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Low Frequency Feedback
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Author:  icenine [ Tue Apr 06, 2004 7:38 am ]
Post subject:  Low Frequency Feedback
Hi everyone,

I have been experiencing a low-frequency hum at my shows. I am using a Mackie CFX12, SM58's, JVC Triple Tray, and JBL G2 speakers.

I have the speakers set out in a few feet in front of the singer and a G2 10' pointed over the singer's head as a monitor.

To combat the problem I have used a technique called 'zero EQ'ing' that was shown to me by a friend who knows better than I. I set all the EQ sliders to the zero point and adjust down from there. For instance, to boost the highs and lows I would pull down the mids, so it is adjusted relatively.

I have also depressed the '100Hz Low Freq Cut' button on all the mic channels.

One more thing I tried was turning the Low Freq pot on each of my JBL's to about 11o'clock (they had been set in the middle at 12 o'clock).

These things have helped but I have a couple of questions for you sound techies out there:
Is using the low-freq cut buttons a good idea? Is there anything obvious that could be causing this 'hum'. Am I handling this the right way? Are there any other things I could try?

Any help would be appreciated. Thanks~ :D
Author:  pkircher [ Tue Apr 06, 2004 8:21 am ]
Post subject: 
The low frequency hum you are hearing may not have anything to do with feedback. I have the same problem from time to time with my powered JBLs. I've always chalked it up to interference. Try a different power source, or power strip. Also you could get interference from different things, especially neon signs do this, however I have also gotten interference from video games, and other things. You might want to try to reposition the speakers somewhere else to help eleminate the problem. At one of my venues If I put speaker A in spot A and it hums like a SOB but if I put speaker B in spot A no problems at all. Just keep trying different things, maybe get better cables with more insulation. Let me know if any of these Ideas helps.
Author:  icenine [ Tue Apr 06, 2004 8:35 am ]
Post subject: 
Thanks for your reply! :)

It seems like feedback because when i TURN DOWN THE MICS IT GOES AWAY.
Author:  KjSammy [ Tue Apr 06, 2004 10:38 am ]
Post subject: 
Mr. Icenine, I believe the pkircher person is refering to a ground loop which it maybe. Keep in mind that ground loops can be caused from the plug in the wall to your video connections to your TV's.

Other than that

Try to take your microphones out ot the effects loop or visa versa. Pull back that send / return back



Sammy
Author:  Victor R [ Tue Apr 06, 2004 5:39 pm ]
Post subject: 
Audio feedback is a loop where sound from a speaker is picked up by a microphone, amplified and then reproduced by the speaker.

The distance the sound must travel from the microphone to the speaker determines it's frequency. Unless you are in a very large room and the most direct path is bouncing off a far wall, the sound is very shrill. Often one path will start the feedback and then others take over. When this happens you hear a shift from lower to higher the frequencies.

Feedback doesn't simply follow the volume control. As you slowly increase the volume control from 0, no feedback will be present until you reach a specific level. At that point is starts with a vengeance. Once started it will continue as you turn the volume down until some point where it stops completely.

Some noise or excitation has to the process. It won't happen in a truly quiet room.

What you describe sounds like good ole' power line hum. AC power operates at 60 hertz (50 in Europe and many other countries). Your Low freq will help because of this.

It sounds like your mic is the source so you might try the following:

1. Check the cable and mic condition. A broken shield or lose connection could be the cause. Try other cables and mics if possible.

2. Make sure mic cables are kept away from power cords if possible. One foot or more if possible.

3. Turn off any other electrical devices in the area which could be causing this.

Good luck.
Author:  BeachHeadBum [ Tue Apr 06, 2004 9:59 pm ]
Post subject: 
Pat, you could be getting this too..
Bad power to the club.. as in the outlets could have a bad ground or wiring.. or be getting "RFI" Radio frequency interference.

A couple solutions:
Definately get a surge protector that is rated to 6,000 Jules (an unit of power) Furman makes several a new drop strip with 6 spaces on it part # SS9B it already has a breaker on it and RFI protection built in. or thier rack power UPS models
(uninteruptable power supplies that have surge protection built in)
But those babies are sure heavy!!! :shock:

Second.. I usually carry a 110volt 15 amp (standard household) style outlet tester the kind with the 3 lights on it to test outlets before we plug into them.. and we do that EVERY TIME!!! to keep gear from being blown up. Also we cary the 2 prong ground lift plugs in case of a 60 hz hum in our system. It prevents chasing 700 line cables and over 300' of snake line, on some bigger shows.

Try that out for size :idea:
Author:  Lonman [ Wed Apr 07, 2004 9:08 am ]
Post subject: 
Sprocket Audio SW wrote:
Also we cary the 2 prong ground lift plugs in case of a 60 hz hum in our system.


While this works, it can be dangereous with shocking experiences.

Brian, that comment surprised me, you should know better.
Author:  BeachHeadBum [ Wed Apr 07, 2004 11:44 am ]
Post subject: 
Lonman,
I digress if most of the KJ's take thier power as seriously as thier music and equipment. They will invest in power protection too I have the furman gear that i can use the 2 prong lifts on and also I can & DO tap DIRECTLY
into alot of club's power disconnects (NOTE TO KJ's THis is not advised unless properly trained and lisensed! I have an licensed electrician on my staff)

And before you reply on this topic I also understand MOST kJ's don't have the budget I do Understood. I'm just trying to give them a TEMPORARY AVENUE to try until a Proper one can be Purchased. Which i gave BOTH options.

Lon, I'm sorry if I confuzzled you. I Now more than ever, really you need to see how my systems work..
Author:  icenine [ Wed Apr 07, 2004 3:36 pm ]
Post subject: 
Thanks for all the replies! :D

A little more info, guys:

I have all of my equipment, except for the powered JBLs, plugged into furman units. I then have the furmans and the JBLs plugged into the same surge surpressor.

I think this rules out any ground loop condition.

My problem seems to be more with the effects and/or EQing. By turning down the effects sends and some of the low frequency sliders, I seemed to get rid of the problem.

As I said, I also depressed the 'low-frequency cut button on the mic channels. I'm not sure if this helps.
Author:  big jimmy c [ Wed Apr 07, 2004 4:40 pm ]
Post subject: 
Ice, Try chocking your monitor so its pointing lower. Pointing it over the head of the singer may catch the mic.
Author:  icenine [ Wed Apr 07, 2004 4:56 pm ]
Post subject: 
Thanks BJC!

The reason that I had the monitor so high was to try and avoid it catching the mic.

But I'll give it a try.

Thanks!
Author:  Victor R [ Wed Apr 07, 2004 7:33 pm ]
Post subject: 
What type of cables you are using between the mixer and speakers? How long are they? If you are using ΒΌ" they should be TRS.

You also mentions turning the sends down but haven't said any thing about any processors or other inserts. These can also be a source of problems. Temporarily remove such devices and see if that solves the problem.

Another question, is the surge suppressor plugged directly into the wall or are you using an extension cord? Any extension cords used for the JBLs should be heavy duty and as short as possible.
Author:  BeachHeadBum [ Wed Apr 07, 2004 11:09 pm ]
Post subject: 
Pat,
Make sure ya don't have the aux's from effect return channels sending back into themselves (Depending on how you have your console set up). It's a mistake i'm sure we all have made it tends to make a low frequency rumble at times.
Author:  Available Sound [ Thu Apr 08, 2004 11:43 pm ]
Post subject: 
Throwing my $0.03 in here from the video side of the techno-universe... ground loops cause "rolls" to scroll across monitors and projectors - VERY annoying and not exactly nice to the gear.

All of the ideas thus far have been good - but there's still a trick or two to try. My lead question is - are you getting the hum with your mics closed?? If so then it's ground-loop. There are a number of products that can address this - Xitel's Ground Loop Isloator (about $30) is a starting point. There are some good proucts made for the automotive audio industry that can help here, too, with a little creative adaptation (cars are naturally noisy with all those electronics and sparks and other evil under the hood). This nifty little article:
http://www.epanorama.net/documents/grou ... ators.html
might be helpful, too, and includes some useful links at the end.

If your hum's coming from microphones, you may have to get serious. Before we ever did our first Karaoke gig we invested in a Peavey Feedback Ferret II. It has been worth the nearly-$500 we paid for it - though we may have put out some ca$h, we also put out the feedback and haven't seen it since. Despite singers best attempts at causing it (dropping mics, standing directly in front of speakers - heck, one even just about put the mic inside the speaker cone when she shrugged her arms during her song!) nothing's escaped our speakers that shouldn't have (bad singing notwithstanding).

Just a few ideas... YMMV ;-)
Author:  BeachHeadBum [ Fri Apr 09, 2004 7:13 am ]
Post subject: 
Shadowfax..
It's NOW in the Furman item I mentioned above too.. :wink: I don't know the cost though.. The Item I mentioned is a stage drop box though.
Author:  BeachHeadBum [ Fri Apr 09, 2004 7:13 am ]
Post subject: 
Shadowfax..
It's NOW in the Furman item I mentioned above too.. :wink: I don't know the cost though.. The Item I mentioned is a stage drop box though.
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