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Bass cab and pa speaker interaction

W

Warmuth

Guest
Aug 6, 2012
241
10
#1
Last night I was having an issue I couldn't figure out that seemed to be an issue with the pa subs reinforcing certain notes from the bass. We were playing a place we regularly gig at and haven't had this issue before.

The first set we start playing and any B or C I hit is resonating like crazy. The stage is shaking, cab is rattling etc and the volume of it is orders of magnitude louder than any other notes Im hitting. I've dealt with room acoustics reinforcing certain frequencies before but this was another level entirely, I couldn't play the note any softer, the sound at any level would amplify beyond control.

We finished the set and tried troubleshooting with just my bass to try to isolate the problem. Changed the phase on the pa subs, the booming note moved to D. We tried different mixes between the cab and pa and the only thing that got rid of the loud notes was removing me from the pa entirely so that the subs were not reinforcing any of my frequencies. Worked great, everything nice and even and my bass sounded as it should.

Once we started playing the offending note became any E but as I wasn't in the pa mix at all the problem wasn't quite as bad. It was still much worse than the typical room reinforcing frequency I've ever run across however. Anyone have any clues to this? Bass cab to sub placement? Would the two of them being too close affect it? The cab and subs were probably two or three feet apart.
 
#3
I usually don't like my bass in the subs anyway, too boomy, and as you have found, can give problems - I like kick in subs, and for my sound my cab and the mains helping distribute through the room.
 
#4
The OP's problem is well-known and has been for decades. You can not run two physically-separated bass sources in the same frequency range and avoid power alleys. Worse, you create all sorts of phase/delay related issues when you run two non-identical cabs in the same frequency range even if they *are* located at the same point. The way to amplify live BG is to run the lowest possible volume on stage, and let the mains handle 99% of the house volume. What helps further is running a middy mix on the stage amp and let the mains backwash fill in the bottom end for you. The louder and/or "bassier" you run the stage amp, the more phase/delay issues you will introduce, which is exactly the problem you were having.

Sound Reinforcement 101 (but no offense intended, honest).
 
#6
It could be something else altogether. Sometimes it's the kick drum mic (or some other mic) picking up vibrations from the stage which travel up the mic stand causing a feedback loop. This sort of thing is much worse when the stage is a big hollow box.

Something else kick drum related is when certain notes hit a resonant peak of the kick drum shell and the mic picks it up.
 

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