Excuse The Delay

Big concerts require big stacks. Those arrays flanking the stage must impress the band enough that they are willing to pay for the tour support. On the other hand, there is the audience these loudspeakers are pointed at. Many of whom are seated further away than the loudspeakers can reasonably reach. The solution may be less visually impressive and more costly to rig, but can provide even the most distant seats with high quality sound.

Delayed loudspeakers are nothing new to sound reinforcement. Most medium and large touring companies have had experience with setting up delay stacks. However, it is surprising how often these arrays are grossly overpowered by the main stacks; not in output level but in audience coverage. It is not uncommon to see outdoor music events in which a large pair of arrays flank the stage, covering the front 20% of the audience and a two or three small arrays set 1/3 of the way back that cover the remaining 80% of the audience that fans out behind the mix position.

This is often based on the mistaken belief that the delay stacks merely "fill in the holes" in the sound emanating from the main stacks. While the latest in Line Array technology may be able to achieve high directivity that can control the mid and high frequency sound aimed at more distant seats, even these arrays have difficulty reaching more that two to four times the distance of the nearest seats at low frequencies. However, less cutting-edge arrays will punish the front rows and starve the back rows much more severely. For many of these outdoor systems, the level will drop to one-quarter volume or more before reaching the edge of the audience area.

If you really want to provide everyone with the same concert, it is necessary to view the each loudspeaker array as covering a specific number of seats. From the main left/right stacks to each delay stack, this ratio must be maintained in order to evenly cover the audience. Spreading those boxes across a number of delay stacks will actually make the concert louder for more people. Of course the advantage is not simply volume, the tonal qualities of the music will be far more consistent when the effects of air absorption and inverse square law do not combine to make the sound increasingly muddy as you move back in the audience (weather permitting).

Typically the delay stack format is configured in a series of concentric rings of loudspeakers that fan outwards in the audience. The distance between rings is determined by the point where the main system (or previous delay stack) falls off by 6 dB or 10 dB, depending on your budget. Of course, do to the greater losses in air at high frequencies, this distance may be determined by humidity and budget. The delay stacks can be few but made up of a larger quantity of high-directivity devices, or more arrays of a smaller quantity of low-directivity devices.

The amount of signal delay used between each ring can be calculated but, because of the aesthetic implications, it is often a matter of judgment or style. If the delay stack provides a matched level and tone to the main arrays, then it is necessary to include both the transit time and some precedence delay (~15 ms) to make the delay fit into the sound from the stage. If the spacing drops the level enough that the array is more than 6 dB louder than the sound from the preceding array, then there is no need for the precedence delay, as the effect is lost anyway. In larger outdoor settings, very little delay is added to the successive delay stacks because the distances become so great that the performers would appear annoyingly out-of-sync with the sound.

A major advantage of distributing the loudspeakers across a large audience is the reduction in FOH volume near the stage. Less feedback potential from the FOH system and an overall reduction in the point source levels (that can generate lobing effects that can worsen echoes and are a major annoyance to the neighbours). Distributed systems place the loudspeakers closer to the listeners, and if the devices are well focused, less sound spills out of the audience area. Distribution of the loudspeakers can also reduce the problem of tall main arrays that bounce sound off temperature gradients (an acoustically reflective air layer caused by dramatic differences in the air temperature at the audience height) and leave an outdoor venue.

Of course there is still the problem of sightlines; setup time; securely mounting the arrays; getting power and audio to each delay stack; having a truck big enough to fit all this gear; etc. But if distributing loudspeakers throughout an outdoor venue were easy, everybody would do it.

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