So you have a sound rig, and every time you push the volume on the desk, an annoying whistle of feedback scuppers your day.
This issue can be made even worse when a microphone is in front of a speaker, or pointed towards it. You may think that theres nothing that can be done to fix the issue, and either brace yourself for glaring feedback every so often, or keep the level lower in the room and often face complaints from the audience.
In order to fix the issue, you are unfortunately going to need to recreate the feedback.
If you have an Analogue Sound desk, you will probably need to purchase a Stereo 31 Band Graphic EQ, (maybe more if you have Matrix sends or wedges). If you have a digital desk there will be Graphic EQ’s built in, its just a case of finding them in the menu, and inserting them over the relevant outputs. For this blog we’ll assume only a graphic over Left & Right, and that its a Analogue desk.
Some good models are DBX 231, BSS 966, Klark Teknik SQ1.
There are two main ways to add a Graphic EQ into your Analogue system, you can either Insert it over the offending channel or output, or simply add it in line after the desk.
Insert
Most desks have the option to insert, like this Zed R16.
Looking more closely at the input and output sections, we can see that Input wise, there is XLR and Jack in, and then an Insert for a Compressor, Gate or Graphic EQ (Red Box).
This can be exceptionally useful if its only one troublesome mic, like a lectern in front of the speakers, and needs quite a drastic EQ, that would make the rest of the system sound terrible.
And Below the XLR outputs, there is a separate 2 track output, and then the Inserts for Left & Right (Yellow Box). This can be used to insert a graphic, external compressor, vintage EQ, limiter.
The back of each Graphic is very similar, they need power, and then have XLR In & Out and Jack In & Out for each channel. Some have extra multipin Ins/Outs to save on cable.
To Insert one channel of Graphic, you would need a stereo Jack to two mono Jacks, or Stereo Jack to 2 XLR’s. If they are wired correctly the Red should be the output of the Graphic back into the desk, and the Grey/White/Black depending on make, would be the signal from the desk into the Graphic. Its fairly simple to workout if its wrong and just requires repatching the graphic.
In Line
This is the simplest way to add a graphic into your chain. Take the XLR L&R Outputs of the desk and patch them into the graphic, then take the outputs of the graphic back into the next item in the chain (Amprack / Speaker Processor / Multicore).
There.. I said that was simple.
Controls
For each of the specially selected frequencies on a graphic EQ, there is a fader that can either give +/-6dB, +/-12dB or +/-15dB of gain to the selected frequency.
Apart from 31 faders for each channel of graphic, there are a few controls to get your head around. Some of the physically smaller rack units limit the faders to +/-6dB of gain, and then give you an overall switch to change to +/-12dB of gain – essentially doubling the range of each frequencies gain reduction.
Gain, how much has the graphic affected the overall sound from before it was patched in to being fully set. Try and set it up at the same level as the unit bypassed.
Low Cut Button / HPF Knob, this allows you to reduce the low end going to the amps / speakers. Helpful if your not using a crossover, or only have tops. Meaning that low end won’t be going to a small speaker thats dealing with the upper frequencies.
Bypass is a useful one, checking that when you have removed the offending frequencies, you haven’t removed too much of the sound. Sometimes when you bypass the Graphic the sound is better, if so start again!
Removing Feedback
As I said above, in order to remove feedback, you have to create it. When you turn up the mic to the point of feedback, and then find that particular ringing frequency, and pull it down on the graphic. One easy way for beginners to learn what frequencies are feeding back is to use an RTA app on your phone.
I use “n-Track Tuner” which is a nice dark display for dark stages. There is a free version, but I use the Pro app, which has several other features and display options. As you can see, it allows you to tell where problem frequencies are and allows you to sweep your finger along the graph to see the exact frequency.
If you want to improve your ears, I can recommend using these apps (link below) on your computer. They are designed as a little game for you to workout which frequency is the tone emitted, and allowing you to learn what different frequencies sound like.
I hope this was helpful. Leave a comment below about your experiences.
Dan
Dan Bamber Audio 