L-Acoustics V-DOSC, Operator'S Manual

Page: 97 / 158

L-ACOUSTICS V-DOSC Manual Version 4 6/29/2005 Page 97 of 158
3.5 SUBWOOFER ARRAYING TECHNIQUES
3.5.1 LEFT/RIGHT CONFIGURATIONS
Any L/R subwoofer array will have problems with the so-called “power alley” effect, i.e., a build up of
low frequency energy in the centre accompanied by uneven low frequency response and impact off-
axis in the horizontal plane. This is easy to understand using Fresnel analysis, i.e., in the centre, the
distance to L/R arrays is identical so that signals arrive in phase and sum while destructive or
constructive interference can occur off-axis, depending on listener position and the frequency of
interest. For example, if on-axis to the L array and a series of Fresnel rings is drawn, the R array could
fall in a destructive ring and cause cancellation (see Appendix 2 for more on Fresnel). In general, L/R
subwoofer arrays are only desirable for long, narrow audiences where the centre build up effect is less
of a problem (since the venue is narrow and we don’t mind focusing energy up the middle).
Past attempts at reducing the power alley effect for L/R arrays have included complementary comb
filtering applied to the left and right signal feeds for the subwoofer arrays (see: “Use of Stereo
Synthesis to Reduce Subjective/Objective Interference Effects: The Perception of Comb Filtering”,
Augspurger et. al. AES preprint 2862, New York 1989). The main limitations of this approach are
reported to be changes in timbre and a potential loss of overall energy. The preferred option is either
an LCR configuration or electronic delay processed systems (see Sections 3.5.2 and 3.5.3).
Despite the fundamental limitations of L/R subwoofer arrays, some recommendations and basic
guidelines are made in the following. Figures 75 (a), (b), (c) and (d) show techniques for optimizing low
frequency tonal balance over the audience while 75(e) shows a non-recommended configuration.
In Figure 75 (a), ground stacked L/R subwoofers are arranged in a block with minimum frontal surface
area and are physically separate from the flown V-DOSC arrays (either INFRA, 4W or X AUX presets
are recommended). When subwoofers are configured in the manner (i.e., with minimum front surface
area or in a vertical column as in Fig 75b) there is reduced center build up and fewer horizontal lobing
problems compared with the results obtained using two L/R horizontal subwoofer line arrays as
shown in Fig. 75 (e). Basically, interference effects between two omnidirectional sources (Figs. 75 a, b)
are less dramatic than between two directive sources (Fig 75 e). Fig 75(b) has the added advantage of
effectively extending the length of the V-DOSC array at low frequencies for improved low frequency
pattern control and throw in outdoor open air applications (X preset recommended).
In Figure 75(c), the L/R subwoofer arrays are configured in an L-wrap around the stage corner in an
attempt to direct low frequency energy offstage and reduce centre build up. Physically curved L/R
subwoofer arrays as shown in Fig. 75(d) are a variation on this configuration.
Figure 75(e) shows a non-recommended configuration since two L/R horizontal subwoofer line arrays
emphasize the center build up problem. In this case, the individual L/R arrays are more directive in the
horizontal plane than the configuration of Figs 75 (a) and (b) so that center build up is greater and
there are more pronounced horizontal lobing effects. However, this configuration can be used if
electronic delay processing is applied to the individual columns of the L/R arrays in order to obtain the
equivalent performance of Fig 75(d).