Rational Acoustics



Dr. J
July 7th, 2010, 12:55 PM
Hi -- I was wondering if you recommend a certain speaker position for single front loaded boxes with 18's in them?

I have TWO TOP speakers with 12's in them and FOUR FANE Sub front loaded boxes. I typically have to stack two subs per side with a top box on them because of a certain club and I even place ALL subs on the floor and couple them together with my tops on poles in venues where I can.

Is this a room dependant type of thing OR can an individual have confidence in a certain spacing (of my tops and subs) that I can go in and know for certain I will be avoiding nulls & cancellations...etc.

My subs are crossed at 100Hz (Acoustic XO shows more like 125Hz) so the wavelength there would be about 11 feet I guess and a third of that would be around 7-7.5 feet.

Any help here would be greatly appreciated.

Harry Brill Jr.
July 8th, 2010, 03:36 AM
The real answer is both. Spacing of the subs from each other or the mains are definitely important issues, but the room interaction is also important. I think you should consider this case by case.

Dr. J
July 8th, 2010, 08:11 PM
Ok Harry -- I guess I should be a bit more specific :D Let me ask this question and see if it will help. I just started running my subs in the middle of the stage with the Hi/Mids up on poles on the outer edge.

So, with a mid/hi cabinet pole mounted on either side of the stage and a row of 4 subs centered and spaced out along the front of the stage, what would be the procedure for aligning the subs to the mid/hi's?

Thanks Harry -- I appreciate your help.

Harry Brill Jr.
July 8th, 2010, 09:35 PM
Same as before. Pick a spot in the room that represents the most people and align them there. You will still have misalignment other spots which is why we call it system optimization instead of system perfection.

Dr. J
July 9th, 2010, 11:11 AM
Thanks Harry! So with four subs in the middle of the stage -- is there a certain amount of space you would place between them so they won't cause cancellations? All subs are single 18's. I am looking for a more even coverage than 4 subs tightly coupled together but don't want to use a spacing that works against me.

Harry Brill Jr.
July 10th, 2010, 01:32 AM
Stay within a 1/2 wavelength of the highest frequency. 100Hz for example has a wavelength of about 11ft. 5.5ft center to center is a good rule of thumb. 2 subs at that spacing will tend to have a figure 8 pattern at that frequency and become more omnidirectional as frequency decreases.

DennisA
July 10th, 2010, 10:50 AM
With a line of four subs positioned at 5.5' apart, center to center across the front of the stage, is there a specific sub that will be aligned to each pole mount mid/hi or will all four subs be "live" for the measurements?

Dennis

Harry Brill Jr.
July 12th, 2010, 04:29 PM
With a line of four subs positioned at 5.5' apart, center to center across the front of the stage, is there a specific sub that will be aligned to each pole mount mid/hi or will all four subs be "live" for the measurements?

Dennis

I'm afraid you will have to make that decision. There will only be a single location in the room where a single sub will be aligned to a signal main perfectly and every other interaction is in error. Our job is to make the decision where to make the alignment perfect so we have the least amount of error everywhere else. Compromise is inevitable. If I have 1 sub with a main stacked on top of it (outside), I can align them well enough the error would not be noticed. As soon as I separate the main and the sub the error will become apparent at one location or another. We minimize the error by picking a spot about 50-75% into the coverage area. What happens if we take our first stack and duplicate it on the other side of the stage? Now we will have interference between the 2 sides. You can see there is no perfect solution, but some solutions are better in some rooms than others. When you bring walls into it, things get complicated quickly. I make it a point to look at each room individually and do my best not to take the easy way out (unless that happens to be the best way).

Dr. J
July 14th, 2010, 10:41 AM
Wow -- thanks Harry. Seems like I have my work cut out for me. Not sure how much pink noise the club owner can handle but we will see.

Harry Brill Jr.
July 15th, 2010, 02:38 PM
Wow -- thanks Harry. Seems like I have my work cut out for me. Not sure how much pink noise the club owner can handle but we will see.

DOn't forget you can capture a measurement right after the averaging buffers fill up, then you can mute it and examine the trace. No need to play the pink for long periods of time. Also the pink only need to be "loud enough." Watch for the coherance trace to go up and you increase the level, when it stops increasing it's "loud enough". No amount of additional level will give you a better measurement. "Loud enough" may be different at different frequencies.

Kip Conner
July 15th, 2010, 02:47 PM
I wouldn't worry about the club owner if you state your case with a variant level of diplomacy. If you can't convince the club owner to work with you on your analysis then he or she doesn't care enough. I know it's a tough one because we tend to have more pride in our work. Asking them to let you in on a dark day to not distrub the dealings ofthe venue shouldn't be that much of an issue.

One thing you might try is a dual source reference. Playing music with a bed of pink noise underneath is a good way to take the edge off and get solid measurements.

Dr. J
July 16th, 2010, 12:29 PM
You are right Kip. Most are ok if I explain it in terms of making it more pleasant to their customers so they will stay. The biggest obstacle is trying to get them to invest in acoustic treatment which most don't see as a valid business investment.

I deal with a lot of concrete walls and metal ceilings in places. I have never ran in a acoustically treated venue before -- but what a rush that would be.

Thanks for chiming in!