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Hello experts,

Jarjar here. Misa confused about Subs causing rumble on the stage.
I worked as stage hand to a concert last week. They had problems with this rumbling movement caused by the subs. The subs were positioned in front of the stage, piled in 6 pairs, so there were 12 subs in total. But the vibration the subs caused was annoying for the musicians on stage.

So, the techs decided to line up the subs instead of putting one of top of another. They took out their calculators and figured that if they put one sub 1mt in front of another the sound would phase up. Also, they said, it would take care of the rumbling on stage. But it didnt do much.

¿How would you solve this vibration if ceiling suspension is not an option?

BTW, it was an outdoor festival with at least several thousand people in it.


dvdhawk Fri, 01/30/2015 - 16:16

I guess they need new calculators. Was this a professional caliber sound company?

I know there are people with much larger rigs than mine around here, but I'll take a run at some of the obvious things.

I think you'll find subs are virtually omni-directional, the lower the frequency - the less directional. That is just as true if they're suspended, just maybe physically a little further away. Stacking subs and/or putting them side by side often allows them to couple in a way that makes them louder without working as hard (which is generally considered to be a good thing). You would have to consider the stage, to see if a crowd of that size would be better served by flying the subs or a ground-stack.

Indoors it's possible to just have too much low-end for the container you're in. (10lbs. of crap in a 5lb. bag as they say) Outdoors you should have no boundaries working against you. This really shouldn't be that difficult.

Some things that might be working against you:

  • Resonant stage decking of portable stages. Decouple the speaker wings from the main stage if you can - or stack the subs on the ground if necessary. (is that what you mean by "in front of the stage"?) Ever see a show where the artist had big persian rugs for all the frontline musicians? Classy, comfortable to stand on, not as slippery as the standard painted plywood decks in your fancy boots, AND good at dampening vibration perhaps.
  • Cheap, broken, or poorly isolated mic stands can be EXTREMELY resonant. There should be an important but unseen cap on the hidden end of the smaller diameter tubing, to keep it from rattling against the inside of the larger tubing below the clutch. The plastic or rubber cap can get brittle and fall out or get damaged over the years. Tapping the culprit stand will sound like a hollow drum, every vibration will course up the stand into the mic(s). If that's the case, thread the clutch off completely, separate the pieces and see if it's missing the cap. If you can pull the small tube clear through the clutch without unscrewing it, the cap is definitely gone. A few wraps of electrical tape to make it just shy of the I.D. of the larger pipe will do just as well. Your mic stands should have rubber feet as well, if they're missing it can be a factor.
  • Insane monitor levels, especially drum monitors. One cheap mic stand, or ringy rum head can get over-stimulated and ruin everything by humming constantly.
  • If there's a dedicated monitor guy who isn't good at his job, he can derail everything. It's really a much harder job than FOH. A good one is worth his weight in gold.
  • Mute every channel one by one until you determine if it's just one or two channels.
  • If it isn't acoustical feedback, it's not out of the question that it could be an electronic fault / feedback loop.

Other than that it's business as usual - low-cut where appropriate, gates/expanders as needed. Honestly, this should not be that hard.

Any pictures?

bouldersound Fri, 01/30/2015 - 17:56

jarjarbinks, post: 424476, member: 40927 wrote: So, the techs decided to line up the subs instead of putting one of top of another. They took out their calculators and figured that if they put one sub 1mt in front of another the sound would phase up.

Look up cardioid subs. If you place them in an end fired array you can make them rather directional over a limited range of frequency. Fortunately subs usually cover only a limited range of frequency so it can work out reasonably well. The trick is to place them a half wavelength apart (for the desired center frequency) in a line from stage to audience, then delay the far sub (from stage) by an amount equal to half that same wavelength. The waves at and near the given frequency will add in the direction of the audience and cancel in the direction of the stage.

If they were doing appropriate delay and positioning then it should have worked well. If not then I'm not surprised it didn't work.

paulears Sun, 02/01/2015 - 14:00

Let's be honest here - 12 subs is a LOT of low frequency energy, and it's going to get everywhere. Cardioid subs mean rows, and space between them, and rely a lot on having plenty of space, because it's based on wavelength coupling, and is quite narrow band and frequency specific. The theory is fine, and the results when done properly are pretty good, but it's unlikely to be made to work. If the stage has space under it, and the low end gets in there, it can couple to everything on it!


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