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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. |
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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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