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Discussion Starter · #1 ·
Hello all. My mobile dj rig consists of two Alto powered 12"s pole mounted on top of two unpowered Harbinger 18"s, a balanced Minidsp, and a Umik-1 USB mic. Audio is normally fed from my NI Audio 10 into a Pioneer DJM-707 and then to the Minidsp and on to the speakers and amps. The Alto's are usually set to a height where the tweaters are about 7' up.

I've had the minidsp for a while and am pretty familiar with all the things that it can do, but only recently did I get the mic and start playing around in REW. I have taken some measurements in my basement with the Altos and have been able to use the parametric eqs to tune their responce fairly flat from where I stand when I practice.

So now that I kind of have an idea of what I am doing, I want to take everything outside and tune the tops and the subs, separately and together. I live on a farm with wide open spaces, so the only reflection will be the ground. I imagine that I would want to tune a single top and sub pole mounted as i normally would with the mic at ear level pointing at the Alto top? How far away should the mic be? Should i consider off axis measurements or just from straight on? What other things do I need to consider?
 

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First of all, welcome to HTS!

You might start out EQing sub and Altos separately at first, with a look at integration, but that integration will vary a lot room to room in actual use. Do the Altos at the height you normally would, with the measurement mic on-axis midway between centers of woofer and - horn? (I am assuming) - at three or four feet away so ground reflection effects are minimized, mic pointing straight at the speaker. Closer to the speaker is better to minimize ground reflection, but as you get closer you are more off-axis re the individual drivers and that will introduce inaccuracy, too.

For a quick view to find that ideal measurement spot, put REW in Spectrum Analyzer mode with pink noise and move the mic around slowly by hand to find the smoothest starting response around the crossover frequency. You might end up favoring the horn a bit because its off-axis changes will be more pronounced than the woofer's. Then switch to REW sweep mode for detailed work.

At 3 feet distance with the mic 6.5 feet above the ground, reflections will arrive delayed about 11 milliseconds, so you will see its effects starting around 100 Hz or so, maybe some comb filter effects above that, do not try to tune that out, just work on the overall FR contour - that's my suggestion anyway.

EQ the sub with mic lying on cloth on the ground at 1 foot distance so no reflection effects. Another approach is a few inches from center of the driver cone. Working outside with no walls around, I would probably put the mic on the ground, get the response from a foot or two away, a little closer to real life, although response in-room will be very room dependent.

Then it might be interesting to see integrated response at 10 feet or so, although ground reflections will have major effect, so take the results with a grain of salt.

Those are a few thoughts. Best of luck!
 

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So many issues to cover, but just to keep it simple and cover the low hanging fruit;

I'd recommend experimenting extensively with your measuring, EQ'ing, etc, ... it's all good. Achieve a good intuitive level of functionality with your loudspeakers, subs, electronics and as AudiocRaver stated, REW.

I'm sure you know, but it's certainly worth repeating, ... be mindful that the environment the system is used in for the performance, is a hugely dominant factor. Yes, you can measure and achieve a level of tonal shaping EQ for the mains. However remember the typical throw distances/listening distances, and where your listeners will be relative to your system.

What I'd focus on is the time signal alignment, and the proper acoustic summation of the mains and subs. Assure that the tops and subs are summing appropriately, whereby the resultant acoustic outputs are in phase and smoothly achieving the desired response. Make sure each section is high pass protected, and this filter is engaged during the system blending. If you're using the subs as bases for the tops, fine. Otherwise, if they're on their own, just remember the relative location ... to the tops.

After physical placement, and adjusting delay and phase, ideally you only use level, and subtractive EQ, but you can get away with a bit of shaping on the plus side if need be. Just experiment.

Now the biggest caveat to quality mobile sound is possessing a solid grasp on boundary interactions, and how to achieve maximum acoustic summation and minimal cancelation effects. Anyone setting up and optimizing portable sound systems really needs to understand quarter wavelength effects, and how to use this to your advantage.

Again, you likely know this, but it's worth revisiting. There's two important interconnected elements at play; the manner in which your boxes sum, and the influences of the boundaries. In a typical sub/top scenario, either place the subs less than a quarter wavelength apart, or more than two wavelengths apart. This applies to the freqs within their coverage. So, ... 1120/freq=wavelength in feet, so if the subs are covering 100hz on down, either keep them closer than about 3 feet, or or more than about 50 feet apart.

All too often two subs spread apart, being used as bases for the tops or whatever, are working against each other as much as with one-another. There's so much precious gain to be achieved by co-locating them. The vast majority of set-ups get this simple fact wrong.

Now the second element is boundary loading. Whenever possible, the acoustic savvy sound man will utilize the surroundings to their advantage. Something as simple as utilizing an adjacent wall can gain you as much as 6dB across the board. Add in another boundary and place them in a corner and you gain another 6dB, totaling 12dB of acoustic gain. Don't be afraid to closely couple the driver, and fire the sub into the wall, or into the corner. This gives you a significant advantage and can entirely transform your otherwise modest sub into a beast.

There's no downside to these techniques, as long as you allow a minimum of several inches between the boundary and the driver, it'll work fantastic. Now you will need to assure proper time alignment and blending within the crossover region. This is easy with your measurement gear/running some alignment signal through the system.

After you consider acoustic advantages, you've got to make sure there's no disadvantages, via cancelations. A great rule of thumb is never place subs anywhere between about 2.5', and around 8 feet of a boundary. The problem is there will be acoustic cancelations within the sub's operating coverage.

Oftentimes other factors intervene, subs flanking the stage, or DJ table, in the typical fashion may sum just fine within an area like the dance floor. But you're on the right track, you've got the measuring gear/software, so experiment away and utilize your surroundings to the fullest advantage possible.


The above techniques/approaches, are a strong force multiplier. :T

I hope this helps
 

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Discussion Starter · #4 ·
Thanks for the info guys! Seems like a friendly forum.

So from the way it sounds, I should ditch the speaker poles and place my subs next to each other against a wall or better yet in a corner. If I point them into the corner, how should I orient them and what will that do to the sound throw from a distance?
 

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Discussion Starter · #5 ·
The top is indeed a horn. I've been reading quite a bit on boundary loading and I read that horns aren't effected as negatively in corners or other boundary reflections. Which is good. I usually do play in corners with the speakers to either side of the table, but from now on the subs will go in the corner. Would it be better to stack them? Maybe even 45° from each other so that one is pointing down one wall and one down the other? Maybe that will produce a more centered force?

I admit that I still have much to learn, but have been trying to learn as much as I can.
 

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Simply placing the subs together is a nice advantage, via mutual coupling. Then, boundary load them to the rear wall behind if possible. Then, a step further is to corner load them. You can experiment with stacking if you want to, as long as the drivers are close together.


Now, those steps help gain you acoustic headroom and overall SPL. But you must be mindful of not creating a distinct and obvious disconnect between your mains and subs, .. so careful adjustment of your crossover freq and relative phase will pay off.

You're right, the horn loaded HF will not be as affected as the freqs on down. Sound possesses size, and it always a good idea to keep that in mind, ie., the wavelength size relative to frequency.

The post above contained some great info regarding boundary interactions.

Good luck, and I hope this helps
 

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You should set your measuring mic flat on the ground. Otherwise you'll get a different measurement at every different height you set it.
 
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