Power Alley

[Original Beef]

I did an outdoor gig with my rig last week. 4 x 18inch subs and 2 mid-tops. The stage was a curtainside lorry. Due to logisitcs I was mixing directly in front of one of my stacks about 20 metres away. During one of the bands set I walked into the middle of the two stacks to have a listen and was overwhelmed with the bass. It was a good 10-12 dB louder in the middle. I know that this is due to comb filtering and fully undersand why this occurs.

 

 

I have a very similar gig coming up in a few weeks and thought I might try putting my subs in front of the stage in the middle and place my mid-tops on the lorry. I am sure this would solve the power alley problem, but would I be best stacking my subs 2+2 or line them up lengthways next to each other? Does anyone have any experience of this scenario?

[Doug Siddons]

I would put them in line across the front equi distant apart

[mackerr]

I did an outdoor gig with my rig last week. 4 x 18inch subs and 2 mid-tops. The stage was a curtainside lorry. Due to logisitcs I was mixing directly in front of one of my stacks about 20 metres away. During one of the bands set I walked into the middle of the two stacks to have a listen and was overwhelmed with the bass. It was a good 10-12 dB louder in the middle. I know that this is due to comb filtering and fully undersand why this occurs.

 

 

I have a very similar gig coming up in a few weeks and thought I might try putting my subs in front of the stage in the middle and place my mid-tops on the lorry. I am sure this would solve the power alley problem, but would I be best stacking my subs 2+2 or line them up lengthways next to each other? Does anyone have any experience of this scenario?

 

The power ally is caused more by cancellation than summation, the two sources can only add to a gain of +6dB, while they can cancel to almost nothing. The power ally is most noticeable when you are mixing in one of those nulls, so you push the low end to compensate. To reduce the lobing due to interference between the boxes you want them as close together as possible. To make the summation in the transition from your mains to the subs as good as possible you want the subs as close to the mains as possible. The two are usually mutually exclusive. Outdoors the power ally is the dominant lobe with subs at the sides of the stage whereas indoors reflections make the cancellations and summations much more complex and the power ally may be less apparent. You may find a center grouping as tight as you can make it is better. That will give you the smoothest coverage and the greatest coupling between boxes, although because of proximity it may be crushingly loud in front of the stage vs elsewhere. If you have the stage height to allow stacking in the center a 2x2 array will have the most coupling.

 

Spreading them across the width of the stage will reduce the concentration down front, but will begin to introduce more lobes, although more complex than the side stacks so perhaps not as noticeable. It may be a good compromise between side stacks and center cluster. ideally speakers producing the same sound and covering the same area should be 1/4 wavelength or less apart for full summation. At 100Hz the wavelength is about 10', so ideally the speaker centers should be 2.5' apart. Below 100Hz the wavelengths are longer so the distances can be greater. Farther apart, less ideal summation.

 

Mac

[Original Beef]

Thanks for the replies. On reflection I could have worded my original post a bit better. Instead of saying it seemed 10-12 dB louder in the middle I should have said it seemed 10-12 dB quieter at the sides, but I digress. If I get time during set up I will try both scenarios recommended above and walk the area, but it seems apparent that either situation will be better than ground stacking either side of the stage.

[the kid]

If you trawl through dave rats blog he has several styles of set up that may or may not work for you.

[Original Beef]

If you trawl through dave rats blog he has several styles of set up that may or may not work for you.

 

Thanks for that. Just checked out his very informative Youtube videos.