Why is my mic giving feedback?

What is Microphone Feedback?

Microphone feedback refers to the loud screeching or howling noise that occurs when a microphone picks up audio from a nearby loudspeaker and amplifies it, creating an audio feedback loop. This happens when the amplified microphone signal from the speaker feeds back into the mic and gets re-amplified repeatedly. The loop results in sustained high-pitched ringing or squealing sounds coming out of the speaker system (Shure, 2022).

Feedback is caused by an audio loop that forms between a microphone and a loudspeaker. It typically occurs when a microphone is placed too close to a speaker it is connected to. The sound waves from the speaker enter the microphone, get amplified again through the system, and are outputted even louder through the speaker. This creates a continuous loop with increasing volume and pitch as the sound gets amplified over and over within fractions of a second (3Dio, 2019).

Common Causes of Feedback

One of the most frequent causes of microphone feedback is positioning the mic too closely to the loudspeaker or monitor. As sound comes out of the speaker, it can be picked up again by the microphone and re-amplified, creating a feedback loop. According to Shure, feedback results from “irregularities in the frequency response and polar patterns of the microphones, the loudspeakers, and the room acoustics” (https://www.shure.com/en-US/performance-production/louder/feedback-fact-and-fiction).

Another common reason for feedback is having the system volume or gain set too high. When the volume is turned up, it increases the chance of audio from the speakers being detected by the microphone and creating unwanted feedback. Sweetwater recommends soundchecking at performance levels to find the right balance between volume and feedback potential.

The shape, materials, and setup of the physical space can also cause issues with feedback. Hard, reflective surfaces allow sound to reverberate rather than being absorbed. Improper speaker placement relative to walls and other surfaces can encourage feedback loops (Sweetwater). Careful acoustic treatment of the room is often necessary to prevent feedback problems.

Feedback Frequency

feedback usually occurs in high, treble frequencies. According to Shure, an audio equipment company, “Whistles” and “screeches” caused by feedback tend to be above 2 kHz. Very rarely does feedback occur below 80 Hz or above 8 kHz. It takes practice to develop an ear for the specific feedback frequencies.” [1]

Big Guy Productions, an audio production company, notes that “If it sounds like a howl its likely in the 250 to 500 Hz range. If it seems less pronounced, like singing your looking at 1kHz. High pitched screaming feedback is usually around 4-6kHz.” [2]

In summary, microphone feedback is most common in the higher frequency ranges from 1kHz to 6kHz, causing whistling and screeching sounds.

Ways to Prevent Feedback

There are several techniques that can help prevent or reduce microphone feedback:

Position the microphone properly – Point the mic towards the desired sound source and away from speakers to minimize pickup of output audio. Get the mic as close as practical to the source to allow for lower gain settings.

Use a directional microphone – Directional mics are more sensitive to sound from the front and less sensitive to sound from the back or sides. This helps isolate the intended audio source.

Use a feedback eliminator – These devices detect and filter out feedback frequencies in real-time. Popular options like the Sabine FBX Feedback Exterminator can drastically reduce feedback.

As explained on the Shure website, “Move the microphone closer to the desired sound source. Use a directional microphone to increase the amount of gain before feedback. Reduce the number of open microphones.” https://www.shure.com/en-US/performance-production/louder/how-to-control-feedback-in-a-sound-system

Acoustic Treatment

One of the most effective ways to prevent microphone feedback is through acoustic treatment of the space. This involves using sound dampening materials on walls and ceilings to absorb reverberation and “deaden” the room. Common acoustic treatment options include:

Sound absorbing foam panels – These lightweight foam panels are installed on walls and ceilings. The porous foam material absorbs mid and high frequency sounds that can cause feedback. Panels are available in various thicknesses and can be custom cut to fit.

Bass traps – Bass frequencies below 250 Hz are harder to absorb. Bass traps are dense panels placed in room corners to absorb low frequency energy. This removes the “boomy” resonances that lead to rumbling feedback.

Diffusers – While absorption panels reduce reverberation, diffusers scatter sound waves to prevent echoes and flutter echoes. Combining absorption and diffusion treatments optimizes room acoustics.

Proper acoustic treatment placement is key. A professional acoustic consultant can analyze the room and advise where to install panels and bass traps for maximum feedback reduction. DIY placement can also help, but may require experimentation to find optimal panel positions.

By absorbing and diffusing sound, acoustic treatment minimizes audio reflections that cause sustaining feedback loops between mics and speakers. It’s one of the best ways to improve on-stage or studio monitoring environments.

Audio Mixer Settings

One of the most common ways to prevent feedback is through proper configuration of your audio mixer or PA system. Here are some key tips:

Lower the gain – Turning down the overall volume on your mixer channels (especially the mic channels) gives you more headroom before feedback occurs. Don’t crank the gain higher than needed.

Use EQ to cut frequencies – Find the specific frequencies that are feeding back and cut them with EQ on the mixer. Sweeping through different frequencies while listening can help pinpoint the problem ranges. A 31-band EQ gives precise control. See How to Control Feedback in a Sound System for more details on this technique.

Use notch filters – Notch filters cut extremely narrow frequency ranges, letting you zero in on feedback spikes. Some mixers or processors have automatic feedback detection that can apply notch filters dynamically.

Pan channels – Panning the mic and speaker sources apart can avoid direct coupling and reduce feedback.

Use subgrouping – Route mics to subgroups and speakers to the mains, so you can lower mic levels as a group. This takes some rethinking of gain staging but can really help.

Automatic Feedback Reduction

One option for preventing feedback in mic systems is to use an automatic feedback reduction system. These systems use digital signal processing (DSP) to detect and eliminate feedback frequencies in real-time.

DSP-based feedback reduction works by analyzing the signal coming into the mixer and identifying sudden spikes in volume at specific frequencies, which indicate feedback. When feedback is detected, the system applies a narrow filter to that frequency to reduce the gain before it can loop and create sustained feedback. This all happens very quickly, without needing any manual adjustments by a sound engineer.

Dedicated feedback destroyer units like the Klark Teknik DF1000 (https://www.amazon.com/Feedback-Eliminator/s?k=Feedback+Eliminator) can be inserted into the signal chain to provide automatic feedback control. Many mixers and PA systems also have built-in DSP feedback reduction as a feature.

The main advantage of automatic feedback reduction is that it reacts faster than a human can to detect and eliminate feedback as it starts. It allows more overall gain to be used without feedback compared to manual notch filters. One tradeoff is that heavy-handed settings can negatively affect sound quality.

Choosing the Right Mic

When selecting a microphone, the pickup pattern is crucial for reducing feedback. Cardioid mics are the most common, picking up sound from the front and rejecting sound from the rear. Supercardioid mics take this a step further with a tighter front pickup pattern and more rear rejection. As this Sweetwater article explains, supercardioid mics like the Shure Beta 58A can help prevent feedback by further limiting the mic’s rear pickup.

Headset microphones are another good option, as their close proximity to the mouth allows for lower gain settings. With the mic element directly next to the sound source, feedback is less likely to occur. Handheld mics require higher gain compensation to pick up the voice from a distance. This greater amplification makes feedback more prone at high volumes. Using a headset mic ensures the ideal mic-to-mouth distance for feedback reduction.

Mic Technique

Proper microphone technique is crucial for reducing feedback. One of the most important aspects is hand positioning. Your hand should grasp the microphone just below the head grille, with your thumb on one side and fingers on the other. Hold the mic loosely but firmly enough to have control. Gripping too tightly can cause handling noise.

Additionally, maintain a consistent distance from your mouth to the mic – usually 6 to 12 inches for dynamic mics. Getting too close increases feedback risk as the mic picks up more sound reverberating off your body. Backing too far off requires turning up the gain, which also feeds back more easily. Find the “sweet spot” distance for your mic where you get optimal sound before feedback.

With good technique, the mic capsule will stay oriented towards your mouth as you perform. This maximizes the sound source your mic is aimed at, improving gain before feedback. Sloppy technique causes the capsule to point away from your mouth occasionally, necessitating higher gain and more feedback.

When to Call a Professional

If you are unable to resolve persistent microphone feedback issues on your own, it may be time to call in a professional audio technician or company that specializes in AV system design and installation. Here are some instances when professional help is recommended:

Persistent Feedback Problems: If you have tried various microphone placements, gain staging, EQ cuts, and acoustic treatment to no avail, then the underlying cause may be more complex. A seasoned pro can troubleshoot issues that are tricky to diagnose.

Large or Complex AV Setups: For large venues, installed systems, multiple mic setups, or situations requiring detailed acoustic analysis, a professional technician has the expertise to fully optimize the system. They have advanced tools and experience managing feedback in complex environments. According to Sweetwater, pros use spectrum analyzers, variable EQs, and feedback eliminators to precisely target feedback frequencies.

System Tuning and Optimization: An experienced technician can fine-tune equipment settings and calibration for optimal gain staging and frequency response. This helps reduce feedback while improving overall sound quality.

Customizable Acoustic Treatment: For problematic rooms, a professional can design and install customized acoustic panels, bass traps, and diffusers to target resonances and reflections that cause feedback. Tailored treatment offers benefits beyond simple foam panels.

Installing New Systems: When installing new sound systems, hiring a professional ensures optimal equipment selection, placement, wiring, and configuration to prevent feedback issues.

In general, for mission-critical events or persistent sound problems, it pays to hire an expert. Their specialized gear and expertise in acoustics and electronic systems makes eliminating feedback much more achievable.