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14.0 Applications
In any large public auditorium it is necessary to arrange the loudspeaker
system in clusters which are distributed around the arena to help obtain a
more uniform distribution of sound level.
For any given position within the arena a listener will be hearing a
combination of both the direct sound from the stage, and that from the nearest
distributed cluster. Because the speed of sound in air is considerably slower
than that of the equivalent electrical signal in the cable, the audible sound
from the nearest cluster will be heard by the listener slightly before that
arriving from the stage. The time delay between these two needs to be
compensated for, otherwise the listener will hear anything from a slightly
unintelligible sound to one that is totally indistinct.
Both the TCS-803 and TCS-804 can be used to cure this situation by
connecting one of its delay outputs to the distributed cluster. The delay time
needed will be equivalent to the distance between the loudspeakers.
Remember that the speed of sound at 20°C is approximately 343 meters per
second. This gives you a rule of thumb of 1 msec/ft, or 3 msec/metre. When
using multiple ‘delay’ clusters or when working in stereo, it will be up to the
user to decide whether it is better to use the TCS-803 or the TCS-804 switched
to mono or dual channel mode.
14.1 Time
Correction for
Audience
Distances
Applications
When a loudspeaker sound system is constructed which utilises different
loudspeaker drivers for separate frequency bands it is not generally possible to
mechanically mount them all such that each sound source is in the same
vertical plane. The effect of this is that phase errors might occur between
drivers which produce a substantial cancellation of the signal around the
crossover region, and there can also be a general lack of transient clarity to
the sound resulting from a poor impulse response due to inaccurate combining
of the wavefront arriving at the listener.
It is the vertical displacement of the loudspeakers versus the wavelength of
the signal around the crossover region that causes the phase cancellation.
Remember that the wavelength of a 1kHz sinewave is 23cm and for a 5kHz
signal it is 7cm. For zero cancellation the vertical displacement must be in
whole multiples of the wavelength. For a good impulse response the zero
phase start point for each loudspeaker drive unit must be in the same vertical
plane. Both of these criteria will probably therefore require each MF and HF
drive unit to be time delayed from the LF drive unit.
Both the TCS-803 and the TCS-804 are ideally suited for this style of
correction. However because you need to delay after the main loudspeaker
crossover, a separate delay channel is needed for each ‘way’ of the crossover
(See Figure 14.1). This can be very wasteful of facilities and not economic for
a typical three or four way stereo system.
14.2 Time
Correction for
Speaker Driver
Placement
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