Many times we use multiple systems to meet our coverage needs, but getting these systems to “play well together” can sometimes be problematical.
A typical strategy when aligning systems is to work from loudest to quietist, for now let’s assume that the speaker manufacturer has done his homework at the component level, so it’s these sub-systems we are integrating: mains, front fills, delays, out-fills, sub woofers, etc.
Although we say to work from loudest to quietest, remember there is no negative delay (yet) so we always have to delay back to the furthest source.
The concept is to get the sound to arrive at the same time to some point from these sources, but where is this point?
As we say in the measurement game you have to pick a point to make (place) your stand. Typically half way through the intended coverage zone is a primary measurement point, but when integrating sub-systems we really need to look at the “overlap” zones where these sub-systems are going to interact.
If we have two sources of the same signal with a time offset between them, the comb filter will be at its worst when the amplitude of the signals is equal (see fig 1). Typically we will see “peaks” of +6dB and cancellations or “nulls” of up to -60dB and the first null can be over an octave wide. (Yikes!)
Fig. 1 comb filter
Reducing the amplitude of one of these sources will reduce the amount of comb-filtering, so we can use this effect to our advantage. (See Fig. 2).
Fig. 2 ripple
For example, let’s say we have a main system (probably flown) and a fill enclosure (ground stacked). The fill enclosures function in this case is as a nearfield to cover the region close or even underneath the flown array. This nearfield is coverage zone, let’s say just the first 20 feet or so, and further back than that and we are then in the main coverage of the flown system.
As we approach the system we walk out of the main flown coverage and into the nearfield coverage and somewhere in the overlap region between them there will be a point where the two arrays are at equal amplitude. This will be the point where any time offset between the sources will produce the maximum comb filter. If we time it at this point, not only will we fix it at this point but the relative amplitude offsets with the varying distances from the sources will also reduce the ripple.
The take-away is it’s important to set level offsets first between the systems and if we change the level of either system you will have to re-time as the equal amplitude position will change.
Next time we’ll look at creating “imaging” from these systems and that bloke “Haas” and the trouble he’s caused.
If you have any questions or suggestions for topics contact me at Ferrit@osacorp.com
About Martyn “Ferritt” Rowe
Industry veteran and OSA’s Director of Engineering Services Martyn “Ferrit” Rowe brings nearly three decades of real-world experience in live event technical services. Ferrit most recently came from Martin-Audio as the technical training manager for MLA, and uses his vast knowledge and expertise of the multi-cellular technology to support client projects as well as support and train engineers and technicians.
Ferrit began his career running cables on a Thin Lizzy “Live and Dangerous Tour,” and then taking on the roles of running monitors, front of house, and system technician for some of the most popular acts in music, such Judas Priest, Ozzy Osborne, Black Sabbath, The Police, KISS, The Who, Elton John, Poison, Bon Jovi and Van Halen.
Ferrit’s training career began in 2000 when the first of the line array’s from V-DOSC emerged and became an instructor on the line array theory and continued that path with various systems over the years before joining the Martin-Audio MLA division, and then bringing his knowledge and expertise to OSA International, Inc.