Term for Mics That Feed Back Into the Local Room
A microphone's feedback is a very annoying and unpleasant high pitch noise that can happen in any situation where there are a speaker and a mic. Understanding what causes feedback is the first step in knowing how to stop it.
Audio feedback happens when a sound coming out of a loudspeaker is picked up by the microphone reentering the sound system again and again in an always increasing loop. The result is a loud rumble or a high pitch ringing noise.
In this post, I will explain several factors that can cause a microphone to feedback so that you can avoid these situations and significantly reduce the occurrence of feedback in your gigs and events. Plus I have included some tips and solutions that will help you stop microphone feedback
Table of Contents
- Main Causes of Microphone Feedback
- Mic Placement in the Stage
- The Solution
- Loudspeaker and Monitor Placement
- The Solution
- The Type of Microphone Used
- The Solution
- Polar Pattern of the Mic
- The solution
- Equalization of the Microphone
- The Solution
- Number of Open Microphones
- The Solution
- The Acoustics of the Room
- The Solution
- Mic Placement in the Stage
- Stopping Feedback – Summary
- Conclusion
Main Causes of Microphone Feedback
Mic Placement in the Stage
One of the main reasons for audio feedback is because microphones are placed in front or too close to a loudspeaker. If loudspeakers are placed behind the microphone, feedback is almost guaranteed because whatever sound comes out of the speaker will be captured by the mic, and the loop will cause feedback.
Placing a mic to the side of a loudspeaker is another potential reason for feedback because sound can bounce back and forth until it reenters the sound system through a microphone.
The Solution
To help reduce the occurrence of feedback, place all open microphones behind the loudspeakers, making sure that there is enough distance between the mic and the speakers.
A rule of thumb commonly used by sound engineers is, if you double the distance between a microphone and the speaker, the volume for that channel can be increased by 6dB, providing you more volume headroom before feedback on that channel.
When setting up your mics, make sure you allow enough time to try different positions until you get the best results. With time and practice, it will become easier because you will understand the behavior of your sound equipment.
Loudspeaker and Monitor Placement
Another reason that causes feedback is the placement of speakers and monitors. The dispersion angles for a loudspeaker varies, but typical angles of dispersion are 90 degrees horizontally and 60 degrees vertically.
Any microphone located in that dispersion area has the potential for feedback. When setting up a sound system, you want to cover as much space as possible so that your audience can hear everything.
This is where it is crucial to place loudspeakers correctly so that they can cover as much area as possible, but staying clear of the pickup pattern of the mic. When setting up microphones on stage, make sure to keep clear of the dispersion area of the speakers.
The Solution
Place the main loudspeakers as close to the audience as possible. That way, you can decrease the volume in the mixer.
Place the monitors exactly behind the mics on stage, especially if you are using cardioid or super-cardioid mics.
A rule of thumb when placing speakers is if the distance between the audience and the main speakers is halved, by keeping the volume constant, the sensed volume by the audience will increase by 6dB. In other words, halving the distance between speakers and the audience increases the sound volume by 6dB.
If you are in a situation where mics can feedback but still don't have the volume you need, try moving the speaker towards the audience. That way, the sensed sound volume increases without actually increasing it on your mixer.
Another solution is to use in-ear monitoring systems; that way, stage monitors can be removed entirely, significantly reducing the occurrence of feedback. The only disadvantage is that in-ear monitoring systems tend to be expensive, but if you can afford them, I highly recommend it.
Allow enough time when setting up everything so that each mic is tested with the main speakers and monitors as well.
Simulate a situation so that you know what is your volume headroom before feedback. In other words, slowly increase the volume of all microphones and generate a sound to see how the equipment behaves at higher volumes.
If you still see that you don't have enough volume for your mix, try moving the speakers to the left, right, and front until you find a sweet spot.
The Type of Microphone Used
Feedback can be caused by the type of microphone you use. There are different types of microphones for live performance, namely dynamic, condenser, ribbon, etc. Each one has its sonic characteristics, and some are more sensitive than others.
For example, using condenser microphones for live vocal performance is the right choice because they provide studio-quality sound, but are way more sensitive to sound. Placement with this type of mic is a crucial factor in reducing feedback due to their sensitivity.
The Solution
Keep to a minimum the number of sensitive microphones such as condensers on stage, use them only for lead vocals and backup singers. For instruments, try to use dynamic mics if possible.
If acoustic guitars need to be miked, place the microphone as close as possible to the guitar. That way, it picks up more of the sound from the guitar and less from the environment.
If possible, use electro-acoustic guitars that come already with installed piezoelectric microphones. That way, you use less open to air microphones for acoustic instruments.
Polar Pattern of the Mic
Choosing a microphone with the wrong polar pattern could be another reason for feedback. The polar pattern of a microphone is a visual representation of how well your microphone picks up sound from different angles. To learn about polar patterns, read this post.
For live performance, the recommended polar patterns are cardioid, super-cardioid, and hyper-cardioid. The reason is that these mics pick up most of the sound at the front and less in the back and sides, significantly reducing the sensitivity to feedback.
The solution
Make sure that whatever microphones you use on stage are one of the mentioned polar patterns. I recommend using cardioid and super-cardioid, especially for condenser microphones.
Equalization of the Microphone
Feedback happens at different frequencies, depending on the volume of the microphone. If the feedback is a low rumble or a high pith noise, you can easily detect the offending frequency. Usually, the offending frequencies can be found between 250Hz and 2kHz and between 2kHz – 8kHz. It's rarely found above 8kHz.
This is where correctly equalizing each microphone comes handy. By doing a few sound tests, you can ring out offending frequencies. Sound engineers use the term "ringing out" as the process of detecting resonant frequencies that can cause feedback.
The Solution
To maximize volume before feedback occurs, ring out offending frequencies. To do this process, you need a graphic equalizer. The process is simple. After mics are placed in the stage, slowly start bringing the volume up until you start hearing feedback.
If the feedback noise you hear is a low rumble, try reducing frequencies between 250kHz and 1kHz. If the feedback sounds like a high pitch screech, try changing frequencies between 2kHz and 8kHz.
You may want to run this test several times until you can bring all your system up to the volume you want without any feedback.
This process requires that your ears are trained to identify which frequency is creating the problem. There is equipment you can get that would do this automatically without the need of a graphic equalizer. They are called automatic feedback reducers.
These systems use a microprocessor that can identify offending frequencies and automatically reduce them before feedback even happens. They are convenient if you run many microphones on stage, and you don't want to deal with checking offending frequencies, or if your ears are not trained to identify problematic frequencies quickly.
Even though an automatic feedback reducer can be very helpful in getting rid of feedback, you can still get it if you use the wrong type of microphone, have speakers in the wrong place, and use an event hall with bad acoustics.
No matter how good of an audio engineer you are, even if you have feedback reducers, there are some places where the acoustics are just terrible, and there is not much you can do about it.
In such cases where the acoustics are awful, you need to be like a hawk watching your mixer to catch feedback while the event is happening, this is not ideal, it's stressful, and is just frustrating.
Number of Open Microphones
As more microphones are open (On), the possibility of feedback increases exponentially. This is because more sound can be picked up and reenter the system, creating a feedback loop.
The Solution
Try to keep only the required mics ON; just use the mute button in your mixer, but make sure you are paying attention so that when someone needs to use the mic, you turn it on.
The Acoustics of the Room
Another factor that affects feedback from microphones is the acoustics of the room or event hall. Sound from the loudspeaker can bounce back and hit the microphone's membrane even if you have followed all the suggestions in this post.
The acoustics of the room is the only thing you have no control over, so you would have to pay lots of attention to your EQ and volume levels.
The Solution
In situations like this, an automatic feedback reducer can be very helpful because the system can be faster in detecting offending frequencies before they get out of control.
In case you don't have a feedback reducer, try placing the main speakers closer to the audience and all microphones as far as possible from the speakers. Keep the volume in check and be on the watch to make sure you can catch possible feedback.
Turn off any mic that is not needed and make sure to use either cardioid or super-cardioid mics.
Perform several tests before the even until you have a clear understanding of what is your maximum volume headroom and don't go over it.
Stopping Feedback – Summary
Conclusion
In this post, I have presented what causes microphone feedback and how to stop it. I have included solutions for each case, but remember, you must practice putting yourself in different feedback situations so that your ear is trained until you can automatically adjust. Plus, explore different setup configurations until you get comfortable.
If you are a sound tech person that does it for a living by renting your equipment for parties and events, or if you are the sound tech in a church or any other venue, training yourself in avoiding and controlling feedback is a must. Remember, there is no substitute for practice when it comes to sound.
I hope this post helped you in some way to understand what causes microphone feedback and how to stop it. If you have questions, please leave it in the comments sections, I will respond within a few days. Thank you for reading my blog.
Source: https://virtuosocentral.com/what-causes-microphone-feedback/
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