JBL Loudspeaker Array Low-Frequency Pattern User Manual

JBL Professional Application Note

Loudspeaker Array

Low-Frequency Pattern Control

using Filtered Array Technology™

1: Overview

Array directivity control theory is not new. Olson’s Acoustical Engineering in 1940
discussed Gradient microphone arrays, and both it and Beranekís 1954 Acoustics
have sections on line arrays. In the 1970s JBL offered two tapered and shaded
line array systems, the 4375 and 4380 designed by George Augspuger. David
Klepper, Topper Sowden, and others have used sophisticated line array designs
for installed sound reinforcement system for several years.

Low cost DSP that conveniently provides the signal processing required for low
frequency control, coupled with customer requirements dictating smaller arrays
with tighter pattern control, has renewed interest in the concept. Simulation
software aided design has further stimulated manufactures to offer solutions to the
market.

JBL researched several approaches to controlling the low frequency pattern,
including constructing full size arrays for proof of concept. Three of the most
useful designs are presented here. Two are Halfwave Line Arrays, which provide

good attenuation at 90∞ off-axis and propagate radiation into the audience area

with half the rate of attenuation of a non “steered” cluster. That is, because of the
directivity characteristics of the array SPL drops 3 dB with doubling of distance,
not 6 dB. This provides more even coverage from front to back at low
frequencies, previously a difficult problem. The third cluster is a Gradient Array. It
uses a steering element behind the main cluster to null the main clusterís radiation
directly below it. This can be very useful in houses of worship and other
applications using lavalier or podium microphones located underneath the cluster.
One other important advantage of this design is its compactness.

Section 2 of this paper describes how constructive interference works to provide
pattern control and Section 3 details the cluster components, construction, signal
processing parameters, and expected results.

2: Demonstration of Principle

In this section we will use very small loudspeakers to demonstrate interference
concepts, and how they can be used for directivity control. The loudspeaker used
is about one third the size of what would be used for sound reinforcement.
Therefore the effects can be scaled in frequency: what occurs at 300 Hz in the
model will be at 100 Hz for a cluster three times as large.

When two sound sources producing identical program are displaced relative to
each other, they will combine to produce a unique interference pattern that
depends on the observerís location. The interference pattern is a result of the
different arrival times from the two sources. For a position directly on the common
axis of the two sources, and equal distance from each of them, the relative
combined response will simply be +6 dB louder than that of a single source alone.
This is known as coherent summation of two signals. They are identical in phase
and amplitude at all frequencies and thus combine to +6 dB.
Figure 1 shows what occurs when an observer moves slightly off-axis of two
sources. This example illustrates two different positions, the first position creates

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a condition where the path length difference results in a time displacement of

0.25 ms, the second 1 ms. The 0.25 ms difference will result in an apparent 180°
phase shift for the frequency of 2 kHz and an apparent phase shift of 360° at 4
kHz. The frequency dependent phase shift introduced by the relative
displacement of the two sound sources means that the sound cancels starting at
2 kHz, is reinforced at 4 kHz, cancels at 6 kHz, and so on every 2 kHz. This type
of response is known as comb filtering because of its resulting frequency
response. If the resulting displacement were 1 ms, then the fundamental
cancellation is at 500 Hz, with summation and cancellation intervals of 500 Hz.

The interference (comb filtering response) occurring in an array of loudspeakers
can be used to our advantage for directivity control. Figure 2 is an example of
four loudspeakers placed in a line with a 23.5-cm (9.25”) spacing, without
filtering.

Here is the on-axis, 45°off-axis, and 90° off-axis response of a four element line
array. The on-axis level goes up by +12 dB relative to a single element, and the

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