Puzzling bass rumble

[wesg]

Played a venue Friday night that has four subs underneath the stage. Bass guitar amp sitting on stage, two 10" speaker and a horn (Mark Bass, 400W head). Active pickups (I think).

 

Here's the weird thing....the bass was shaking the stage and drowning out the other instruments. Bass was turned down low, as low as our rehearsal hall. Third set he turned down real low and that helped a fair bit.

 

Other than the bass, stage volume was good. Rumble was inaudible in the audience, but loud on the dance floor. Rumble was at its worst playing open strings, and it seemed to "build", so if he stopped playing for a second, it would go away but come back again after a few seconds of playing. Bass player describes it as some kind of feedback.

 

Ideas? Sound guy high-passed the stage monitors at 100Hz. Rumble was definitely lower. Drum kit was on a riser. Leslie was 15 feet away from the bass amp. No bass amp or Leslie in the monitors. I was going to try flicking the low-cut switch on the RE20 but forgot.

 

Never had a problem here before, but for some reason the bass player closer to the middle of the stage instead of the edge like he usually does. Dunno if that's relevant or not.

 

Typical stage plot: http://drbombay.ca/drbombay-stage-plot.pdf (note bass player and lead guitar switched around on Friday for some reason?!. Also, Leslie mics are no longer oriented that way)

Photo shot that night: https://scontent.xx.fbcdn.net/v/t1.0...79&oe=579D7F45

 

Stage volumes in this band are loud, but not stupid loud. And they have been dropping, Friday was actually really good other than the bass. They guys know the quieter they play, the better my intonation on background vocals. Seat of the pants estimate would be around 90dB. Nowhere near ear-tired loud.

 

Thanks for any suggestions. I'm really scratching my head on this one. Should I recommend an amp stand for the bass amp? I was thinking about trying to find four paint can lids at one point to see if I could reduce it. It was LOUD. I could barely play my 40W Leslie loud enough to hear myself. We're not back there 'till September, would be good to have a solution in mind my then.

 

Wes

[RoadRanger]

Was the bass in the subs? Kick drum mic gated and/or not stoopid loud? If your using the venue's "sound guy" you might be SOL - he's probably cranking the gains to make up for the lack of stage volume (assuming he's shooting for stoopid loud)..

[Tomm Williams]

Was the bass cab mic'd or direct ? If the stage or drum riser is flimsy enough, you could get some vibration into the kick mic as well. Take a look at the mounting method for the kick mic, a weighted bag would help with vibrations. HPF every single input and then disengage (if desired) to see if the culprit shows itself. Also HPF the RE20 while you're at it. Do you have much LF material in the wedges?

[agedhorse]

How much bass was in the PA (especially subs?)

[Craig Vecchione]

"Bass player describes it as some kind of feedback" seems spot on. The fact that the rumble doesn't begin immediately, but builds shortly after he begins playing, is the clue here. I've had this problem when the kick mic picks up the bass guitar either via floor coupling or if the bass amp/speaker is too close to the kick. I've also had as Andy is alluding...the bass was simply stupid loud and mic'd, and was feeding back. But note that almost any mic on stage could be the culprit. Cut one mic at a time to see if it stops the problem.

[wesg]

Hi, guys! Thanks for the great follow-up questions. Answers:

 

- Yes, the bass was in the subs. No choice, venue is very large. Venue sound man is really a DJ, so unlikely the kick was gated. His mixes tend to be appropriately bass-heavy for a classic rock act in a dance club. The fact that the sound man is really DJ (who, to his credit, tries hard, has decent ears, and is very open to learning) is why we couldn't solve this during the performance...he's not used to troubleshooting problems, and I can only ask for so much over an open mic. (I also refuse to say anything negative over an open mic).

 

- Bass amp has a direct out. We do not mic the cabinet.

 

- The only LF material in the wedges is from the digital piano and synth, and I play very, very few low notes. Wedges get keys submix, vox, acoustic guitar (one of those Godin semi-hollow things that also has a coil pickup).

 

- Kick mic is an EV model (PL33?), placed inside the drum on a small pillow or towel or something like that.

 

Craig's hypothesis is very interesting. I will add the observation that, since the bass player was 3 feet closer to the center than usual, his amp was also roughly three feet closer to the kick mic. See pic - it's right behind him. The drums are on a riser. I'm not sure if the riser is on the stage, or part of the stage. I suppose it doesn't really matter.

 

Thanks,

Wes

[aaronm72]

Had this happen to me recently in a venue I mix quit a few shows a year in. Found that the front kick head was resonating off the bass guitar cabinet on stage. It didn't help that the reso head on the kick was ripped and buzzing like a chainsaw. Was using an external Beta 52 and fixed it by moving the mike a bit further away from the port hole. I know you said the mic was internal but maybe my experience could be a bit more information/clue to where your issues stem from? That said if the gain structure and gating is a mess well then good luck with that!

[Tomm Williams]

Not knocking DJ's but most seem to think that sub frequencies are the feature part of every song. I would have to be at least curious if his mix and EQ boosting on the lows is way out of context for the music.

[Dogoth]

Anybody think about stage (or possibly room based on where the subs were) resonance? If it was loudest with open strings, Id bet one of those open notes (probably the lowest one) is the resonant freq of the stage. You said louder on stage and still noticeable on the dance floor? Again it sort of points to stage resonance. Yup. Gating the kick can help a lot. Otherwise isolating things can help a bit. Eliminating the resonance (cross beams screwed into the stage from below is a start) is the best fix. Moving the subs to another location would probably help as well (maybe not practical though - depends on the room layout).

 

My .02 worth

[dbMontana]

I've been holding off on a post so as not to thread-jack Wes' question but think this may be at least tangentially relevant.

 

My normal venue is looking to increase the size of the indoor stage. Presently is it only about 12' deep and the intent is to add another 30"-36" to that dimension. Presently there are two 18" subs center-clustered in front of the stage and to avoid taking up more of the limited floor space in front the intent is to raise the stage enough so that the subs can go underneath (as Wes mentioned in his case). That raises major questions for me regarding how to isolate the thump and keep it from rumbling the entire stage deck. I'm not saying that is the problem in your instance Wes but it can't be helping.

 

So I know under-stage subs have been done elsewhere but what's the best reasonable way to provide acoustic isolation from the stage deck? I've somewhat seriously proposed building a 2" concrete bunker to encase the subs (it is a concrete floor underneath) or... I dunno. What do the "big boys" do? Thanks in advance for any input.

 

..dave

[wesg]

Heard back from the sound guy - thinks maybe there was "some gate" on the kick, Hm.

 

Working theory is now that the bass amp was getting into the kick drum, which caused the stage to rumble with the subs, possibly building up to a resonant frequency, which was in turn picked up by the kick mic (or maybe the Leslie lower mic) and fed back into the kick.

 

I didn't notice the "out front" mix as being particularly bass heavy, but I only had about 20 seconds to listen from FOH all night. The organ was mixed a bit quieter than I would have, but it was very present and sounded good. Plenty of sound guys bury keyboards; not this guy.

[Pro Sound Guy]

Live sound check and pull one channel or two out of the mix at a time to isolate the culprit. Try it with monitors on and monitors off.

Also, typically subs under a flimsy stage can be a real issue with a live act with DRUMS. D.I. bass bleed into the drum mics is probably not the problem. So I have couple theories. Was a HPF mentioned in this thread?

 

[wesg]

I wish this had popped up at sound check. No issues then, or (to my recollection) the first set. I wonder if FOH levels were boosted as the crowd grew? We couldn't really stop the show when it was (from what I was told) not audible at FOH.

 

HPF was not mentioned. That said, there were no drums or bass in the monitors. I'm assuming all the vocal mics were high passed. Hopefully guitar cab mics, too.

 

My working theory so far is that the bass amp was acting to keep the gate on the kick open longer than usual, allow the rumble from the subs/stage to build to resonance. But it's just a theory, and I have precious little experience with these things.

 

Next time we're there (Sept, I think) maybe I'll see if I can get the house sound guy to push the levels up "too high" during sound check and do a drums-n-bass thing. If we can replicate the rumble during sound check, we'll have a much chance of doing something about it.

 

[Tomm Williams]

Hey Wesg

 

I mentioned HPF's about 8 posts up......it may also help to use a weight bag on the kick mic stand. I carry a pair of small cordura bags filled with #8 lead shot. They keep the stand from moving and dampen any vibration.

[wesg]

Is vibration like that usually an issue when the mic is inside the kick, resting on a pillow?

[Tomm Williams]

Is vibration like that usually an issue when the mic is inside the kick' date=' resting on a pillow?[/quote']

 

I could not imagine it would be but applying a HPF as a test can't hurt. As we all know, sometimes it's the strangest thing causing an issue.

[Craig Vecchione]

Is vibration like that usually an issue when the mic is inside the kick' date=' resting on a pillow?[/quote']

 

It shouldn't be.

 

I think you've got a good lead on the problem. Since the bass guitar signal is often pretty constant in many songs, the speaker being close to the kick drum will pretty easily set the drum heads in motion, and all it takes is one hit to open the kick mic gate...or possibly the bass amp is enough to open the gate on its own. As soon as the PA subs are in the equation, the ball is in motion and you've got a nasty feedback loop.

[Phil O'Keefe]

What was the stage itself made of? Was it a concrete stage, or an elevated platform?

 

I've run into stages that only "went off" (acoustical sympathetic resonance) once you got the room full of people and really cranked the mains up...

[Tomm Williams]

Another simple thing you could try is to place the bass amp on a very short stand so it isn't coupled to the stage floor. From your own observations and many of the suggestions here, I think you'll conquer this quickly.