How to Remove Audio Feedback through Equalization

Use your EQ controls for knocking down those feedback frequencies.
Photo provided by piovasco

Not all feedback is eliminated in the same way.  Feedback typically occurs when a microphone and a loudspeaker are too close together thanks to a singer dropping the mic to their side.  In this case, let’s call it user error.  But have you ever had multiple people use the same microphone and suddenly you hear the ringing of feedback?  Have you ever created feedback by altering the EQ of a channel?  Let’s find out why.

What is feedback?

Audio feedback is the sound created when a sound loops between an audio input and an audio output.  A simple example is a microphone and a monitor.  The monitor is broadcasting sound the microphone then picks up.  The monitor then is amplifying that sound and broadcasting it back out where the microphone picks it up again.  Eventually, when the volume going into the microphone is the same as the volume coming out of the monitor, feedback begins.

The first frequency that feeds back is the one that requires the least amount of energy to excite resonance.  Resonance is a vibration of large amplitude caused by a relatively small stimulus of the same or nearly the same period as the natural vibration period of the system.

What are the common reasons for audio feedback?

  1. Microphone located too close to a monitor
  2. Gain structure set too high so as frequencies primed for feedback

What can be done to stop audio feedback in these cases?

  • Move the microphone
  • Move the monitor
  • Use a more directional microphone to meet your mic’ing need.
  • Turn down the monitor volume
  • Turn down microphone gain (likely the person needs to hold the mic to their lips if singing)
  • Watch for reflective surfaces that might be bouncing the monitor sound to a microphone not directly in line with the monitor.  Then, make changes then using one of the above.
  • Simple but common…turn off microphones when not in use.  A stage arrangement can change for an event and create the right conditions for an open mic to cause feedback.
  • Equalize the microphone channel signal, lowering the frequencies which are causing the feedback.
  • If you are constantly dealing with feedback problems, then check out my guide, Audio Essentials for Church Sound. The guide covers all aspects of audio production including the stage and booth work necessary for preventing feedback before it starts.

How does the equalization-for-feedback process work?

In the first part of the article, I mentioned the frequency that required the least amount of energy to excite resonance.  Let’s lasso that one to the ground!

Frequencies by their sound:

  •     Hoots and howls:  Likely caused by a feedback frequency in the 250 to 500 Hz range.
  •     Singing: The range is in-line with 1kHz.
  •     Whistles and screeches: most likely above 2 kHz.

Determine the likely frequency range and then apply a cut to that range by 3dB.

What about creating feedback when EQ’ing a channel?

It’s that very EQ process where we can cause feedback ourselves.  For example, one time I had choir mic’s all set and EQ’ed to my liking.  During a specific song, I decided to try boosting the mid-range EQ a bit more (that 1kHz range).  That’s when the feedback started.  I quickly cut that mid-range frequency back before anyone (except my sound guy, Jeff) noticed.

The keys to feedback control

Eliminate the conditions in which it can appear.  Teach singers to hold the mic right up to their lips…and never drop down next to a monitor, establish proper gain structure, and turn off unused mic’s.

When it does appear, know that you have an immediate alternative to turning down volumes…you might just be able to EQ it out.

Question(s): What have you done to control feedback in your environment?

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Comments

  1. An Engineer-in-training says

    Thanks for the tips to eliminate or avoid feedback. I understand that the relationships between the microphones and monitors/speakers, but what about a guitar player who was using his own amp which was mic’ed that caused terrible feedback throughout the performance? He kept signaling us to turn up the volume for his mic, but we had to avoid getting the feedback. How do you handle this dilemma? Would it better if he did not mic his amp but provided him with a monitor and used a DI-Box connected to his guitar and adjusted the volume without using the microphone?

  2. Chris says

    THey might be having a problem either because of the location they are in the room, or more likely, because if the eq'ing.  Here's the thing, if you are pushing the HI's to max and don't hear a different, then there are a few possibilities;

    1. You need your hearing tested.  No joking.

    2. The singer isn't holding the microphone close enough so what you are hearing in the room is their natural voice, not the amplified voice.  You should not have to max the gain.  I'm surprised there wasn't feedback just because of that alone (not always but still).  Mic should be up to their lips.  Check the channel pad button as that can cut the level of the incoming signal.

    Might I suggest you talk with a sound guy at a local church and have them come in and help/watch you during a practice. 

  3. Dan says

    Chris,

    Thank you very much for your thorough reply. It really helps a lot.

    I will take your advice and have someone set it up for us properly so that I have a kind of template to work from. For our house mix, I wanted to have a kind of guide to start out from on a channel to what is technically ‘sound’, and then work on a good sound.

    1. On the soprano voice for instance, the Sennheiser mic she uses does not give the nice sharp tone that her voice naturally has and I wanted to add a little crispness to it. In my limited knowledge, I increased the frequency to maximum (20Hz) and the gain on +18 (max as well) to try and get the sound we should expect. When I increased the gain on the High, it did not really make an impact until I lowered the frequency which changed the tone. Is this ok? So for me it is to strike the balance like you said of not changing too much from the natural sound.

    2. I have also noticed that in our concerts which is more or less 2hrs long, some people have been rubbing their ears! Now I know some of you reading this will laugh your heads off! It was not the kind of ear rubbing as to give you a hint that you should sing the last song and make a run for it (we hope not!). I just noticed every now and then that perhaps they are doing this because some of the frequencies might by at a very high resolution which might not be ‘user’ friendly on the ears. Is this a ‘duh’ thing or is there any substance to this?

    Thanks again

  4. Chris says

    Dan, you ask a question I asked years ago.  Long answer short…it doesn't work like that.  For instance, if you look at this frequency chart, you'll see all the different frequencies an instrument or a voice occupies.  Does this mean you should tweak the EQ settings to maximize those frequencies?  No.

    EQ'ing is about sculpting the frequencies of each channel so that the instruments sound great but even more important, they fix together to for a great song.  For instance, I might EQ a great bright acoustic guitar sound but when mixed in with the rest of the band, it doesn't sound right.  It might be the cymbols or the hit hat have a bright sound that covers or clashes with the guitar's higer frequencies.  And in the case of that particular song, it might be best for the cymbols to shine through and therefore I would cut those high frequencies in the guitar.

    What a frequency chart can show you is the frequencies in which an instrument or a vocal might center.  So the kick drum's fundamental frequencies are below 500 Hz.  Therefore, that's where the power of the sound lies.  Boosting 8000 Hz of the kick drum isn't doing anything.  However, if you have other instruments like the bass that are in the same range, now you know what you are mixing against.

    Something I learned a long time ago about equalization is this…don't change the EQ settings as a way to asking "now does this sound good?"  EQ with the sound already in your head that you want to hear.  This way, the EQ knobs are turned so you will hear what you want to hear.

    As for a "main mix" frequency settings, I'd gather you are talking about setting the house EQ.  There is a lot that goes into setting house EQ including the type of music played, the type of use of the system (music and the spoken word), as well as the room acoustics.  If you're house EQ has never been set, look for a local REPUTABLE AND TRUSTED audio company to hire to come in and set it for you.

  5. Dan says

    Just an add-on to the first post. Where do I find the ideal frequency setting for the ‘Main’ mix. I can’t believe that I have been mixing for a few years and only asking this question now, but I suppose rather late than never! Please help. Thanks.

    • John O'Keefe says

      Definitely agreen with Chris here. If you are talking about the house EQ. Don’t do that work yourself. It requires a lot of testing. Checking the aquoustics of the room, the equipment your working with, the sound your church is looking for…

      Definitely hire out for this one time job and then put a locked grate over it. The rest can be done from the board.

  6. Dan says

    Hi guys,

    I am so encouraged to hear and read of all these topics. It helps a lot and it is good to see that I am not the only one struggling with some of these topics!

    My question is: does anyone know where I can find like a ‘standard’ frequency chart I can set the different channels to? For instance I just read that for a soprano mic, the setting should be between 260Hz – 1150Hz. That helps already, but what about Alto voice, Tenor voice, keyboard, Sound Module and Backing track i.e. CD signal or Itunes track.

    Please could you help to refer me to a kind of simple – non-fussy website that just cuts to the chase – that would be very helpful. I found the ‘behindthemixer.com’ website a great tool. Thanks to whoever does all the write ups.

    Kind regards

  7. Chris says

    I should note that using the EQ method to stop feedback when it occurs might be best done by first slightly lowering the channel volume in which it's caused and then using the EQ.  This way, the feedback itself is removed ASAP and then you can tweak the EQ so it doesn't happen again when you boost the volume back up.

  8. says

    ditto to the “turn off mics when not used”

    - mics set up the previous week for for musicians absent this week are often still in the monitor mix. Unplug or mute.
    - simmilarly, turn off the monitors which point to seats that aren’t occupied this week.

    We just did our Christmas production: 10 radio mics and 4 cabled. Great way to demonstrate gain before feedback, luckily there wasn’t too much “why can’t you just leave them all turned up?”!

  9. Danny says

    Hey I’ve been hearing a lot about these frequencies pertaining to the EQ. I have an analogue soundboard, it’s pretty old but it keeps me busy when I’m running sound for my church. I was wondering how I can find the frequencies. My soundboard has a two high knobs, 2 mids, and 2 low for the EQ. I also noticed that there was this machine that measures sound in decibels or Hz, but don’t know how to use it. Can anybody help me out here? :) I’d greatly appreciate it!

  10. Chris says

    By the sounds you hear the feedback making, pick the high/mid/low where it appears to be.  Then, as it sounds like you have a sweeping and a cut/boost knob for each, proceed as follows:

    1. Cut the sweeping knob a little.  If the feedback is coming from this frequency range, this should stop it.  If not, move to the next range (mid/low).

    2. Once you identify the range, cut the signal a few with the cut/boost knob and then turn the sweeping mid back to where you had it. 

    dB Meters are for sound level measurements.  You can always post up a photo or comment here with the make/model and then we can find out more.

  11. Danny says

    We have a Yamaha MC32/12 (http://www.zzounds.com/item–YAMMC3212)

    But I think our feedback comes mostly from limited space. Our stage is small, but spread out wide and thin like a rectangle. Our two stage monitors are in the center front (if you cut the rectangle in half). One side has the synthesizer and electric guitarist. Other side has the bass and the drums. The worship leader stays in the middle with vocals by his side. The worship leader stands right in front of the podium. Right above the podium, about 40 feet has two small bose speakers with a subwoofer in the middle (this is our mains; its a mono type).

    The problem usually is that the worship leader can’t hear himself. And when we have two speakers with similar mic standing directly underneath the house. So when I try to turn up the speakers because their too soft, feedback usually becomes a fighting obstacle.

    Our sanctuary is probably only 100 feet by 100 (being able to fit about 200 people).

    I’m trying to figure out how I can use a compressor that’s been lying around for years. But I’m not sure if this will help, but I know it’ll make a huge difference from what my electric guitarist is telling me and research online. Someone also setted our EQ (long ago) for the room. But now I’m thinking that if I use our thing that measures dB and set it to 80 and try to keep it down at 0 then perhaps it’ll help? It should be loud enough right?

  12. Noel says

    Our church has a pretty elaborate sound system, but nobody knew how to run it. I am new and am learning and trying to help. The main speaker channel as well as the various monitor channels all have equalizers before the amps. How can you best adjust these to the auditorium? I have recently found an oscilloscope app for my Droid X. Can I use this to help me out? Thanks

  13. Nathan says

    The best way to use the EQ’s then, is to grab a dynamic mic (Shure SM58) place it in the center of FoH Speakers or where the singer stands at the fold back. Turn up the gain to what you would normally use during service, slowly turn up the fader until it feedback’s (monitor, FoH) find the offending frequency on the EQ and reduce by 3dB. Turn up the fader again and repeat. Do this until multiple frequency’s feedback. Turn the fader down. Now you have a usable setting thats EQ’d to the room/venue. Now put a song through the system (a song that you know how it sounds on multiple systems) and fine tune the EQ.

    Example
    If you find the bass rumbles alot, bring down the 63Hz slider on the Graphic EQ. If there’s don’t sound full and warm bring up the 250Hz slider on the Graphic EQ. One thing to remember, ALWAYS TRUST YOUR EARS.

  14. Chris says

    There are a lot of factors to consider in your situtation.  Right off the bat, I'd look at one thing;

    Work on using one of the monitors to better work for the worship leader.  Move it closer, give them only the sounds they need.

    While you could work on the house EQ, it sounds more like an overall setup issue.

     

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