Have you recently started using a microphone and experienced a high-pitched squeal, whistle, hum, or some other unwanted noise? You may instinctively know that it was feedback, but you may be at a complete loss of why that horrible sound happened.
If you are not very familiar with using audio equipment, you may need some help solving microphone feedback issues. So, let’s get straight to it as we take an in-depth look at what is microphone feedback and how to eliminate it for good?
What is Microphone Feedback?
First of all, it’s important to understand that feedback is the result of a feedback loop. Each rotation adds more frequency to the initial signal, which compounds on itself. So, in very basic terms, microphone feedback is a frequency loop, commonly called a “feedback loop.”
Here’s a general diagram of a microphone feedback loop.
As you can see, the output from the speakers becomes the input for the mic. The result is a horrendous sound that is powerful and, in the worst cases, even deafening. Based on the diagram above, how a feedback loop develops involves a few simple steps.
- The microphone acts as the audio input and sends the captured sound as a frequency signal to the audio mixer.
- The audio mixer sends the signal to the amplifier, which amplifies the signal and sends it to the loudspeaker.
- The loudspeaker is the audio output that is then picked up by the microphone as a greater audio input sent again to the mixer and amplifier. Therefore the gain of the frequency further increases.
- More and more sound is captured by the microphone creating a cycling loop that overloads the system and ends up in the squealing, squelching, whistling sound we know as feedback.
That’s a basic description, but, in fact, microphone feedback is a rather simple phenomena. On the other hand, the simplicity of what a microphone feedback loop is raises a simple question. Why don’t microphones feedback all the time? It should come as no surprise that the answer to that is a simple one as well.
The reason why microphones don’t feedback constantly is something called “gain-before-feedback.” This refers to the amount of gain the signal and the audio equipment can handle before you start getting feedback.
In simple terms, if you turn up the volume enough on a microphone, amplifier, loudspeaker, or any other component in the signal chain, you get feedback. Therefore, going past the feedback threshold will cause feedback.
However, the gain-before-feedback threshold is not the same for all audio capture and reproduction equipment. Let’s look at a quick example.
Bullhorn vs. Public Address System
A person with a bullhorn on the street will often have short bouts of feedback. Conversely, a singer in a stadium will almost never experience feedback. Even though they are surrounded by loudspeakers and the decibels are much higher than the bullhorn.
This is due to the feedback threshold of the audio equipment being used, which depends on the amps and volts of the equipment. More amps and volts means greater gain capacity and, therefore, a higher feedback threshold.
Another comparison is a wedding reception speech and a Metallica concert. Which situation is more likely to experience microphone feedback? If you said the wedding speech, then you understand how gain-before-feedback levels can differ. However, the feedback threshold of audio equipment by itself is not the reason why microphones feedback.
What Causes Microphone Feedback?
If you want to get rid of microphone feedback, then you need to know what causes microphone feedback in the first place. There are several factors that cause a microphone to feedback. I have already discussed the gain-before-feedback threshold, and that leads us to a major cause of microphone feedback.
Gain and Volume Levels
When you increase the amplification, or gain, of the microphone, you are adding more to the signal sent to the loudspeakers. Additionally, increasing the microphone gain increases the mic’s sensitivity and noise floor. However, the more gain you add to the mic signal, the greater the risk of feedback becomes.
Likewise, increasing the volume of the microphone channel on the mixer or amplifier is something that increases the chance of feedback. This is because increasing the volume level on the mixer/amplifier pumps more of the microphone signal to the loudspeakers.
The same is true for the volume level of the loudspeakers. With greater audio output from the loudspeakers, the more likely the microphone will pick up the sound and start to generate a feedback loop.
Distance Between and Position of Audio Input and Audio Output
This is most easily understood when you think about guitarists deliberately creating feedback. They move their guitar towards and away from the guitar amp to generate feedback cycles. Have a look at this video on how distance and position affect feedback with a guitar and amplifier.
The distance between a microphone and a loudspeaker affects the signal in a similar way. But, there will be a difference in the kind of feedback sounds. It’s why you can use feedback “artistically” with a guitar and an amplifier, but you cannot with a mic and a loudspeaker.
This again relates to the feedback threshold of the different components. But it also highlights the difference in how a microphone captures sound and guitar pickups capture sound.
All that said
The general rule is the greater the distance between a mic and a loudspeaker, the lesser the chances of feedback. This is due to the Inverse Square Law of soundwaves. It states that for each doubling of the distance, there is a 75% decrease in volume intensity.
Furthermore, one of the benefits of placing mics and loudspeakers further apart is that you can increase the gain levels on both components. Remember the example of the singer in the stadium? The distance between the PA speakers and the mic is quite large, which helps reduce the possibility of microphone feedback.
What Is Microphone Feedback And How To Eliminate It For Good – Microphone Sensitivity
The kind of microphone you use plays a big part in creating feedback problems. That’s because microphones can have different directionalities, or “directional sensitivity,” based on the polar patterns of the mic.
As a result, we have terms like unidirectional, omnidirectional, bidirectional, cardioid, supercardioid, and hypercardioid. They refer to the positions/directions that different types of mics will pick up. Below is a basic diagram of microphone polar patterns.
From the diagram, you can see that the cardioid pattern picks up the least amount of directionality.
These kinds of microphones are less “sensitive” than the other ones. Making them the ideal mics for live performances because monitors can be placed in front of the mic, and the sound will not get picked up by the mic.
It’s not polite to point
Similarly, you can see why omnidirectional and bidirectional microphones are not used in live performances. The chances of those mics picking up the sound from loudspeakers and floor monitors is almost guaranteed. Those mics are essentially always pointing at the loudspeakers.
Furthermore, if you point a unidirectional cardioid mic directly at a loudspeaker or floor monitor, you will get feedback. That’s why loudspeakers are positioned to the sides and in front of the mic. It eliminates the potential for feedback by removing directional and positional conflicts.
Microphone Frequency Response
In addition to the directional sensitivity of a microphone, the “tone” of the mic plays a part in feedback. Tone being the frequency-specific sensitivity of the microphone. Some mics have a high frequency response, and others have a low frequency response.
High frequency mics don’t handle low-end sounds well. And low frequency mics don’t do too well with the high-end sounds. The result of using the wrong frequency response is greater feedback potential.
For example, using a high frequency microphone to record a kick drum. Kick drums produce low-end sounds, meaning that the frequency response of the mic needs to work with that range. Otherwise, you are likely to end up with some feedback. And this is why dynamic microphones are the preferred live microphone because they have a wide flat frequency response that reduces the possibility of feedback.
The Physical Space
The last entry on the factors that cause microphone feedback is where the microphone is being used. The shape and size of a physical space contribute to the acoustic profile. When the sound can bounce around the physical space, you increase the likelihood of feedback.
This is why recording studios use soundproofing, and concert halls are big and spacious. Additionally, it’s why rock shows in actual basements often unintentionally feature feedback. The former reduces sound reflections, and the latter tends to increase the reflections.
What Is Microphone Feedback And How To Eliminate It For Good – How to Prevent Microphone Feedback
Now that we know why microphone feedback occurs, we can look at ways to get rid of it for good. It isn’t hard to do, and the ways to fix microphone feedback don’t require a degree in audio engineering.
Don’t Go to 11
The easiest way to reduce microphone feedback possibilities is to turn the levels down. It’s why mixing boards and all manner of audio equipment have those green, yellow, and red lights that make up the level meter.
If you are in the red, your sound will be dead. So, one of the quickest solutions to microphone feedback is to bring the mic level down. If that doesn’t work, adjust the levels on the amplifier or loudspeakers if they are powered.
Everything in Its Right Place
If your levels are good but there’s still microphone feedback, the next thing to do is check position and direction. Make sure the microphone is not pointed at the loudspeakers or the floor monitors.
You can reposition the mic or speakers, and remember the Inverse Square Law when you are figuring out where to place loudspeakers to eliminate feedback.
Also, you can eliminate microphone feedback by placing the microphone close to its audio source. This is so you can reduce the gain levels on the mic needed to pick up the sound source. As another general rule, the closer the mic is to the sound source, the clearer and cleaner the signal will be.
Use the Correct Microphone
This means you need to pay attention to the feedback threshold and the directional sensitivity of the microphone you are using. So, determine how loud the sound source is and the frequency range it occupies.
Then you can find the right microphone to use to eliminate feedback by staying under the gain-before-feedback threshold. As well as handling the proper frequency range of the sound source.
Be Mindful of Your Surroundings
Realize that if you are playing Heavy Metal in a tiny basement club, you may need to turn the volume down. You may not like that option, but it will actually improve the quality of your music by reducing space-related microphone feedback.
Understand the physical dimensions and material composition of the space where you’re using a microphone. You may need to add some kind of sound dampening such as rugs or foam panels to help cut down on the number of sound reflections.
In other words, make sure to do a soundcheck before you go live.
Technology to the Rescue
Finally, if you want a great way to eliminate microphone feedback for good, there are sound filters that can make a huge difference.
One of the best filters to eliminate microphone feedback is with a Noise Gate. It sets levels where if the audio signal goes above those levels, they are cut off. Or the “gate is closed.” Along with a noise gate, you can use a Limiter which is almost the same thing but less effective.
You also have the option to use Compression to reduce the high-gain frequencies. And there’s always a plain old Noise Suppression filter or making adjustments to a graphic equalizer, especially if you have a narrow band version, that can get rid of feedback potential.
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What Is Microphone Feedback And How To Eliminate It For Good – Final Thoughts
Hopefully, this article has made it clear what microphone feedback is and how you can get rid of it. It’s a common problem that can plague even the most experienced audio engineers. So, don’t be surprised if it happens to you.
However, you should now know the steps you can take to avoid microphone feedback in the first place. Additionally, you should have no problem on exactly how to eliminate microphone feedback if the problem occurs. It’s not that complicated once you understand the basics of microphone feedback.
All the very best with your feedback control!