WHEN AND HOW TO USE DIFFERENT SUBWOOFER ARRAYS

These days it’s just as common to find a contemporary church as it

is a historical one. Regardless of which you are operating out of,

both styles will pose challenges to gaining consistent and even

audio coverage. Understanding the different ways your PA system

can be configured is key; whether you’re building a system for modern,

angular features or traditional intricate design work.

Part of this configuration is the subwoofers. Depending on placement,

your subs can either enhance the final sound quality or be a detriment to

your system. Knowing the various subwoofer arrays, along with the correct

scenarios to use each one, will ensure your parish receives crystal clear

sermons no matter the acoustical challenges.

Omni-directional Arrays

One of the easiest choices you have for subwoofer arrays is omnidirectional,

which involves a simple 1-over-1 stack of subwoofers. Both

elements in this case are facing forward. The results are as expected:

they sum all around. The array has very little directivity and is nearly

omni-directional. This configuration is great for any church with open

architecture, where you need increased SPL that you can’t achieve with just

a single subwoofer.

End-Fire or “Shotgun” Arrays (Option One)

For larger venues where you need to throw audio a longer distance, an

end-fire array may be your best bet. Here, one subwoofer is placed behind

another, with polarity inversion and delay applied to the rear loudspeaker.

This configuration yields nearly identical forward output as compared

to a standard two-subwoofer stack yet provides approximately 20dB of

broadband cancellation behind the array.

End-Fire or “Shotgun” Arrays (Option Two)

There is another array that goes by this same name, but it provides a

drastically different performance. Using the same physical configuration,

straight delay (without a polarity inversion) is applied to the front

subwoofer to align it with the energy from the rear subwoofer. Within a

narrow frequency band (say, the ?-octave around 100Hz), this might look

effective, and we are inclined to ask, “What’s the difference between this and

our previous end-fire array?”

The main disparity between these two arrays can be seen across

the low-frequency band. Though the delay does produce appreciable

cancellation, the effect is very narrow in bandwidth. A ragged frequency

response on-stage can significantly reduce gain-before-feedback and

create an uncomfortable listening experience for the artists. Certain bass

notes are emphasized and ‘boomy’ while others are nearly inaudible. Thus,

more is demanded of the monitors to compensate, but these are already

handicapped because of gain-before-feedback issues. Thus, the vicious cycle

continues, and nobody (artists, engineers, or audience) are satisfied with

the result.

Stacked Cardioid Arrays

Like an end-fire array, a stacked cardioid array features one subwoofer

placed behind another. Ideal for churches with major acoustical challenges,

such as problematic surfaces, the stack cardioid array can be a valuable

approach. The sub directivity can be manipulated depending on which sub

is facing the rear. From top to bottom, the rear sub is in the middle, on the

bottom, then on the top. This allows you to adapt your system depending

on the layout of your environment.

Subwoofer Arrays on a Larger Scale

While each type of directional array has been examined on the smallest

scale, it is important to mention the degree to which each configuration can

be scaled to achieve greater output and/or directivity. It is now possible to

add additional subwoofers in line (with progressively more delay) to create

an increasingly narrow beamwidth.

Additional cancellation can be achieved when adding more subwoofers,

however it cannot exceed the extreme frequency-dependence of the system.

No matter the result of this approach, there will always be an inconsistent,

peaky response in the cancellation region.

Fortunately, you can also scale true end-fire or ‘shotgun’ arrays – it just

takes a little more than with a straight delay and polarity inversion. This

configuration is physically identical to that of the second end-fire system,

just measured. All three subwoofers were able to achieve total summation

in the forward direction, while still achieving broad-bandwidth cancellation

behind. But what was done differently, if no physical changes were made?

To achieve this, the same technique was utilized as with the two-sub endfire

(delay and phase shift). This is easy with two subwoofers, since nearly

all processors can apply 180 degrees of phase shift (i.e. polarity inversion).

In this case, typical alignment delay was applied to the rear subs as

before, but with 120 degrees of phase shift. If you are aiming to use this

solution within your own church, you can determine the phase shift by

dividing 360 degrees by the number of subwoofers (or rows of subwoofers).

Ensure that this configuration is setup in addition to the physical alignment

delay that is applied progressively from front (no delay) to back (most

delay).