Feedback Prevention: A Closer Look at Microphones and Monitors

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Feedback Prevention: A Closer Look at Microphones and Monitors

It’s all about location, location, location.
Photo provided by cricava

Monday’s post on feedback prevention prompted a good question from Ryan, a reader.  He said,

“…there was one phrase that jumped out at me.  Specifically, ‘Regarding floor monitors, vocalists should be very close to their floor monitor.’  I wanted to ask what the reasoning is behind this?  I would think the further away the singer is, the more the sound is diffused; ergo, feedback is reduced.”

One of the hard parts of live audio production is taking into account all the different factors that are needed and finding the balance in using them all to create the best sound.  Monitor usage is a perfect example.  A musician needs monitors for hearing sounds to be in key and in time.  The sound tech needs the musician to sing into a microphone using the right technique so they can get a solid sound for mixing.  The audience / congregation needs to hear a great house sound.

Looking specifically at a fictional vocal singer, we have five factors to take into account to make all of this work;

  1. The microphone they use.
  2. How they use the microphone.
  3. The monitor they use.
  4. How they use the monitor.
  5. What they hear in the monitor.

1. The microphone they use

Cardioid (named so because of the heart-shape. Picks up primarily in the frontal section with reduced pickup on the sides.)

Microphones come in a variety of types with a variety of frequency responses but you need to focus on their polar pattern for now.  The polar pattern is the area around the microphone head and how a particular microphone picks up sound around it.  For example, an omni-directional polar pattern picks up sound all around the microphone.  A cardioid (see image) picks up sound in the front and sides but not behind the microphone.

Let’s give the vocalist a common, albeit aging, Shure SM58 microphone.  This is a dynamic vocal microphone with a cardioid polar pattern.  As an aside, you need to find out the polar pattern of your microphones.  They will either have a small picture of it on the microphone or you can check out the manuals. Most microphone spec’s are available online so don’t fret if you don’t have the manuals.

2. How they use the microphone

Vocal microphones need to be held right up to a singer’s month, tilted at about a 45’ish degree angle up to their mouth.  This way, you can get the clearest sound without getting other stage noises in the mix.  While you can’t get total sound isolation in the live environment, by having the sound source so close to the microphone, you can get pretty darn close.  Let’s say our vocalist is properly holding the microphone.

3. The monitor they use

Focusing on the source of this question and the original topic of feedback, two monitor types are to be noted; floor monitors, also called floor wedges, and spot monitors.  Spot monitors are smaller and are usually placed on a stand.  The vocalist, and any musician, sees the monitor as a necessity.  If they can’t hear other singers, they can’t harmonize.  If they can’t hear the right rhythm instrument, they can’t sing in time.  It’s for this reason musicians might spend a lot of time asking for the right monitor volume and mix.

Let’s put our vocalist in front of a floor monitor.

4. How they use the monitor

Proper monitor placement.

Our vocalist needs to be standing on-axis to the floor wedge.  The moment they are off-axis, that is to say they are outside of the sound cone radiating from the monitor speaker, then they are hearing significantly less volume.  But how close do they need to stand?

This leads us into the topic of stage volume.  Stage volume is the amount of sound on a stage.  Sources of stage volume would be acoustic instruments, singers, drum kits, and monitors.  These are all different sounds that can be heard on stage that don’t emanate from the house speakers.

Our vocalist needs to stand as close as possible to their floor wedge because of two reasons;

  1. They need to clearly hear their monitor
  2. You need to keep the stage volume to a minimum.

Stage volume, when excessive, destroys the sound in the room because, at that point, the stage volume is exceeding the house volume.  Excessive stage volume also makes monitor mixing a nightmare.

5. What they hear in the monitor

Without going into detail on monitor mixes, I’ll hit a few highlights as it would relate to our vocalist.  First, they need to hear voices for harmonizing and a rhythm instrument or two for timing.  They DON’T need every sound in their mix.  Second, they need to hear all of these in the right relationship to each other.  Think volume.

Only now can the question be answered…

“‘Regarding floor monitors, vocalists should be very close to their floor monitor.’  I wanted to ask what the reasoning is behind this?”

Bringing this all together and remembering we are scoping this around feedback…

Feedback, in the case of monitors, is produced when a sound is looped between the microphone and the monitor to the point where the volume looping between the two begins to significantly increase.  So why do we want our vocalist up close to their monitor?

The biggest point that I can make is that when you properly set the gain for that vocalist’s microphone, they keep it up to their lips, and you have ONLY enough volume coming out of the monitor, then feedback won’t be a problem.

Where feedback does come into play is usually when;

  • The microphone is too hot and picks up more monitor than necessary.  This goes back to setting the proper channel gain.
  • The monitor is too loud and the microphone is looping the sound.  Going back to our stage volume discussion, we don’t want to negatively impact the house volume or produce too much volume from one monitor that it affects others on the stage.

This all leads back to the original article and the three ways to easily prevent feedback;

  1. Place microphones in the right relationships to monitors.  Close enough the singer hears the volume they need when they are standing on-axis to the monitor speaker cone.
  2. Teach musicians to use the proper vocal microphone technique.  The moment a vocalist drops the microphone from their mouth to their chest or their stomach, a significantly greater amount of gain is required which can produce feedback and will pick up more sounds on the stage.  And dropping the microphone to their side will certainly throw everything into a loop because of the close proximity from the monitor the microphone.  They need to keep the microphone up to their mouth or step away if they drop it to their side.
  3. Use the proper gain setting technique.  The channel gain setting is foundational for all your work.  It gives you the amount of channel signal you need and is the best aid in preventing feedback.  A channel that’s too hot will cause just as many problems as a channel that’s not loud enough.

The Take Away

You have to keep several areas in check when producing live audio.  These areas include stage volume, feedback, and monitor usage.  To Ryan’s question, the farther away the vocalist from the monitor, the more monitor volume and thus stage volume.  But when you place the vocalist in the right relationship with the monitor and they use proper the microphone technique, then they hear everything they need, you get the right sound signal you need, and the congregation hears the sound coming from the house speakers, not from the stage.

*Tip: If you are having problems with feedback when the monitors are set and the musicians are properly using the microphones, check the type of polar pattern of the microphone.  You are likely using an omni-directional microphone wherein a cardioid microphone would be better suited for your situation.

Looking for all the details behind what makes microphones unique?  Check out this guide to microphones with a focus on vocal microphones.

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Comments

  1. Chris says

    Feedback is a complex subject. It occurs at the frequency at which the closed-loop gain is greater than unity, and the phase is reinforcing rather than cancelling (i.e. the path from themicrophone through the mixer, amp, speakers and acoustic path from the speakers back to the mic diaphragm). That’s why you can change feedback by putting a notch in the eq at the frequency of operation, or use the phase invert switch on the channel. Adding notches knocks down each feedback mode one by one, but you need a digital desk to get more than 2 or 3 notches…

    The microphone has quite an effect on feedback – more expensive mics can have a better polar response, as the cardioid or hyper-cardioid response is produced by an acoustic labyrinth which presents a phase-cancelling signal to the back of the mic diaphragm. Because the size of this is limited to the size of the mic body, it determines the cutoff frequency below which the cardioid mic becomes an omnidirectional mic – look at the mic spec sheet for the polar response at different frequencies. In addition, some mics have resonance peaks (like the famed SM58 presence peak) which can contribute to ringing, etc.

    In general, the flatter the frequency response the better; a condenser mic may have higher conversion efficiency (i.e. sound pressure to millivolts) but it also has the advantage of less mass to move (no attached coil) so the HF response is usually flatter and extends higher. It’s easy to turn down the channel gain to compensate for a more sensitive mic – it’s harder to cut out resonances.

    That is in addition to the ‘sound’ of a mic, of course – I love the projection of the AKG C535, especially on female vocals, but that’s another story…

    • says

      Chris, thanks for adding those details. I’ve been at events where every time a person spoke, I could hear a bit of feedback. They finally notched it out after about 5 minutes.

  2. Chuck Farley says

    Oops inadvertent submission … My last point was that I use stands for wedge monitors especially for worship leaders at the keyboard since they need a pretty full mix. This allows me to keep monitor levels lower.

    • says

      I don’t like the cluttered look of the spots on the stands but you’re right, stage volume was never a problem when the band I mentioned used those. I’ll take the trade-off any day. :)

      • Cajundaddy says

        I agree that near-field spot monitors add visual clutter. When we use them we mount them to the music stands to clean up the look.

  3. Chuck Farley says

    A couple of other issues that I have run into has been monitor EQ and competing monitor mixes. Obviously, we base our gain structure and channel EQ on our house mix. The stage acoustics are rarely similar to the house so as we boost gain, we may be boosting a frequency that is well absorbed in the house, but can be a problem on stage. Stuffing a bunch of musicians, vocalists, and monitor mixes, an inidividual may not even know what he is hearing or where it is coming from. Stage set-up is critical to providing clean, lower volume monitor mixes. A big help for me has been the use of stands for wedge monitors especially for lead vocaliststs(that varies on a weekly basis adds some interesting

    • says

      Chuck, I was working sound at a church that had spot monitors on stands. They had a rather good implementation as to how they routed three aux sends for about 7 spot monitors. One aux send went to the drummer and bassist. One aux send went to the lead singer / Backing vocalists. The last aux send went to the rest of the musicians (guitarists).

  4. Cajundaddy says

    Training the performers how to use the tools comes first, carefully setting gain structure and monitor levels is second, and if you do these jobs well, live performance feedback will not be an issue. We use a simple ghostbusters analogy: “never cross the streams” (response cones). For practical purposes we suggest that a mic picks up sound primarily in a cone shape in front (ok not exactly but people tend to understand this visual), and speakers output sound in a cone shape from the speaker. Anytime the two cones point towards each other we will have an uncomfortable result (screeching feedback). Teaching performers to sing close to the mic and point the tail end (with the cord or antenna) of the mic towards the monitor and never ever stand in front of a house speaker goes a long way towards making your job easier and the resulting performance feedback free.

    As a performer, my personal preference is near field powered monitors. Low stage volume, good audience connection, and the ability to get my monitor volume just right with my own volume control are the reasons. I don’t really need the added bass provided by a floor wedge due to omnidirectional bass wrap from the house speakers and I find in-ear too isolating even with a dedicated audience mic. I also prefer a fairly simple mon mix with only my instrument, the lead instrument, and the vocals. In-ear Aviom systems just seem like complete overkill for my needs. Other performers will surely have different needs and wants.

    • says

      IEM’s have their place. From my experience using them, I found I played better but also, I found it was better if the worship leader used a wedge.

  5. Quaid says

    Good dicussion.
    I’ve been dealing with this for over 3 years. The gentleman with the hearing problem, during his time in the military, had something explode close to one of his ears. So, naturally, the hearing loss inhibits his singing.
    In addition to this, he’s a soft vocalist. So, the solution has been depending on me to give him loud monitors, which messes with my house mix.
    Other group members complain about him being deaf. But, if I don’t crank him and the track, I’m inhibiting him, and thus, the whole group’s performance.
    He’s recently obtained an in-ear system, which will make everything much eaiser for all of us. (Yay!)
    We haven’t implemented it yet, but when we do, I don’t think anyone will want to go back to the way we’ve been doing it.

  6. Anonymous says

    My understanding of how microphones work is this. I absolutely don’t mind being wrong, but am I?

    Nothing on the mixing console affects what the mic picks up, only how much that signal is amplified by and how much of that signal is sent to other places.
    An unplugged SM58 sitting on the stage floor is on and working and picking up signal, that signal just isn’t going anywhere. or being amplified.
    The only way I know of to make a mic pick up more signal is to fire louder sound at it, move it closer to the source etc (anything to cause a higher SPL at the diaphragm) or move it so the polar pattern is directed to receive the maximum amount of that sound. The opposite would obviously be true for making it pick up less.

    My understanding is that the amount of gain being applied at the input pre amp or at any other point in the system has no effect at all on what the mic picks up, only how loud that signal gets. It could of course cause the mic to pick up more sound if this causes a louder output from a speaker but that would be caused only by a louder sound arriving at the mic.

    I’m not sure from reading the article whether or not we are saying the same thing. It’s really this bit that confuses me.

    “The microphone is too hot and picks up more monitor than necessary.”

    Are you saying that this happens because it picks up more sound because it is “hotter” or because being hotter is causing the monitor to be louder (because of more gain between mic and monitor)?

    I agree totally with the practical side of this, I just have a different understanding of why it works.

    Thanks,

    Chris

    • says

      The short answer is, the hotter the microphone, the more that all the sounds being picked up are thus amplified. Therefore, the feedback frequency would be more likely to be excited.

      The long answer…

      A “hot” mic can be called as such for two reasons; it’s a highly sensitive microphone OR it has too much gain applied to it.

      In the first case, easy examples are dynamic mic’s verses condensers. The condensers are much more sensitive to sound and thus can pick up a wider range and produce a more nuanced sound. Even more so with ribbon microphones. The more sensitive a microphone, typically the more directional it should be. Even within the generic type “condenser,” you can have a range of sensitivity. This is why some are naturally hotter than others. For example, I usually use a channel pad on my condensers because of the hot signal they send.

      I was discussing this question with a friend because I wanted to make sure I was covering all my bases when I wrote a response. He brought up a secondary point; any given mic has pre-amp parameters that make it have a sweet spot for gain-before-feedback. Dynamics have a higher gain-before-feedback point than condensers because they need more sonic energy to move the diaphragm in the capsule. Therefore, they aren’t as hot.

      In the second case, a mic can be “hot” because one has applied too much gain. You are taking the input signal and via the system amplifying ALL frequencies which are picked up in that input signal. And that’s when it’s easy to give enough energy to the feedback frequency which then gets you that oh-so-wonderful feedback.

      In retrospect, I should have detailed a bit more about the “hot” microphone. In regards to that particular line, a “hot” microphone, that is highly sensitive, will pick up more sound and thus needs the gain to be set accordingly. But a mic can be also be “hot” because the sound system is amplifying the sound that has already been input into the system and can therefore set off the feedback frequency.

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