Hitting It Head On

In Dimensional Focus: Horizontal Plane, we looked at the potential advantages of toeing main and sub hangs in or out rather than straight ahead for the purposes of coverage uniformity. This time we’ll examine the same idea from a different perspective: mix position.

Let’s start by imagining a situation in which we only need mains (no sides) to cover a relatively narrow audience geometry - say, a theater - narrow enough that we can add some toe to the main hangs without creating an under-coverage situation on the sides of the room - and consider the effects of such a decision. Perhaps we have a 60 foot (18 meter) wide proscenium, which means our Mains are hung +/- 30 feet (9 meters) off center. And let’s place mix position about 85 feet (26 meters) back into the audience (Figure 1).

FIGURE 1 - An unnecessarily large graphic showing example venue geometry that places mix position 20 degrees off-axis of the mains.

If we point the mains straight ahead in such a situation, the mix engineer will be listening to the system from about 20° off axis. We generally audition sound systems and loudspeakers on axis, right down the center of the horn, which is often where systems engineers are taught to tune systems from, and where many manufacturers make their primary voicing decisions. Nevertheless, it seems that the mix position, the place where the critical mixing decisions are made on behalf of all audience members, is more often located off axis to the sound system. This disconnect between the way loudspeakers are typically voiced and tuned and the way they’re typically used in the field is a bit amusing to think about.

But surely being just 20 degrees off axis doesn’t make too much of a difference? Well…

EASE GLLViewer allows us to study this “directly from the horse’s mouth,” by examining loudspeaker balloon data released directly by the manufacturers. We used this tool in Priority Shift to examine the polar behavior of loudspeakers in the horizontal plane, and we’ll do a similar exercise again here.

The program allows us to view normalized magnitude data, which means we use the 0 degree on-axis magnitude response as the flat-line reference and the curve we see shows the change in tonality that occurs by listening from 20 degrees off axis.

Figure 2 below shows the 20° tonality deviation for two commonly used large-format line array cabinets, both made by the same major manufacturer. On the left, we see a cabinet that exhibits more or less “textbook” horn behavior, showing a gentle HF roll-off as we move off the center off the horn. On the right of Figure 2, we see a not-so-well behaved horn, as moving just 20° off center introduces some substantial deviations in the high frequency - this horn is actually brighter on the sides than it is in the middle. (And I will say that I find this particular facet of this product’s behavior to be quite audible indeed.)

Figure 2: Two normalized (using 0° on-axis as a flat-line reference) magnitude traces at 20° horizontally off-axis for two commonly used large format line array cabinets from the same manufacturer. The left shows a well behaved HF roll-off while the right shows a horn with undesirable brightness “spikes” off axis.

If you’re thinking that’s quite a substantial change for just a 20° angular shift within the loudspeaker’s coverage pattern, I tend to agree with you. And this is not an isolated case. Figure 3 below shows the normalized 20° magnitude traces for three other commonly used line array cabinets from various manufacturers, all of which exhibit some substantial deviations from their on-axis response, and all in different ways. Bear in mind, this is the data as self-reported by the manufacturers, what’s being used under the hood to calculate acoustic predictions.

Figure 3: Three more normalized magnitude traces for 20 degrees off-axis of commonly used line array elements from various manufacturers. They all deviate substantially in the high frequency range, in very different ways.

The first thing we can glean from this is that the polar behavior of each particular loudspeaker is a major player in the severity of this effect, as the above figures illustrate, so it follows that there is substantial value for system designers to get to know the axial behavior of the loudspeakers that comprise their tool set (“Priority Shift” is a deeper dive into this concept), and to consider those behaviors when choosing between loudspeaker options for applications in which axial uniformity is a design priority. Critical listening, manufacturer data and measurement are all helpful avenues here.

Secondly, the 20 degree figure we’ve been using thus far wasn’t arbitrary - it is a reasonable figure based on typical mid-sized venue geometries - but we must bear in mind that there is a bit of a range here as well. I pulled up model data from various venues I’ve worked in recently, and calculated the axial offset of mix position with respect to the main hang for each. I got a range of offsets between 15 and 30 degrees, which with some loudspeaker models is quite the difference, totally speaking. Wider displacement between mains will increase the offset at a centrally-located mix position, as will a mix position closer to the stage.

In my experience, adding a small bit of toe-in (about five degrees) to the mains seems to improve the situation at mix position markedly with some models of loudspeaker and has been well received by the mix engineers I work with regularly. Figure 4 below shows Mains hung with an 8-degree toe-in, which is about the most I typically would do for an arena configuration. In such cases, care must be taken to aim any side hangs or outfills appropriately to avoid a main-side gap, and to make sure we’re still extending coverage to the edges of the required listening area. With some models of loudspeaker, the horizontal horn dispersion may not be sufficient to reach the 180-degree line with toed-in mains, and the venue aspect ratio and stage location are also ingredients in this stew, so prediction plays an important role. And of course, my solver spreadsheet (available on my Resources page) has a handy two-point toe calculator to turn that "5 degrees” of desired to into X/Y coordinates for the riggers.

FIGURE 4 - An example small arena design with Mains toed in by 8 degrees.

Regardless of the severity of the issue or our level of freedom to accommodate for it in the design, one thing is certain: the mix position is where our FOH engineers will be making their tonal decisions, and so we can bank on the fact that they will adjust (or ask us to adjust) the tonality of the system from that listening position until it meets their expectations. For this reason, in addition to whatever measurement and listening work we’re doing on-axis to the loudspeakers, we should be carefully evaluating the resulting system tonality from mix position as well. Once the alignment is completed, I like to start my listening work at mix position, season to taste from there, and then walk the entirety of the coverage and make sure it holds up.