weird feedback issue -- drums themselves feeding back

[morgansterne]

We had a gig this saturday, a venue we've played a dozen times over the past few years. We had to run our own sound since our usual sound guy couldn't make it. We're a little out of practice so the other four guys in the band got together at the venue around noon, set up the whole PA and did a soundcheck, then leave again. (I live much farther away from the venue than they do so they didn't even bother mentioning this plan to me.)
The only snag being they wanted to use the sub, which I have at my house.
So I get there early and hook up the sub. We use a separate rack style PA head to power the sub -- just plug the kick and bass guitar into this and keep them out of the main PA. It's a 500 watt speaker, no crossover, so essentially you can run a separate PA for the kick drum and bass guitar.
I start turning up the PA slowly and we get a low frequency hum coming out of the speaker, even with the sub very far away from the drums and the volume at a low level.

TL;DR
the feedback was NOT from the microphone, it was from the kick drum (resonant) head itself. If I put my hand on the kick drum head, the feedback instantly stopped. There's no port in the head and the drummer apologized and suggested that's the reason it was feeding back. Even if we'd had a gate on the channel, it wouldn't have mattered as the feedback started at very low volume settings.
eventually we reconfigured the setup to put the kick through the main PA, to take advantage of the better EQ and dial out the offending frequency (250 hz -- no matter how we set channel and main EQ on the rack PA, we couldn't get rid of that frequency enough to make a difference. Only the channel parametric EQ on the main mixer did the trick). This meant we had to steal the aux send I use for my in ear monitors, so the night was a little rough on my ears.
anyone experience this problem much? would having a front hole in the kick drum have prevented this (and helped save my hearing)??

 

[walterw]

So I get there early and hook up the sub. We use a separate rack style PA head to power the sub -- just plug the kick and bass guitar into this and keep them out of the main PA. It's a 500 watt speaker, no crossover, so essentially you can run a separate PA for the kick drum and bass guitar.

yeah, don't do it that way.

the kick and bass need to be in the main PA too, and the sub needs to have everything above like 100Hz taken out of it.

 

[morgansterne]

right. I know this is the low budget way to do it. We've never had a proper crossover. We hardly ever run sound ourselves anymore so I don't fancy upgrading our PA at this point.
We ended up with an aux fed sub, which is a good way of doing things, but even then it was feeding back until the parametric EQ took out a huge amount of 250hz.
Our usual method certainly wasn't the right way to do it, but it wasn't the central problem. That's not what was causing the feedback issue.

 

[Silas Stingy]

With no crossover in the setup there is a potential to create a low frequency boost, where the bottom end of your top speakers frequency range combines with the top frequencies of your sub, this could be right about where your kick drum is tuned.

 

[ggunn]

Is the sub under the stage? Sometimes subs under the stage can acoustically couple with the drums and set them resonating.

 

[jimfist]

I've experienced this problem on occasion. Coincidentally, it only happened with one drummer using the same kit (a double kick drum kit), but only on ONE of the kick drums, but not both, single mic'ing the outside head sound hole of each drum. I've never heard this problem with any other drummer or drum kit.

I'm generally very much on-top-of trouble-shooting problems in live sound situations. This one had me scratching my head for a while, especially when all of the typical remedies didn't work as well as I thought they would. Thus it leads me to believe that there is an odd low end coupling (between the room/stage, subwoofers, drum tuning and placement) going on that sets off the drum as very specific frequencies.

These days, I'm able to "mult" a kick drum into 2 or 3 discrete inputs of a digital mixer, with each channel handling a different frequency region, and then use different EQ and dynamics settings on each channel. An alternative would be to have a PZM mic mounted in the kick drum to capture attack (high end) of the batter head and a second mic at the outer resonant head to capture the low end push. Then you do the same, treating each mic differently with respect to EQ and dynamics. You can gate the "low end" mic path more heavily this way in order to address the runaway feedback issue, but I do believe the real answer lies elsewhere, with the drum tuning/damping, mic placement, and subwoofer placement.

 

[LiquidMidnight]

Yes, you definitely don't want your bass guitar going through just a sub without tops. The frequencies produced by the sub are such a small part of the bass's range. Even though it's called a "bass" guitar, it's really a low-mid oriented instrument. You really don't even want a kick going through just a sub, as it gets its attack from much higher in the frequency spectrum.

 

[morgansterne]

The frequencies produced by the sub are such a small part of the bass's range.

the "sub" is a 15 speaker with no crossover. There's no tweeter or mid speaker but it can cover some higher frequencies. We've used it plenty in the past for kick, less often for bass guitar. Keep in mind we're not trying to fill a huge venue with it, just trying to give the main PA a little break. Honestly I think our tops are still distorting a bit with vocals, they're 15" JBLs but I don't think they can handle the volume we're trying to get out of them.

Is the sub under the stage? Sometimes subs under the stage can acoustically couple with the drums and set them resonating.

The sub was set at least 15 feet from the drumset, but the drums were on a small stage that's hollow underneath. (room for drums plus one other person, it's more of an oversized drum riser -- pretty strange.) I suppose that contributed to the drum head feeding back.