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Boundary conditions
When sub woofers are placed on or near a floor, wall, ceiling, solid stage apron etc.,
these solid surfaces form a boundary that can restrict or reflect a significant portion of
the subwoofers' off-axis radiation.
Some of these boundaries will inevitably reflect this off-axis radiation back towards
the audience – but with a small time delay. The audience will hear a combination of
direct and reflected sound which, if the direct and reflected paths are within ½ a
wavelength of each other, will tend to add positively – but with a slight phase lag
caused by the delayed reflective component.
The above illustration shows an inset polar diagram with the direct LF (red) summing
to a lower magnitude - and phase shifted – reflected LF (orange). The resultant direct +
reflected signal (brown) is higher in magnitude but rotated anti-clockwise to indicate a
phase lag.
This phenomenon makes precise subwoofer alignment difficult – especially if the main
line array system and the sub woofers are affected by different boundary conditions or
adjacent systems.
The null method recommended on the previous page assumes that the main arrays and
subwoofers are set-up using the recommended crossover presets and that they have
been time aligned to compensate for any distance offset. It works on the basis that two
sources (the line array and the subs) will cancel each other in the crossover region if
switched to opposite polarities – but only if perfectly time aligned. Once the main-to-
sub array delay has been fine adjusted to create a null, the polarity reverse is cancelled
so that the two systems sum in phase.
Note that, although cardioid subwoofers produce less radiation directly to the rear,
they are still affected by floor and wall boundaries and fine alignment is still
necessary.
All material © 2007. Martin Audio Ltd. Subject to change without notice.
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