Keep It Down
A man goes to the doctor and says “Doctor, it hurts when I do this!” The doctor says, “well, then don’t do that!”
It’s a joke that’s been slung by Dads for generations, but beneath its prima facie silliness, there might be some wisdom lurking.
Case in point: I was tasked with designing and deploying a sound system for an event held at a casino and resort. These types of venues often feature a variety of popups and events on their grounds as entertainment for the folks staying in the hotel as well as visitors coming to the site specifically for the event. This particular event was a “pool party,” in which a concert would be performed on a large mobile stage overlooking the resort’s expansive pool area. By necessity, this meant that the stage - and therefore the sound system as well - would be firing directly towards the hotel’s main tower, just behind the pool area.
This raises a pair of considerations: first, the sound quality for the people attending the performance and floating in the pool. Any acoustic energy from the sound system will readily bounce off the concrete and glass exterior of the hotel tower and back into the listening area, creating a highly undesirable slapback echo. As annoying as this can be for the audience members, it’s even more distracting for the artists on stage, which can negatively impact the performance. In extreme cases I’ve also noted “second generation” slapback, when the returning echo is so severe it finds its way back into the open microphones on stage and gets amplified again. It’s like a fancy delay plugin, except more terrible in every way.
Second, even though the show happened during afternoon and evening hours, it still seemed prudent to reduce the nuisance noise factor for those staying in the hotel who might be trying to rest. While this wasn’t explicitly requested by the client or the vendor as a priority, it is both a first principle of sound system design (send the sound towards the audience and not everywhere else) and surely appreciated by anyone trying to sleep.
So in this case, our goals for the audience, artist, and neighbors are well aligned (which is not always the case). The best way to keep the PA from slapping off the building is, of course, not to face the concert towards the building in the first place. Beyond that, we can take steps to minimize it, but to some degree it’s a fact of life in this situation.
Design Approach
What tools do we have at our disposal to accomplish this?
For starters, if we happen to be using one of the loudspeaker systems that allows for “hard avoid” beamsteering, great. There is no magic bullet here, but these systems generally do a pretty good job when programmed (and deployed!) correctly. If that tech isn’t available to us, what can we do?
Maximize Trim Height
Fly the PA as high as practical (on this particular mobile stage, that means somewhere around 25 feet / 7.6m to the hook), and then angle the array more steeply downwards to minimize the amount of energy “skimming” across the ground at a low angle of incidence. Rather than the typical deck chain links between the chain hoist body and the array’s flybar, I used a pear ring, providing just enough clearance for the chain bag to cleanly hang without blocking the array. This nets a bit of additional headroom on the rigging which translates into a higher achievable trim.
Less Overshoot
Usually I design with enough overshoot to fully keep the last row of seats well within the low-variance high frequency coverage. In this case, we will allow the HF to taper off just a bit towards the very rear of the pool / bar area, where the casual attendees are not highly focused on the show, which in turn gives us a bit more room for the energy to roll off before splashing against the side of the building.
Smaller Splays at Top
I usually steer clear (no pun intended) of splay angles below 1 degree, especially when the array is rigged in tension rigging (the majority case in the US), as those sub-1 degree angles can collapse to effectively zero, and in general I find that they tend not to sound very natural. The extremely small splays put a harder edge on the vertical dispersion pattern in the highest octaves, causing it to narrow before the midrange, which causes a disproportionate amount of tonal variance for those listeners on the top edge of the coverage. In this case, however, containing the high frequency output is exactly what the doctor ordered (well, right after telling us not to blast a rock concert at the side of a tall glass building with people sleeping inside). The half-degree splays at the top of the array help minimize the spill of the high frequencies past the coverage area onto the building facade, in an area where we’re not highly concerned about tonal variance (folks in this area of the pool don’t have much of a view of the stage anyway, so people who are deeply invested in the performance aren’t watching from this location). This, combined with the reduction of overshoot (see #2 above) means a substantial reduction in high frequency splashing off the hotel glass.
Upside-down Cardioid Subwoofers
As discussed in “The Crossroads Array,” I often use two-element stacks of subwoofers to allow a vertical steering component to my cardioid subwoofer deployments. We can note that when the array is deployed “upside down,” with the rear-facing element on the top of the stack, we steer the rear-ward cancellation vertically, at the cost of nose-diving the forward output a bit into the ground, instead of angling up into the air. In this particular circumstance, that “shortcoming” is actually a benefit, because less energy gets sent up towards the building, and instead hugs the ground.
If you spend some time in prediction (or in the real world!) evaluating the effects of these design decisions, you will notice that none of them are truly night-and-day in terms of how much energy makes it to the hotel building behind the listening area. However, they all help a little bit - if we can get a dB or two here, and a dB or two there, the combined benefit is a substantial improvement over not giving any consideration at all to the issue. Especially in situations like this, incremental improvements are often superior to the status quo.