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Drivers must always be time delayed in horns so that they
are time aligned. If the bass cabs are on the ground with the mid top cabs
on top of them then start with the closest driver, usually the compression
driver and delay it so that it is in alignment with the furthest driver,
usually the bass driver. So if the bass driver is in a horn and the driver’s
cone is 150 cm from the front of it’s cabinet and the compression driver
is 30 cm back from the front of the mid top cab then delay the compression
driver by 120 cm. If the compression driver was flush with the front of
the mid top cab then you would have to delay it by 150 cm for it to be in
alignment with the bass driver, but as it is already 30 cm from the front
of the mid top cab you can subtract this from the distance it has to be
delayed to be in time with the bass driver. Next the mid driver. Say this
is 50 cm from the front of the mid top cab, you would have to apply a delay
of 100 cm to align it with the bass driver. The mid driver is already 50
cm from the front of its cabinet and another 100 cm of delay would bring
it to 150 cm from the front of its cabinet. The mid driver would now be
in alignment with the compression driver as well as the bass driver. The
front of the mid top cab would now look as if it was 150 cm behind itself,
which is where the bass drivers cone is. So as long as both the bass and
the mid top cabs are on top of each other and both cabinets’ fronts are
flush everything will be in alignment.
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If the bass cab was on the floor and the front of the mid
top cab was flown 1 meter above the bass cab, then you would not have to
delay the mid driver at all. The mid driver would be 100 cm from the front
of the bass cab plus 50 cm down it’s own horn which would be 150 cm. The
compression driver is 20 cm in front of the mid driver and so would be 130
cm away from the front of the bass cab, apply 20 cm of delay to the compression
driver and the system would be in alignment again. The delay settings for
this system would be zero delay on the bass, zero delay for the mid and
20 cm of delay for the compression driver.
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If the mid top cab were flown 4 meters above the bass cab
then we would have to start delaying the bass cab to be inline with the
mid top. So the front of the mid top cab is 4 meters from us, the mid driver
is 450 cm from us and the compression driver is 430 cm away from us. First
delay the compression driver by 20 cm, this will align the mid top cab.
Next work out how much closer the bass cab is than the mid top drivers.
The front of the mid top cab is 4 meters from the front of the bass cab
and the drivers are 4.5 meters away, so if we applied 4.5 meters of delay
to the bass cab they should align up. But remember that the bass driver
is already 150 cm from the front of the bass cab, so we take that from the
4.5 meters and get 3 meters. So applying a delay of 300 cm to the bass cab
will now bring the whole system into alignment. The delay settings on this
system would be, 300 cm of delay on the bass, zero delay on the mid and
20 cm of delay on the compression driver.
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All the delays here have been set in distance but could
have been set in time. 150 cm of delay is the same as 4.37 ms (milliseconds).
100 cm is the same as 2.91 ms and 20 cm of delay is the same as applying
0.58 ms of delay. It also matters where your audience are. If the bass cab
is small and on the floor and the mid top cab is 1 meter above it but at
the same height as the listener’s ears then you will have to take that into
account. All the above examples are made with the bass cab being one a stage
or with the top of the bass cabs being at or just above the listener’s ears
height. That way when the mid top is 1 meter above the bass cab it will
also be 1 meter above the listeners ears. It also matters at which angle
the mid top cab is flown. If the front of the mid top cab has a lot of downward
tilt and is facing the listener square on then all of the above applies.
It the mid top cab has no tilt and is pointing out parallel with the ground
then the compression driver will be further away if all your audience is
close to the stage, so you can probably ignore the 20 cm of delay that has
to be applied to it in most of the above cases.
Most digital system processors like those made by BSS, XTA, Ashley and DBX
will allow you to apply delays to all the bands independently, and some
like the BSS 388 will allow you to plug in a temperature and humidity probe
so that the unit can automatically adjust the delay settings for each band
when the temperature or humidity changes. As the temperature and humidity
changes during a gig the speed of sound also changes. This can make it appear
that some of the drivers are at different distances than they were when
you aligned them earlier at 20 c. The unit takes all of this into account
and realigns all the drivers for you all night. What more could you want.
Also note that if one driver is in font or behind another, there will be
a difference in phases between the two. A difference in time or distance
is also a difference in phase, although only at the point at which both
drivers are playing in the same frequency range. If drivers cannot be aligned
then steep filers to stop any passband information will help to keep the
phase variant intact over a narrower and so less obtrusive range. |
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