How do I stop feedback on my PA system?
PA System Feedback
Ah, feedback – the unwelcome guest at any gig, conference, or public address event. If you've ever heard that ear-splitting screech or howl emanating from a sound system, you've encountered feedback. It's not only disruptive but can also be a sign of underlying issues with your audio setup.
Whether you're a musician, DJ, a public speaker, or someone who sets up sound systems, knowing how to stop feedback on your PA system is crucial. This article will delve into the common causes of feedback, how to set up your PA system to avoid it, and whether feedback can damage your speakers.
What causes feedback on a PA system?
Feedback typically stems from poor microphone and speaker placement. When a microphone is positioned too close to a speaker or pointed directly at it, the sound from the speaker enters the microphone, gets amplified and sent out of the speakers, gets picked up by the microphone, and the loop begins. This is especially common in small venues or spaces with reflective surfaces that bounce sound around.
Another frequent culprit is excessive gain. Gain refers to the adjustable voltage level of your input signal. It’s an important adjustment that lets you raise the level of a particularly quiet input device such as the weak signal of a microphone and match it to the stronger line-level signals from instruments. If your microphone gain is set too high, the signal will be saturated when it's amplified which introduces distortion and clipping, and this boosts a lot of undesirable frequencies in the process which your speakers put out and the mic then picks up.
You also have the issue that some mics are just overly sensitive to specific frequencies, and much more likely to pick up the output from your speakers. A standard vocal mic can have a +/- 3dB output variation, so proper EQ and gain control at the mixer are essential as every mic is different.
Lastly, room acoustics play a significant role. Hard, reflective surfaces like walls, floors, and ceilings can reflect sound waves back into the microphone, causing feedback. Poorly treated rooms with no acoustic dampening can exacerbate this problem, making feedback more likely and more pronounced.
The image here shows a great example of how NOT to set up your PA. You not only have a 50s-style mic which will have a large capture pattern, but both speakers are set back and firing across it.
How do I set up a PA without feedback?
Setting up a PA system without feedback starts with proper microphone and speaker placement. Position microphones as far away from speakers as possible and never point microphones directly at speakers. On a normal live stage setup, the speakers should be at the front of the stage facing outwards, with everything else behind them.
Use dynamic directional microphones with cardioid patterns for vocals in a live environment as they are designed to pick up sound from a specific direction only and reject the rest, thus reducing the likelihood of capturing unwanted noise from the speakers or the room.
Adjusting the gain of microphones is equally important. Start with the gain on the mixer set to zero and gradually increase it using the mixer level meter for reference while listening for any signs of feedback. Use the PFL (pre-fade listen) if available as it routes each channel to the VU display for a visual reference, whilst remembering that you are increasing the signal strength of the mic's output into the preamp (gain is not volume!). The goal is to find a balance where the microphone's signal into the desk is at a usable level without causing distortion, as it’s those boosted frequencies that are most likely to be picked up from the speakers by the mic and looped back.
Utilising equalisation (EQ) can also help manage feedback. By identifying and reducing the frequencies that are prone to feedback, you can minimise the chances of it occurring. Many modern PA mixers or amplifiers come with built-in feedback suppression or compression technology, which can automatically adjust frequencies to avoid feedback.
Does feedback damage speakers?
Feedback, while mostly a nuisance, can indeed pose a threat to your equipment. Prolonged or intense feedback can damage speakers by causing them to overheat or by pushing the speaker cones beyond their normal range of motion. This can lead to reduced sound quality and, in severe cases, permanent damage to the speakers.
High-frequency feedback is particularly harmful as it can cause the tweeters in your speakers to blow. Tweeters are designed to handle high frequencies, but when exposed to continuous high-pitched feedback, they can quickly fail. This is especially true of piezo units and compression horn drivers that aren't always protected through a crossover network.
Moreover, feedback can also damage your amplifier (be it built-in or external) or cause it to shut down. A feedback loop can quickly cause a large increase in signal amplitude, and the common howling effect means spikes in particular frequencies, which push the amp's input stage into hard clipping. This can cause significant component damage depending on the amplifier's design, especially if it happens abruptly at high volume.
That said, it’s one of those ever-present possibilities with live audio. You can't be fearful of it happening, but just be aware of the techniques to best avoid it, and know how to make quick adjustments to kill it off quickly if it does occur.
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Understanding and Managing Audio Feedback in PA Systems
Let's take a further look at some of the main causes of audio feedback and how to deal with it should it occur.
Microphone Sensitivity
There is a lot of confused language and misinformation online around the adjustment of microphones, even on manufacturer's sites. A standard dynamic microphone is a passive device, meaning it produces a signal without external power. What this means is that you have no control over the electrical sensitivity of the pickup, regardless of what anyone tells you.
The sensitivity of a microphone pickup capsule does of course vary from mic to mic, but this is purely down to variances in manufacturing tolerances. You may end up with two identical microphones and one is simply more sensitive than the other, which results in a hotter output signal.
The gain control on the mixer's microphone preamp is there to add or subtract amplitude (voltage) to the signal it's receiving, which allows you to compensate for these variances, but it does not make the mic more sensitive or able to detect a wider area! It's physically and electrically impossible for that to happen in a standard microphone.
What is Feedback?
To truly grasp how to stop feedback, it's essential to understand its cause. Feedback is an acoustic phenomenon that occurs when a sound loop exists between an audio input (like a microphone) and an audio output (a speaker system).
This loop can be thought of as a vicious cycle: the microphone picks up the sound of your voice (and anything else), which is then amplified and played through the speakers. If the microphone then captures this already amplified sound again, it gets re-amplified, leading to an escalating cycle that results in that characteristic screeching sound.
Identifying Feedback
Not all feedback sounds the same. Feedback can occur at different frequencies depending on its source, and are typically those that the room or the system is most resonant at.
The feedback caused by a vocal microphone for instance is often a high-pitched howl, whereas acoustic feedback from surface reflection can present itself as anything from a low rumble to an unusual echo or phasing effect.
For more advanced system tuning or troubleshooting, these frequencies can often be identified using a spectrum analyser or frequency measuring tool. By pinpointing the exact frequencies that are causing feedback, you can adjust your system or alter the room layout to reduce or eliminate them. This is actually a common part of setting up a larger system such as a nightclub or theatre.
As fun as that type of frequency hunting sounds, realistically for the average DJ or live band it's not going to happen, and won't help you in the moment anyway. So for mobile rigs, all you need to know really is it’s 99% likely to be microphone-related and fairly easily rectified, even if you have little to no control over the room layout.
Room Acoustics and Treatment
The acoustics of the room where you're setting up your PA system significantly affect feedback. Rooms with a lot of hard surfaces – like concrete walls and tile floors – can cause sound waves to bounce around more, increasing the chances of feedback.
When designing the layout or decoration of a venue or stage area it’s important to consider materials and the positioning of performers and equipment to minimise sound issues. Many larger venues will carpet cover a stage for deadening purposes, and high-ceiling locations employ sound baffles suspended from the ceiling to help with runaway frequency and echo cancellation. Some performers even take carpet or rugs with them as part of their stage setup for this reason.
Feedback can be tricky to work around if you are only doing a single gig or event and the venue has particularly poor acoustics. Absorbing panels, bass traps, and acoustic diffusers in front of instrument amplifiers can help scatter sound waves, reducing the likelihood of feedback, with the rest requiring electronic solutions such as a feedback destroyer or compression system, along with the careful EQ of instruments and microphones during setup.
Speaker and Microphone Placement
Proper placement of speakers and microphones is crucial in avoiding feedback. Speakers should be placed in front of the microphones and angled away from them. This setup helps ensure that the microphone is not picking up the sound coming directly from the speakers, and this includes any floor monitors too, not just the main speakers. Speaker placement varies with the space available at each venue, and can be extra tricky if using subs and tops as a smaller venue especially may just squeeze you into a corner.
As most vocal microphones will be stand-mounted and easily moved, this will usually be your first stop to rectify any issues, and even half a metre of movement or less can be plenty enough to knock any unwanted howling on the head.
Using the Right Equipment
It generally goes without saying, but investing in high-quality equipment can make a significant difference. Dynamic microphones, which are designed to pick up sound from a specific area only, are less likely to capture the output from the speakers than those designed for studio or recording use, and the better the quality of the mic, the better its frequency response and pickup characteristics will be.
Cheaper mics with a wider ‘cover it all’ pickup pattern are far more likely to cause unwanted problems on a loud stage as they are only really designed to be used as budget home karaoke options. Ideally, it will have a metal body which helps ground body static and mute any noise caused from holding the mic in your hand. It will also have a detachable XLR connection to allow for a balanced, noise-reducing cable to be used, and ss a rule, any mic with a fixed cable and 6.35mm phono jack should never be considered for professional work.
The mixer is also an important factor, especially for a live band. You should be considering a mixer with options such as pre-fade listening (PFL) to each channel, and PAD buttons for reducing the input signal when trying to mic up a particularly loud source. Some mixers may also include a selectable high-pass filter button (HPF) to its channels which removes unwanted low frequencies (good for vocals, hi-hat, snare etc). Some mixers include features such as DSP for sound shaping and vocal effects, and many now feature built-in sound cards for easy USB linking to a laptop for recording.
Connections between all stage equipment should be balanced where possible for noise rejection, and particular attention should be given to power distribution and the use of too many extension leads if you want to avoid ground hum loops.
Effective Use of Equalisation
Equalisation (EQ) is a powerful tool in preventing feedback. By cutting the frequencies that are prone to feedback, you can reduce the chances of it occurring. Start by using a graphic equaliser to identify and reduce the problem frequencies. Many sound engineers use a technique called "ringing out" the system. This involves gradually increasing the volume and cutting the frequencies that start to feed back.
Compression is another form of EQ, and many mixers include it in their microphone preamps already. A compressor reduces a signal's dynamic range between the loudest parts and the quieter ones to give a more consistent sound and can work wonders with certain live vocals, though it can also increase the risk of feedback if not dialled in properly. It’s one of those things that can really transform some performer's sound or just isn't required for others, so it's well worth experimenting with.
The Role of Gain Structure
Proper gain structure is essential for preventing feedback. Gain structure (also known as gain staging) refers to the levels set at various points in your audio signal chain. If the gain is too high at any point, it can cause feedback.
Stage and studio equipment, in fact any electronic device used for audio, has a specific signal voltage and amplitude it’s expecting to see at its input. If you give it a signal that’s outside of those parameters, it will do what it can but generally with poor results, and it then passes this to the next thing in the chain.
When linking audio equipment, be it a mic to a mixer, a DJ controller to active speakers, or even an electric guitar into effects pedals, it's a careful balance between tone shaping, gain boosting or reduction, and volume level, while keeping the output signal at an acceptable level for the next device. This process is actually the essence of an audio mixers core functioning outside of providing somewhere to plug everything in. It allows you to balance out the levels of wildly different signals into a tamed and controlled singular output that an amplifier or other device can easily deal with.
Monitoring and Adjusting During an Event
Even with the best preparation, feedback can still occur during an event. This is why monitoring is crucial. Given most DJs or bands won't have a sound guy on hand, assign someone to keep an ear out for feedback and be ready to make adjustments as needed. This might involve lowering the volume, adjusting the EQ, or repositioning microphones and speakers. Being proactive can prevent minor issues from becoming major disruptions.
Training and Experience
Finally, there's no substitute for experience. The more you work with PA systems, the better you'll become at identifying and preventing feedback. Hands-on experience and continuous learning are the best ways to improve your ability to manage feedback effectively.
It's also imperative that you become familiar with the controls of your equipment, understand what each pot or knob does, and preferably do this before you put yourself into a live situation. On-the-spot panic can cause you to overreact and this can be fatal for speakers and sensitive equipment if you are just grabbing at buttons.
Take the time to read manuals, do test setups at home or in a practice studio, and get your head around gain adjustments and EQ tweaking to get the sound as professional as possible.
Conclusion
Feedback is an inevitable challenge when working with PA systems, but with the right knowledge and tools, it can be managed and eliminated effectively.
Understanding the common causes of feedback, setting up your PA system properly, and using the correct equipment and techniques can help you minimise or eliminate feedback. Remember that preparation is key: proper microphone and speaker placement, consideration of room acoustics, and effective use of equalisation and gain structure between devices are all crucial steps in preventing feedback.
With practice and experience, you'll become adept at avoiding feedback completely or managing it when it appears, ensuring smooth and professional audio experiences for your audience. Whether you're a seasoned sound engineer or a beginner setting up your first PA system, these strategies will help you keep feedback at bay and deliver clear, high-quality sound.