Specifications

TI 323 (6.0E)
11 - 36
4. Acoustical parameters
4.1. Power and bandwidth
The C4 system (C3, C4-TOP and C4-SUB) is a flexible
modular sound reinforcement system.
The TOP/SUB ratio can be adapted to the power and
bandwidth required for the specific program material:
− speech only reproduction
2 x C4-TOP/1 x C4-SUB or
2 x C3 in LFC mode
− light music program
1 x1 C4-TOP/2 x C4-SUB or
2 x C3/4 x C4-SUB)
− full range music
4 x C3/4 x C4-SUB/1 x B2 or
4x C4-TOP/4 x C4-SUB/1 x B2
Modular hardware design and arrayability enable the
scaling of a system for any coverage and SPL
requirements.
4.2. Level requirements
It is difficult to give a general recommendation on the
sound pressure level (SPL) needed for specific
applications. The suitability of a system is linked to a
number of factors such as the program material and the
nature of the acoustic environment - the size, shape and
surfaces of the room, loudspeaker and audience
positions etc. However, simple calculation can help clarify
which type and numbers of speakers are needed to
achieve a desired SPL at a defined distance.
The level produced by a loudspeaker system diminishes
with distance; the propagation loss in free field conditions
is –6 dB with each doubling of distance (known as the
Inverse Square Law see chapter 5. Sphere or Line ?). This
rule is also valid for line arrays since the 3 dB loss per
doubling of the distance happens only with straight line
sources.
A pair of speakers used left and right, as with a stereo
system, will raise the total SPL by an average of 3 dB in
their overlapping coverage (incoherent coupling). Given
a stereo system where an average direct 100 dB SPL is
required at the mix position 20 metres (65 ft) from stage,
then the level produced by each side of the system needs
to be 100 dB –3 dB +26 dB = 123 dB. An additional
12 dB of headroom should be allowed for the peak
levels of dynamic program. In this case a system capable
of producing 135 dB SPL per side should prove adequate
(coupling of multiple systems is described later).
distance rel. level
2 m (6.5 ft) – 6 dB
3 m (9.8 ft) – 10 dB
5 m (16 ft) – 14 dB
10 m (33 ft) – 20 dB
20 m (65 ft) – 26 dB
30 m (98 ft) – 30 dB
40 m (131 ft) – 32 dB
80 m (262 ft) – 38 dB
4.3. Coverage
The coverage is a decisive criteria for the perception
of the quality of a sound reinforcement system.
Naturally, the system coverage pattern needs to be
sufficiently wide to take in the entire audience
(Democracy for Listeners). Narrow coverage systems
can be arrayed to provide wider coverage. This
works well, provided the systems used have a well
defined constant directivity (CD) characteristic and
can be physically arrayed at the optimum relative
angle.
On the other hand it is important to reduce the
diffuse sound by limiting the coverage angle of the
system to audience areas only. The diffuse sound
energy in a room remains largely constant whereas
the direct sound energy from the loudspeakers
diminishes with distance. Narrow coverage systems
produce less diffuse sound energy and therefore
provide a higher direct to diffuse level ratio at
greater distance. This maintains a higher intelligibility
and is often described as a greater 'throw'.
The three following illustrations, created in an acoustic
simulation program, show the differences in the
coverage of a listening area using wide, medium and
narrow dispersion loudspeakers. In each simulation
two stereo cabinets cover a listening area 20 m (65 ft)
wide by 25 m (82 ft) long. The coverage angles, which
decrease from the left, are 90° x 50°, 60° x 40°, 35° x
35°. All are at equal level in the audience area right
in front of the speakers.