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Off Center

I hinted at this a little bit the last time I was on Church Tech Weekly so today I want to touch on some of my thoughts on the mix position or FOH location for live sound reinforcement. I might have written about this in the past, and if that’s the case just consider this my current feelings on the subject.

Believe it or not, the best place for the mix position is actually debated amongst engineers more than you’d realize. I mentioned in the podcast how I don’t like FOH being off-center so let’s start there.

One of my main problems with being off center has to do with low frequency coverage. When we start spacing sound sources with overlapping coverage more than half a wavelength of their highest frequency apart, the two sources will no longer sum together in nice ways, and destructive interference often occurs instead. We typically refer to this by it’s street name: comb filtering.

To review, Comb Filtering occurs when we have a sound produced by two sources where there is a time arrival difference between the sources. When we start spacing our sources apart farther than 1/2 wavelength apart, the difference in time arrival of those two sources creates phase cancellation when we’re not standing equidistant to the two sources. As you can imagine, in the case of two loudspeakers, only a small percentage of listeners in any space are going to be able to be in a position where they’re equidistant to both speakers.

While the comb filtering in the mid- and high-frequency ranges can cause issues in some circumstances in sound reinforcement, in my experience it doesn’t become as problematic as with low frequencies. If was problematic, we’d probably all be going out of our mind at what happens to center-panned sources such as vocals. I have some theories as to why that is, but I’ll leave it for the more educated to explain that one while I talk about the potential low frequency issues.

In most large PA’s, it’s often difficult to get all low frequency sound sources close together. These might be subs, but they could also be the mains or “tops”. Let’s use subs, though, since it’s a little easier to look at.

Think about the traditional side-stacked subs. Now observe the photo above. This is a model of subs side-stacked approximately 50′ apart at 80 Hz. Notice that the distance between the subs creates areas of destructive interference within the listening area. You can see these nulls that occur as dark blue in the heat plot. The darker the color, the greater the null or cancellation.

Notice where the darkest nulls occur?

Just off-center.

If I ran models at more frequencies within the bandwidth of those subs, we’d see the nulls occur in slightly different places moving out from the center. The one thing I can guarantee, though, is that the largest nulls would all occur right off-center almost right on top of each other.

Now picture our intrepid FOH engineer mixing just off-center in the room. He’s meticulously tweaking away trying to get just the right amount of feel and low-end punch out of the subs to give the music the depth and weight it needs for emotional impact. But he’s off-center standing in the 1st null of the subs so no matter how much he cranks the low-end, he just doesn’t hear what’s happening. Meanwhile as he turns and turns the knobs trying to figure out what’s wrong, his trusty assistant standing just to the side of him in the coverage of the subs is on the verge of a bowel movement along with the other 80-90% of the audience sitting outside that first null.

OK. OK. Maybe this is a bit of an exaggeration because acoustical reflections in the room generally keep complete cancellation from happening in those nulls, but those nulls will still be an issue. If you look in that picture above again, you can see the box where our current mix position sits. Our sub configuration is quite different these days so that null isn’t nearly as deep, but I did mix for several years with that configuration and it was definitely a challenge to get the low end consistently right.

Now, this low frequency issue is a particularly sensitive issue for me because these days I spend the vast majority of my time mixing in a church, and what is the biggest sound complaint most people have in modern church’s?

That’s, right! You guessed it! Too much low frequency content.

So my personal preference for mix position is to be in the center of a two-channel/stereo system because it’s the one place where everything adds together. It’s also, theoretically, the one place where all frequencies will be audible if the system was properly designed and implemented. If I’m going to make mix decisions that impact everyone’s experience, I want to be in the place where I know I’ll be able to hear all the frequencies that would make someone uncomfortable: namely the low-end and upper-mids.

Being in the center, however, is not without its downsides. Since it’s the place where everything tends to add together, we often have what we refer to on the street as power alley. Since those subs add up real nice together right in the middle, they kick more in the center than throughout most of the room. The whole PA may also be a bit louder right in the center. Sometimes this can lead to some engineers undercooking things since they are getting so much more than the other 90-98% of the room.

Another issue I’ve experienced dead in the middle is the comb-filtering that happens in the mid- and high-frequencies can become an issue from just the little bit of movement you naturally do when standing. It’s not always the case, but I have had been in situations where standing right in the middle of the left and right PA stacks has been weird.

Some engineers dislike the middle just because of the simple fact that barely anyone listening is in that center sweet spot in the room. So part of the thought process is to get off-center to be more in the real world of the rest of the audience.

These are all fine reasons to dislike mixing in the middle of the room, and if you’ve been mixing for a long time and know where you like to mix from, I’m not looking to change your mind. Really I just believe you should strive to mix from the place where you are most comfortable.

Personally, I think the mix position can be a bit of a moving target. You are always the only one standing where you are standing so the rest of the audience getting a different perspective than you. How different that is will vary depending on each situation. The point is there is no perfect anything. Getting outside of your mix position to listen to things and get another perspective is definitely beneficial. I even do that when I’m in the studio. However, to me that should just be a spot-check situation which makes the mix position very important to me since it’s the primary location I monitor from.

If I had to pick my ideal mix position, it’d be on the floor and not under a balcony about 2/3 of the way back in the room in the center of a stereo PA. In a really big room, I’d probably want to be somewhere in the 75-100′ range away from the stage. In a small room I want to be as far from the stage as possible and ideally not up against a wall.

So a few more quick thoughts on this.

If I’m too close to the stage in a big room, the stage volume could influence the mix to the point that I get it wrong for the majority of the room that doesn’t have that stage proximity. If I’m too far away, though, I could end up hearing the mix too late–remember, distance=time–so my mix moves don’t sit in the pocket anymore.

Dead in the middle between the front and back walls of the room can be problematic because it’s typically where room mode issues all like to party together making for more weirdness. And being too close to a wall or under a balcony also typically causes weird frequency buildup issues. Being under a balcony would probably be a little easier to deal with, in my opinion, than being up against the back wall, though, because fills might be used to help with the issues. On the other hand, you can’t really do anything about the bass build-up when you’re against a wall, though. Although, I’ve also been in situations where having that bass bump from the wall was nice.

I guess the bottomline is I believe as a mixer you have to find the spot where you’re most comfortable mixing. There might be some better positions than others, but the ideal ones get debated. Given that, I think it’s important to understand what you’re trading by setting up in one spot over another. All too often I see churches that arbitrarily just stick the mix position somewhere that looks relatively OK. In my opinion, that’s a foolish move considering the person mixing will have an effect in some way on every person in the room’s experience.

Now there’s one last thing I want to say on all this. A few years ago I did some research on where the big chair engineers I admire have their mix position. I can’t remember seeing one of their consoles set up off-center. I’m just saying….

3

Mixing Mindset: Range for Impact

So I’m gonna break one of my rules today because I want to do a little illustration. I feel like a lot of newer engineers miss this boat sometimes when mixing. They make it all about loud when they should really be going for impact. I believe there’s a time to be loud, but loud is just another tool in our arsenal.

So I had my little iPhone camera at FOH earlier this summer, and I had it running through one of the music sets. Unfortunately you can’t see much in this because once everyone stands up they block the stage. I stand to mix, but sadly, my little iPhone was not that tall. However, you can see something here that I think some of you might find interesting. At the bottom of the frame is my RTA along with….I can’t believe I’m doing this…the A-weighted SPL of the room at FOH.

I’ve taken my FOH mix and slapped it on the video so your ears don’t fall off from the distorting iPhone mic, and then I mixed the audience mics into it a little. For full disclosure’s sake, you should also know there was an L2 on the mix doing about 1-2 dB of limiting at the loudest part of the song and Brainworks’ bx_solo plugin widens the whole thing just a hair. My point is, this is essentially an untouched 2-mix from FOH with some audience added. It’s not my favorite mix, but I remember it felt good in the room and it’s a good example for illustrative and educational purposes.

As you’re watching this, notice the RANGE of SPL in use and how it follows with the emotional impact of the song. I don’t have any kinds of limits on this stuff week to week. Nobody sitting in the seats or from leadership came up to me at any time to say it was too quiet or too loud. In fact, as memory serves, this was a day when I had more people than normal coming to FOH after the service to say how good it was.

Now, I’m not saying you should be running your mixes as loud as we do or with as much range as we do. You might want to run less, or in fact, depending on your situation you might benefit from running louder with even more range. Who knows. You have to figure that out. The point I just want you to understand is that music is not about hitting some target number.

When I’m mixing it’s about music and impact. Just think about what happened to recorded music when the recorded industry started pushing everything closer to digital zero. When you target a number, the music almost always suffers, in my opinion.

Parenthetically, one thing that really sticks out to me listening back to this several months after actually mixing it is the way I’m riding the vocal. I can really hear how I was pulling it back and sinking it into the crowd in the room especially at the start of the first verse. I’ll save talking about that for another time, though.

As you’re watching Smaart in the video embedded below, you’ll notice the RTA running there as well. Smaart’s RTA is running at 1/48 resolution here with a 5 second average. My Target Curve is overlaid on the measurement, but you’ll probably notice I don’t exactly adhere to it. Again, that’s because mixing is about how it sounds and feels and not always how it measures. That RTA is another tool I use all the time, but I rely my ears and gut far more than the RTA.

3

The Physics of Concrete

https://farm4.staticflickr.com/3728/9714172926_9a3c5948f9_m.jpg

That’s what I’m blaming. The concrete. So here’s what happened.

This week we did a little experiment with our subs. Our subs are good, but in truth I have never been as satisfied with them as I feel like I should be. Playing with the d&b B2’s at last year’s almost-Concert on the Lawn and then the L-Acoustic SB28’s this year hasn’t helped my satisfaction, either. Oh, and then there were the J-Infra’s my buddy Zito had out with OneRepublic this summer. Those were quite nice. Quite. Nice.

Now, I know our Meyer M3D-Subs aren’t any of those, and that’s OK. Our subs are good, and most people would probably question why I’d even consider changing anything. However, it would make me happy if our subs performed a little closer to what I’ve been experiencing with some other boxes. Plus, a couple months ago when we were working on the outfills for the West PA, my friend Ed Crippen made a couple of tweaks to the West sub array that got them much closer to what I’ve been wanting so I’ve been even more motivated as of late to try and move our East subs up a notch. This week we took a shot at it.

I had three primary goals in reconfiguring the subs:

1. Tighten up their performance and punch.

The current horizontal arc of the array works great for even coverage without the fingers of destructive interference from stacks spaced on either side of the stage. However, arc’ing and/or using incremental delay on a horizontal sub array has a tendency to induce time smear in the listening area resulting in decreased punch and impact. Things can be punchy in the near- and mid-field of our sub coverage, but as you transition into the far-field, that punch goes away. And wouldn’t you know it, mix position is right on the edge of that. Moving the mix position 3-4′ forward would be all it would take to get that punch at FOH, but then FOH would be too close. So my hope was to tighten up the overall sub response throughout the coverage area so everybody wins.

2. Increase headroom.

We have 10 M3D-subs in the room, but because they’re cardioid they don’t seem to sum quite the same as traditional subs, and on a typical week I mix them right at the edge of excursion. You don’t hear it at mix position, but if you get right down in front they don’t sound so good in excursion. Plus it’s probably not so good for the longevity of the speakers, either. I typically have Meyer’s RMS up on my machine so I can keep an eye on what’s going on with the subs, and I’ll back down my feed to them as necessary. So my next hope was to get the subs to sum a little better giving me a little more headroom in the overall sub performance.

3. Widen the coverage.

Horizontal arrays are great, but with an array length as long as we’ve got we end up with pattern control as wide as the actual array. It extends out beyond the array simply because of the arc, but that still leaves sections of the room on the far outsides with lesser coverage. In my opinion, they are not good seats, however, the people sitting in them are still important so I would have loved to increase the coverage.

The challenge in reconfiguring the subs is there’s not a lot of real estate to do it. They have to live under the stage, and that area was really designed for the subs to be in bays. So the structure doesn’t offer much flexibility for alternate configurations. There is a narrow path running behind each bay, though, so I measured the space under the stage several months ago when I was originally thinking about this. I plugged those measurements into MAPP so I could play around with some different designs knowing roughly where the speakers could fit.

So let’s look at the models.

Subs current

The photo above is our current configuration. Every color change represents 3dB so you can see how the coverage cools off on the outsides. You can also see how the level drops right at mix position just right of the center of the room.

Now here’s what I was envisioning. My idea came from conversations with Steve Bush from Meyer Sound when he was onsite last year helping with the install of the West PA. Since the subs are cardioid, my idea was to tight-pack them and position them so each side of the room would get its own stack of subs. I then aimed each stack so there would be minimal coverage overlap between the two. My thinking was the close proximity of boxes would tighten things up and aiming them this way would keep the coverage smooth on each side of the room. Here’s the model:

Subs new

You can see in the proposed idea that the coverage in the room gets wider and more consistent in level. Now let’s check out some Virtual SIM measurements.

03 Current  FOH
Current Configuration @ FOH

04 Stagl  FOH
New Configuration @ FOH

Virtual SIM measurements showed an increase in level relative to our current configuration of about 4dB at FOH, 2.5dB in the front row, the back of the room stayed virtually the same in level, and the front sections on the far sides would get around 6dB more level. I figured this was just the extra little bit of headroom I wanted without blowing away the front of the room to get it.

The model looked interesting to me, so I bounced it off some friends who were also intrigued, but there was still some skepticism. You see, the thing that MAPP isn’t accounting for is that the subs are under a concrete stage. And one thing we know is that when you put cardioid subs under a stage, they don’t quite work the same as if they’re in open air. So I knew the only way we would know for sure was we would need to actually move the subs.

We spent most of the afternoon on Monday moving the wiring and subs around under the stage. Fortunately, most of the area under the stage is carpeted so with the help of a crowbar we were able to get furniture movers under each 400lb. sub. With the furniture movers in place, the subs move relatively easy especially when two people were pushing and pulling them.

Of course, things didn’t go entirely to plan. The first issue that bit us was somewhere my measurements of the space went off a bit so things didn’t quite fit the way I had hoped they would. We ended up a few degrees off on the aiming of each side, and the back row of subs weren’t able to land exactly where I wanted them. Incidentally, the back row wasn’t an attempt at an end-fired array; that just seemed like the best place I could put the subs. Ideally, I probably would have had those two subs stacked on top of two of the front row boxes, but there’s not enough room under the stage, AND there’s no way any of us would have tried lifting them under there even if there was room.

I came really close to scrapping the whole plan when we couldn’t get the first side in position exactly the way the model was, but I figured I would always be wondering if it worked so we went ahead and moved them all into place. Of course, all hope drained away after I re-timed them to the tops and finally got some music going.

Everything measured quite nicely, but the subs felt worse than before. I tried turning the back row off and flipping polarity but nothing worked. My measurements showed me the same, if not more low frequency level at FOH, but I didn’t hear it. There was no punch and the deep lows I was used to were missing. Some of the guys who helped move the subs were wondering if I was wrong in my initial assessment on the new position. After mixing on this system for 4 years, I knew what it could do, and the new configuration was far from that. I really wanted this to work, but it didn’t.

So on Tuesday we took a couple hours in the afternoon to move things back into the original positions. I was very thankful I had spiked the original locations of the subs because as soon as we got things back up and running it was quite clear that our original setup is the optimal one for now. I’ve still got some other ideas I might explore, but the subs are staying put for the foreseeable future.

Now some might look at this whole escapade as a waste of time, but I don’t see it that way. This was a priceless learning opportunity. I’m still a very strong proponent of computer modeling and using measurements for loudspeaker optimization. However, even if things look good in computer models and measure nice, you still have to put your ears on things. In this case, I liked the model and measurements, but I didn’t like the way it sounded. If we hadn’t actually moved things around and given it a listen, I never would have known how the setup actually worked. Personally, I think the cavity underneath the stage thwarted things because it was a missing variable in the models, so I’d still be curious to hear a setup like this in an open air environment to see if it’s something I should rule out for all-time or not.

There was one other great thing that came out of this, though. Through the course of rewiring our RMS lines to our subs, some of the RMS issues I’d been having from time to time cleared up and some missing boxes that were up in our tops finally showed up in RMS. If you’ve played with RMS at all, you can probably guess how happy that alone made me.

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