Monday, January 11, 2016

Subwwofer Setup for Bassnectar NYE 360 degree

In my last post I outlined the goals of the sound system design for Bassnectar's New Year Eve 360 sound setup and covered the main flown system decision and configuration (goals #1, 2 and 3). In this post I will cover the subwoofer side of things

The goals of the system design were:

  1. The sound system must be optimized for 360 degree arena coverage
  2. The system should not block sight lines or distract from the visuals in any way
  3. The system must cover the entire arena as smoothly and uniformly as possible
  4. The sub woofer must be powerful and immersive to a very low frequency
  5. The sub woofers and main system must not reproduce excessive low frequencies in the center of the room where the artist is located. (This has been a significant issue in the previous years of doing 360 degree shows before I was involved)
Achieving smooth loud low frequencies covering a large area can be quite challenging. Time alignment issues causing cancellations and huge audible holes in the coverage are quite common in all but the most well thought out configurations. To cover the venue evenly, a ring of subwoofers around the stage optimum. Unfortunately, the subwoofer sound will be the loudest at the any point that is equidistant from the most sources. With a 360 degree coverage subwoofer ring, the equidistant point is dead center and exactly where the artist is located. In past 360 coverage shows, the low frequency volume levels in the center were so loud that vision was blurred and the control equipment needed to be strapped down to prevent it from rattling, falling and there were issues with hard drive malfunctions.

Here is a coverage map of a ring of subwoofers 38 feet in diameter. The grid lines are spaced at 50 feet. In the prediction below with a single ring of 8 clusters of 5 subs, 40 total, we are able to get 103 db at 300 feet away where I marked the level. The prediction specs are listed on the left side of the image measured 20 to 80hz. The actual venue is much smaller so the actual levels were much higher for even the farthest audience member.

But, now lets look at the level in the center where Lorin will be DJ'ing from::

Yikes! 134 db is crazy loud and over 30 db louder than the sound 300 feet away. So, the challenge is to reduce the level in the middle significantly and also if we can get more level far away, that would great. Additionally the solution should look cool and be physically feasible to achieve without disrupting the super smooth coverage we already show.

So to do this I added an inner ring of subs that were also facing outward. the inner sub has a diameter 14 feet smaller and is 8 clusters of 3 subs totaling 24. 

The reason clusters of 3 were chosen rather than more or less was that an inner ring of 24 generated a volume level dead center that most closely matched the volume level that the outer ring generated dead center. The idea is to have both cluster create as close to identical signals dead center as possible. 

The reason a spacing of 7 feet was chosen is that 7 feet is 1/4 wavelength at 40 hz, which is the center frequency between the 20 and 80 hz.and the frequency I wanted to optimize for. For more info on setting up sub arrays I have 3 youtube videos on sub setups:

Ok, so since the most critical purpose of this sub setup is to drastically reduce the level in the center I used a polarity reversed cardioid setup. Once the volume levels are matched as close as possible measuring from room center, then the arrival times of from both the inner and outer subs are matched as well. For this setup at 25 degree C and a spacing of 7 feet, it required the inner ring to be delayed 6.2 ms for maximum summation room center. Then I inverted the polarity of the inner rings and rather than maximum summation, maximum cancellation was achieved. Here are the predictions with the inner ring:

Ok, that's good, we gained a few db at 300 feet, the prediction still shows smooth coverage all the way around so lets look at the level in the middle.

Awesome! 93 db in the middle and  over 10 db quieter than the 105+ db level at 300 feet away. Super cool and exactly what I was looking for. Of course there was a lot of messing around to get to this point. Paul F at Rat helped me with some skills using soundvision software, which I am not overly well versed at. Also Jason Brandt not only assisted in refining the predictions, he also was invaluable in implementing the setup and getting it to perform as predicted. 

And an up close

The real world outcome was quite interesting. The subs covered quite well throughout. No significant holes, low end naturally drops with distance but we were able to get a pretty good level to the rafters. When walking to the center, the low end became oddly decentralized between the rings and when you got up into the central booth, you could feel low end and hear the rotating structure rattling but the it sounded distant and there was an almost surreal drop in level.

If all that info is not enough for you or you are versed in Soundvision software, here a link to download the file with all the info:

And here a link to download Soundvision, though you probably will need a training course to use it.

Cool cool, hope this is interesting stuff, hit me up with comments on stuff you like or want more of.  Gonna try and get back into the rhythm of posting and sharing.