Loudspeaker Array Low-Frequency Pattern Control using Filtered Array Technology™

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Figure 3 (below) shows the result of applying bandpass filters to a four-element system.
Element 1 is operated full range, Element 2 is operated only in the octave about 750 Hz, Element 3 the octave about 273 Hz, element 4 the octave about 215 Hz. The 90° off axis attenuation is now nearly 14 dB across a wide bandwidth with a smoother response. The 45° response is about 10 dB down and exhibits smooth response. Note the trade off for this array to the typical line source is that the maximum on axis level is 3 dB down from the driving all array elements full range.

A gradient array is very useful in that it allows the designer to “tune” the attenuation null. The simulation in Figure 4 (below) shows how varying the delay to the steering element moves the null.

A full range loudspeaker or a cluster of loudspeakers is arranged in front of a “steering element”. The steering element is driven over a limited bandwidth of ~150 Hz, at -6 dB from the main speaker, with inverted polarity. The front element(s) have signal delay, the amount of delay determines the null location. Construction of this array can be very compact requiring an envelope as small as one cubic foot. An array capable of a medium size house of worship requires only cube three feet per side.

Section 3: Practical Implementations of Filtered Array Technology
This section describes three arrays; a Three Element Bandpass, a Four Element Bandpass, and a Gradient. Information on construction, signal processing and performance is given for each array. JBL Sound Power model loudspeakers, their arrangement, and DSC260 signal processing parameters are presented. On-axis, 45° and 90° off-axis frequency response curves are also included. The response curves provide both the best method to evaluate the arrayís suitability for use with open microphones, and a convenient performance verification check requiring only three measurements.

Please note rear wall and ceiling reflections will interfere with pattern control. It is recommended that the cluster be at least 20 feet from rear wall and 10 feet from ceiling. The gradient array can be mounted directly against the ceiling, or be spaced a minimum of 10 feet away.

3 Element Bandpass Array
This simple array combines moderate under cluster attenuation with uniform coverage to the rear of the room. The example shown provides 120° horizontal coverage but other loudspeakers can be used to cover as narrow as 60°, or up to 360° of horizontal coverage. When using other configurations, care should be taken to balance the output of both low frequency steering elements with the cluster’s output to maintain desired coverage.

Coverage Angles: 120° Horizontal x 50° Vertical (For Array Shown)
Under Cluster Attenuation: > 6 dB
Cluster Configuration: 2 Unit Arc + Vertical Low Frequency Pair
Array Elements: 2 x SP215-6 + 1 x SP215S
Signal Processing: 1 x DSC260
Amplifiers: 4 Channels, 2 x MPX600

4 Element Bandpass Array

This more complex array combines moderate under cluster attenuation with very uniform coverage to the rear of the room. The example shown provides 60° horizontal coverage by 95° vertical but other loudspeakers can be used to cover wider than 60° of horizontal coverage. When using other configurations, care should be taken to balance the output of both low frequency steering elements with the clusterís output to maintain desired coverage.

Coverage Angles: 60° Horizontal x 95° Vertical
Under Cluster Attenuation: > 6 dB
Cluster Configuration: 3 Element Arc + Side Low Frequency Pair
Array Elements: 3 x SP215-6 + 2 x CSP115 (Custom Shop Model)
Signal Processing: 1 x DSC260
Amplifiers: 6 Channels, 2 x MPX1100, 4 x MPX600

Gradient Array

This very simple array provides greater than 10 dB under cluster attenuation with control of null location and very uniform off-axis frequency response. The example shown provides 120° horizontal coverage by 50° vertical but other loudspeakers can be used to provide up to 360° of horizontal coverage. When using other configurations, care should be taken to balance the output of both low frequency steering elements with the clusterís output to maintain desired coverage.

Coverage Angles: 120° Horizontal x 50° Vertical
Under Cluster Attenuation: > 10 dB
Cluster Configuration: 2 Element Arc + Rear Steering Element
Array Elements: 2 x SP215-6 + 1 x CSP115 (Custom Shop Model)
Signal Processing: 1 x DSC260
Amplifiers: 3 Channels, 1 x MPX1100, 1 x MPX600