Providing Superior Sound in Burlington

The Event and Venue

I was contacted by my friends in the GrooveBot band about providing sound for a concert they were performing in Burlington at the Visitors and Information Center, run by the city. This facility features a small amphitheater out back with a covered stage. The City of Burlington presents their Summer Nights concert series here, and GooveBot was set to make their debut at this series.

GrooveBot's problem: A large, outdoor area to cover with sound, and a desire to make their debut impressive! A space this large required a larger and more powerful speaker system than the band had.

Why is this tricky?

There were several aspects to this venue that presented challenges. The first problem is the layout. Take a look:

In concerts and other events, it's important to maintain sight lines on stage; we want equipment to be as invisible as possible so it doesn't block the audience's view of what's happening on stage. To that end, subwoofers typically will go on the ground in front of the stage, or will be flown to either side of the stage. Main speakers can be flown, stacked, or pole-mounted to either side of the stage. Stage monitors, well... They typically go right there on stage, but sit low enough that they don't usually block the audience's view.

We have a few problems, here, preventing an ideal deployment:

• Though the stage is raised, there are bushes immediately in front of it, and a sidewalk in front of those. We can't place subs here.

• The stage is covered and has a solid back wall. Bass energy from the subs radiates in all directions, including straight back, and that sound will reflect off the wall. The distance is such that the reflected bass wave will cause destructive interference out front, leaving the audience with muddy and uneven-sounding bass.

• The solid rear wall and roof of the stage also concentrate sound energy on stage, resulting in very uncomfortably loud (and potentially hearing-damaging) conditions for the performers.

• To keep the main speakers off of the stage requires they be deployed on tripods; we can't put a tripod on the left side of the stage because of the sidewalk and stairway.

Now, these limitations certainly don't stop anyone from deploying speakers, like in this photo of another event:

We see a spaced, stereo pair of subwoofers, with large main speaker stacked on top of them. This deployment certainly will get the band heard to the entire audience, but with all the problems described above: this deployment will produce uneven bass because of reflections off the back wall, very loud bass on stage, and the subs being spaced this far apart will cause interference between them that will actually cancel sound for portions of the audience, and augment it in others; in other words, some people will hear no bass, and others will hear very loud, pounding bass.

North Sound Audio's solution

North Sound Audio uses a pair of very-high-performance subwoofers that we built ourselves. Just two of our dual-18" subwoofers will provide more than enough low end for this venue; the throw to the back row is only 100 feet (about 30 meters). Here's our deployment:

We placed the left speaker on the grass just to the left of the sidewalk; we used leveling blocks to keep the tripods level, and ran the cord under some cable ramps for safety. We also placed the subwoofers together on the right-hand side of the stage (audience perspective). Close spacing of the subwoofers like this provides even bass coverage without "power alleys" and "power valleys." But let's look closer at the subwoofers:

One of the subwoofers is turned around and pointing backward. Why? This is what we call an "inverse cardioid" arrangement. We use system processing in the Powersoft amplifier to change the timing, level, and phase of the rear-facing subwoofer, which cancels out sound to the rear of the subwoofers! We get 20-30 dB of sound reduction to the rear of the stack, which means much less bass to reflect off the rear wall and ceiling, and we lower the volume on-stage for the band's comfort and well-being. So why are the subs on the stage at all, instead of next to it? Well, that's a compromise we chose to make. We ideally want the subs to be centered for proper coverage, and placing them off the side of the stage would result in one side of the audience receiving noticeably more bass than the other side.

Was the event a success?

Yes! We heard from several people that the sound we provided for GrooveBot was "the best sound we've had all summer"! The band appreciated a better-sounding stage, and especially appreciated our expertise and knowledge to deploy a speaker system like this.

We called it a good day!