3 Secrets to Eliminating Microphone Feedback

Nothing can make a performer cringe and occasionally dive for cover like the whine and squeal of microphone feedback through a PA system. The mixing engineer can also get pretty uncomfortable as every head whips around to look in their direction, as if to say, why would you let that happen? Obviously, none of us starts a show intending to damage people’s ears or the speaker components with microphone feedback, but the challenges of operating a speaker in a room with open microphones can be complex. Here are some best practices to minimize the danger of microphone feedback that everyone should be aware of, whether behind the console or in front of a microphone.

What causes microphone feedback?Three effective tips for eliminating mic feedbackUnderstanding your microphone’s characteristicsRinging out the room to rid microphone feedbackAutomatic microphone feedback suppression

What causes microphone feedback?

Microphone feedback happens when a speaker and a microphone recirculate sound multiple times and continue to boost a specific range of frequencies. Mic feedback happens because no microphone or speaker is completely flat in actual operation. If a microphone has a peak at 6kHz to aid in clarity and you have applied a “smile” EQ curve to your speaker system, both pieces are boosting those higher frequencies. When you speak into the microphone, the speakers put sound into the room, which is also heard by the microphones and reamplified. Because this sound is often a reflection off a surface in the venue, it is later in time than the original signal, so this process can actually repeat several times.

Because this level typically drops a lot on each trip through the system, it is usually not a problem. However, the room you are operating in will also emphasize certain frequencies based on its geometry. These are called room modes and they are developed based on the length, height, and width of a room. The closer the room is to a cube the more exaggerated this effect is because each dimension builds up energy at a certain frequency, and if they are all the same then this frequency can resonate loudly. A room that is a 16′ x 16′ x 16′ feet cube would build up a lot at 35Hz, and also at the octaves of that frequency, at 70Hz and 140Hz. If you are a math geek, the formula for figuring this is pretty simple.

Divide the speed of sound in feet per-second — which is 1,130 ft/s — by the room’s measurement in feet, multiplied by times two. The formula looks like this when using a measurement of 16 feet′:

1,130/(16X2) = 35.3125Hz

Microphone feedback can become a significant problem when the room modes land on the same frequencies as the bumps in the microphone and speaker responses. This means that frequency gets increased multiple times and generates that noise we all dread.

Three effective tips for eliminating mic feedback

Understanding your microphone’s characteristics

The best thing to do when it comes to avoiding microphone feedback is to prevent problems before they start. 1dB of prevention is worth 16dB of cure. Obviously, we want to point speakers away from microphones and set up a performer behind the main speaker system. If you are using a small system and are placing it behind the microphones for the performer to hear better, you are behind the eight ball before you ever turn the microphone on!

Additionally, understanding microphones and their characteristics can make a big difference in preventing microphone feedback. Most people are comfortable taking advantage of the directional characteristics of a microphone to help avoid microphone feedback. Everything else being equal, a cardioid microphone is less likely to feed back than an omni microphone because it is less sensitive as you move away from the front of the microphone. Keep in mind that hypercardioid and supercardioid microphones like the Shure Beta58A or Sennheiser e 935 are actually more sensitive behind the mic than a typical cardioid mic, so they might be a poor choice if you are using open wedges on a stage.

Figure 1: The Shure Beta 58A is still pretty sensitive at 1kHz and 500Hz on the back side of the microphone.

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However, an omni microphone tends to have a flatter frequency response than a cardioid mic, so if you make a cardioid into an omni by holding it around the ball of the microphone, it will actually be more prone to microphone feedback. Also, moving a microphone that is feeding back farther away from a vocalist’s mouth will actually exacerbate microphone feedback, as the sound engineer will have to apply more gain to keep the level the same. Training a performer in proper mic technique can really help with microphone feedback!

Unfortunately, physics cannot be trained. All microphones on stage are receiving signal from all sound on the stage. A source that is farther away is lower in volume, but it also has a delay, roughly one millisecond for each foot of distance (1,130 ft/sec divided by 1,000 milliseconds = 1.13 ms/ft). If you have two vocalists standing right next to each other, those microphones could be about 18″ apart. Both mics will “hear” both vocalists but with a time delay between them. Even though the vocalist who is farther away is lower in volume, that small delay means some comb filtering will happen, which will make the frequency response more ragged. This means having more open microphones on stage increases the likelihood of microphone feedback and having those microphones closer together makes that potential worse.

The way to fix this is to mute unused microphones and keep open mics as far apart as is realistically possible. Remember that the inverse distance law tells us that doubling the distance between a source and a receiver results in a 6dB decrease in level, and that is a lot! Use this fact of physics to your advantage. If you have two mics 18″″ apart — or you could alternatively set them up 3’′ apart — then the bleed between those sources and the comb filtering that is created would go down by 6dB, making the sound more linear and less prone to microphone feedback.

Finally, remember that all directional microphones exhibit what is called “proximity effect.” This makes the mic have 10dB–15dB more output in the low- frequency range when you are very close to the microphone. If a vocalist is right on top of a mic like a Neumann KMS 104 or a DPA 2028 and you are in a 20’x20′ (same format as the 16′ x16′ x16′ above), then your system is already building up a lot of energy around 100Hz, and it can easily get out of hand. Using the highpass filters on the console or mic can help control that energy and limit the risk of feedback.

Figure 2: You can see that the Neumann KMS104 has about 12dB more output at 100Hz when you are right on top of the mic.

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Ringing out the room to rid microphone feedback

It can be fairly obvious if your room is building up a lot of energy when you look at all of your EQs and see that you are cutting the same frequency on most of the channels! It may be time to ring out your room. This will help you identify problem frequencies for your microphones so you can cut down on microphone feedback. You can hire a professional to come in and tune your room to address this, or use an automated processor like the dbx DriveRack 260 which will tune your room and do automatic feedback suppression.

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The process for eliminating microphone feedback is actually pretty simple if you want to attempt it yourself. Most digital consoles have an RTA built into them that will allow you to look at the response of incoming audio. If you have this and a graphic or paragraphic EQ on the output of your console, you can start today.

Start by feeding pink noise to your speakers. If you don’t have a way to generate it, you can download a file here to play through your system.

Place a simple measurement microphone like a PreSonus PRM1 at the listening position in the room and run it into a console channel with no EQ applied, then look at the RTA for the channel the mic is coming into. Pink noise is an even distribution of energy, so if you see any big spikes in the response, these are indications of a buildup (resonance) in the room, which can cause microphone feedback. If the bump is at 300Hz, use the graphic EQ to pull out 3dB of level at that frequency. Note that if you have big holes in the response curve, you cannot do the same by adding EQ at these frequencies because it is also caused by room geometry. Adding output EQ won’t help. At that point, you will need to move the speakers, but that would require a different article!

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Ringing out microphone feedback can be done once you finish this. Turn off the pink noise and open all of the microphones on your console. Slowly turn up the main fader until the system starts to give mic feedback. That frequency will stand out on the RTA, and you can pull down that frequency on your output EQ. If you don’t have an RTA on your console, you can download one on your phone. Sometimes I will sing a feedback pitch into an RTA on my phone to set a quick read on what frequency it’s at, if I don’t want to re-create that feedback during a show. Repeat this process as necessary.

Automatic microphone feedback suppression

Sometimes manufacturers decide to package a microphone feedbackn expert in the box with the equipment. If all of this just seems a little too overwhelming for you, a mixer like the QSC TouchMix series gives you both automated room tuning and a set of automatic feedback suppression filters. This feedback suppression is so easy in execution, literally one push of a button once you hear microphone feedback and it finds and eliminates it. We should have this on every live console! Even if I don’t use the mic-feedback wizard that is built into this mixer, I will often use its frequency recognition to cut those areas when I have purposely caused some feedback before the band gets there.

Figure 3: All the Touchmix mixers feature eight channels of automated feedback filters.

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Don’t settle for microphone feedback

There are lots of tools and software apps to help you through the process of making your sound system more stable and less prone to microphone feedback. Give your Sweetwater Sales Engineer a call at (800) 222-4700, and they can connect you with the right equipment that will make you sound like an FOH pro in no time.
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