throughout all of the seating areas. Psychoacoustically,
the sound will appear more natural to the listeners if the
source is elevated and in front of the audience. Weather
proofing techniques must be employed in the loud-
speaker manufacture and/or in the loudspeaker installa-
tion process.
A distributed system may be powered in a number of
ways. All of the power amplifiers may be located at a
single central point, in which case, long cable runs must
be made on 70 V or 200 V lines to the distributed loud-
speakers. This is convenient from the standpoint of
monitoring or servicing the amplifiers, but is enor-
mously expensive to install. Rather than locate all
amplification at a single position, power amplifiers may
be located at several subpoints throughout the stadium.
Less costly low-level signal wiring connects the
subpoints and high-level power runs are shortened and
hence made less expensive. Alternatively, powered
loudspeakers are available from which to construct the
individual loudspeaker clusters. Less costly low-level
signal wiring can now be run to each cluster. Ac power
must be made available at each loudspeaker location
under this option. This expense, however, is now shifted
to the electrical contractor. This individually powered
option is the least expensive initially but may present a
servicing nightmare in the future.
Regardless of the technique employed for installa-
tion, any reasonable design will include provisions for