Q: After we EQ our PA and tune our Feedback Eliminator we notice there is always a low rumble coming through the system. We can bring down the level of the vocal microphones and most of it goes away, or we can lower the low frequencies on our graphic, but then the PA sounds thin and wimpy. What is causing this rumble and what can we do about it?
A: It’s hard to pin down exactly what you mean by “low rumble” from the information you give. There are many different sounds that can come out of equipment ranging from feedback to ground loop hums that may sound like a “low rumble.” Sometimes if there is excessive boosting going on at low frequencies in an EQ it can generate its own rumble. If the rumble is being generated by a piece of equipment in your system, the best way to track it down is by using common trouble-shooting techniques. Namely, disconnect things until it goes away.
It is also possible that you simply have too much low-frequency boost going on in your equalizer. You can EQ the PA to sound great with a particular CD, but sometimes this is just too much low end to be practical for live, uncompressed instruments. It really depends on the equipment you have and the venue. The low end of a PA system can be tricky to deal with because if you equalize every instrument to have a big, full, bottom-heavy sound the whole mix will probably turn to mush when you combine them all. This is especially true on lower quality PA systems that aren’t designed to handle large amounts of low-frequency information.
It is generally better to equalize the PA so it’s a bit light on low frequencies, and then put the lows back in on the few instruments that need them (bass, bass drum, etc.). A live vocal mic does not benefit from having excessive boost below 100Hz. In fact, it is usually a good idea to cut low frequencies on all vocal mics. They may sound a tiny bit thin by themselves, but they’ll sit better in the mix and you will reduce stage rumble/feedback problems.
Remember that vocal mics are going to pick up a little of everything onstage, and are a huge source of extra garbage getting into your mix. So anything you can take out of them with EQ is only going to help you in the long run.
Mic stands are also a notorious source of stage rumble. They will sympathetically pick up the vibrations of the bass rig or bass drum through the stand, which gets reproduced as rumble. A better mic, mic stand, or shock-mounted mic clip can really help this, but it’s also effective to simply cut the low end on those channels. You don’t need it for vocals and you will usually find that the overall mix sounds better without it.