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.  Have you ever had people share a microphone and the ringing of feedback only happens with one person?  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 requiring 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.  Stick with me, it gets easier.

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 microphone with a directional polar pattern such as a cardioid.
  • Turn down the monitor volume.
  • Turn down microphone channel’s gain.
  • Watch for reflective surfaces that bounce the monitor sound to a microphone not directly in line with the monitor.  Then, make changes 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.
  • EQ the microphone channel signal, lowering the frequencies which are causing the feedback, which leads to…

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 is the feedback frequency.  Let’s lasso that one to the ground!

Frequencies by their resulting sound:

  •     Hoots and howls:  Likely 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.  Using a digital mixer, tighten up the frequency range of the applied cut so only a small range of frequencies is cut.

If you are constantly dealing with feedback problems, then check out my guide. The guide covers all aspects of audio production including the stage and booth work necessary for pro-actively preventing feedback:

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. Tony Diver " Crazy Train" Rock & Blues band says

    We recently experienced a bad echo even when the monitor was switched off. It was a small pub venue but we’ve never had it happen before. We changed channels and made sure that all effects were off. We eventually went back to the original channel for the singers Mike and it was still there. We then unplugged all leads and put them back in the same sockets and the echo went away although there was a very faint trace of echelon but we were able to continue, The problem is we still don’t know what caused the echo so we are concerned that it may happen again. Any thoughts?

    • says

      Really bizarre and hard to guess without reproducing it. You said you changed channels. Did it go away then? I’m wondering if a channel is going bad on the board. Also, I’ve had standing waves occur in a room but that was with a drum in a very square shaped room. That type of venue should haven’t that problem.

  2. Kevin says

    In our church we have a few girls that want to sing but when they are on stage they will go in whisper mode. Then complain they can not hear themselves in the monitor. Turning up the gain , volume or both always produces feedback. The other ladies tell me to turn these girls up so they can hear there parts but we always struggle with feedback. Is there something I can do as a sound board operator to help. These girls will move the microphone further away from there mouths or like I said just barely whisper there parts and then when they come to a part they have confidence in they belt it and everyone looks at the sound booth like I turned them up. Frustrating

    • says

      They are in-experienced singers who need the worship leader to teach them how to hold the microphone and how to consistently project. Their isn’t much else you can do.

  3. Myles Hamlyn says

    Hi!

    I’m involved with a touring production at the moment, and I have the ultimate sound man nightmare – school halls (mostly) with omni lapel mics that suck. Any tips? Would really appreciate it.

    Myles

  4. Rosalio Zapata says

    I am a church member at church and recently the Pastor is preaching and in the background is a local radio station. None of the musicians know what is causing it. Your articles at least give a place to check for damage, breakage, short or basic cable inspection. Please any help is appreciated or advise for this specific problem. We are a small church and we are self supporting, so expenses come out of pocket. I do not know the system (yet) they use. Thanks!

    • says

      Start by isolating the source of the problem. When the pastor is preaching, are any other mixer channels turned on? If so, mute then until you find the one causing the problem and then look into things like replacing cables and checking any wireless unit that might be used. If it’s only through the pastor’s wireless, then again, look at replacing cables and changing the transmission frequency. If you are right under a radio tower you might not be able to use wireless microphones at all. I’ve seen such scenarios.

      • Anonymous says

        I would suggest changing channels on the wireless setup, making sure the receiver and hip pack, or microphone, are set on the same channel. Sometimes I get radio stations/interference on mine as well, and usually changing channels helps. Try changing it to one that’s as far away numerically from the one causing problems as possible (example: if channel one is causing problems and your wireless gas 12 channels, then move both the receiver and the mic to channel 12 and see if that helps shut out the radio signal)

  5. 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?

  6. 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. 

  7. 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

  8. 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.

  9. 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.

  10. 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

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