What is Feedback?
Acoustic guitars are highly susceptible to feedback, that eardrum-bursting high-pitched howl that has been the ruin of many a live show. Most people will have experienced feedback at one time or another, from the low-end drone that you feel in your bones to the high-pitched howl that feels like it’s threatening to pierce a hole in your eardrums.
I can distinctly remember being back at high school and on many occasions when a principal or teacher addressed the school using the PA system there would invariably be feedback, shortly followed by loud groaning from the students.
This could have been prevented, but unless you understand what causes feedback you have little chance of stopping it.
So, what is feedback and why does it occur?
Feedback is a ‘looped signal’ that occurs when an output source e.g. the amplified sound coming from a speaker is also being used as input e.g. being picked up on a microphone or guitar pickup and re-amplified, increasing the signal.
The frequency (pitch) of the resulting sound is governed by numerous factors including the resonant frequencies of the speaker and amplifier, the microphone or guitar pickup, the distance and angle between the input and output source, and even the acoustics of the room itself, along with a number of additional factors.
In the case of what was happening back at school, the microphone was picking up the sound coming from the powered speakers due to the angle of the speaker. The speaker directed the sound toward a smooth, reflective surface (concrete walls on either side of the stage) back toward the microphone amplifying it again, and again and again, and … you get the idea.
Hence the name ‘feedback’ as the sound coming from the speaker is continually feeding back into the microphone. To stop feedback, therefore, the looped signal must be interrupted.
Acoustic guitar feedback can be influenced e.g. made worse by:
- How resonant the guitar is
- The size of the guitar body e.g. smaller body guitars have less trouble with feedback
- Gain (input volume) and volume (output volume e.g. how much of the signal is sent to the speaker)
- The distance between the input (microphone, guitar pickup) and output (speaker, amplifier)
- The direction of the input and output in relation to each other
- Sound reflection e.g. sound waves reflecting from objects back towards the microphone
In the case of guitarists, feedback can be produced by a microphone e.g. a guitarist is positioned in front of a mic to amplify the guitar or through the use of a pickup, or often a combination of the two.
While electric guitarists occasionally use feedback to their advantage e.g. guitarists such as Pete Townshend and Jimi Hendrix deliberately created feedback by holding their guitars close to the speaker. Acoustic guitars and feedback, do not play well so together as acoustic guitars are far more prone to feedback than their electric counterparts due to how they are constructed.
Why Acoustic Guitars are more prone to Feedback?
As we know, acoustic guitars rely on resonance to amplify vibrations detected from the strings. Resonance is the transfer of vibrations at either a fundamental or overtone frequency measured in hertz (HZ).
What is HZ (aka Hertz?)
A measure of frequency per second. E.g. 100HZ is 100 vibrations (or cycles) per second. The human ear detects sound between 20 and 20,000HZ.
A vibrating object (in this case guitar strings) vibrates and displaces air molecules creating sound, however, due to the small surface area only a small amount of air molecules are displaced.
However, the vibrations from the strings cause the soundboard to resonate at the same frequency which in turn creates sound waves which displace more air molecules (due to the larger surface area) resulting in more volume being detected by our ears. These sound waves bounce around within the internal cavity of the guitar most strongly before exiting via the soundhole.
Resonance and Feedback
A great acoustic guitar will be highly resonant, meaning the soundboard timber is selected due to its flexible strength and lightness.
As a result, the ideal soundboard material will be light and strong e.g. able to vibrate easily but strong enough to maintain its structure over time.
Unfortunately what makes an acoustic guitar resonate strongly when played acoustically is also the reason the acoustic guitar is far more prone to feedback issues when amplified.
High-end acoustic guitars are fairly light, emphasizing their resonant capabilities, especially on the very low-end frequencies.
You will often feel the wood resonating when you play a more expensive acoustic guitar, as opposed to a guitar with a laminate top.
If the output source (speaker) is close enough to the guitar, sound waves emanating from the speaker will cause the strings to resonate which causes the soundboard to resonate, creating a loop.
The sound waves from the speaker will also hit the guitar top directly creating further resonance. This is also why you can stop most feedback simply by muting the strings and preventing them from resonating.
This is also why a classically trained singer can theoretically break glass with their voice. If the singer’s pitch matches the resonant frequency of the glass, the glass will vibrate and when subject to sufficient volume, break.
How to Prevent Acoustic Guitar Feedback?
So what’s an acoustic guitarist to do? Feedback may suit some styles of music e.g hard rock and metal but this is mostly the domain of electric guitars that do not rely on resonance to produce sound.
The good news is there are a number of things you can try even if you have very little experience with audio engineering. The first thing you should do is make sure you are using a soundhole cover.
Soundhole covers / Feedback Busters
Covering the soundhole prevents much of the air resonance of the guitar but also predictably has an impact on volume and tone, depending on your pickup system.
You can experiment with different types from the standard soundhole cover, to vented versions along with lute style ornate soundhole covers to get the right balance between tone and preventing feedback.
Choose an appropriate microphone
If playing in front of a microphone, you will generally find yourself in front of an omnidirectional mic, cardioid microphone, super-cardioid microphone, or a bidirectional microphone.
Microphones can be differentiated by their ‘polar patterns’ which relate to their sensitivity to sound from specific directions.
Omnidirectional microphones detect sound equally from all directions. This means they also detect a lot of ambient sounds, meaning they are not ideal for micing up acoustic guitars because they can not be directed away from the direction of a speaker and cannot be aimed at the ‘sweet spot’ of your guitar, generally around the 12th fret.
Cardioid microphones detect soundwaves almost entirely from the front and as a result, can be aimed at the source of sound (the acoustic guitar) and directed away from the direction of unwanted sound.
Super cardioid microphones detect even less unwanted sounds as they are more tightly focused than cardioid mics. They also detect a small amount of sound from the rear.
Bidirectional microphones detect sound in a figure 8 pattern e.g. they are sensitive to sound from both the front and rear.
If you have control over the microphone being used to amplify your acoustic guitar, choose a cardioid or bi-directional microphone as these can be aimed away from the sound coming from the speaker and can be more tightly focused on the sound coming from your guitar.
Volume and Microphone Placement
Along with ensuring you are using a microphone that is less prone to feedback, the output volume of the speakers along with their proximity and the direction they are facing are also important considerations.
The more volume coming from the speakers the more likely feedback will be an issue.
Switch of unwanted microphones
While this may appear an obvious tip, if you are following another act it’s possible there are going to be more microphones on stage than you require. If so, ensure they are switched off.
Move the microphone
Keep your microphone as far away from the speaker/s as you can. By increasing the distance between the input and output you can decrease the possibility of feedback occurring. This may not be something you can control if playing a larger venue but for intimate settings, this is a quick fix that can save you a lot of trouble.
It is also helpful to move the microphone closer to the guitar on stage. Less gain will be required due to the proximity of the mic to the source (e.g. the acoustic guitar).
Change the angle of the speaker
As previously mentioned, if playing in a larger venue this won’t always be possible but if you can, try changing the angle of the speakers so they are less likely to reflect sound in the direction of the input e.g. microphone or pickup.
Get foldback speakers off the ground
Sound travels through the ground. This is why in those old western movies the tracker would put his ear to the ground to detect the sound of approaching hooves.
A simple fix for this is to simply get the speaker up from the ground, using a stand or cushioning the bottom of the speaker to absorb the sound waves.
Acoustic treatment (also referred to as sound treatment) is often confused with soundproofing but they are two entirely different things.
Soundproofing involves reducing the amount of sound that escapes the room you are performing in while acoustic treatment relates to preventing unwanted sound reflection within the room itself.
As we have already discussed, one of the contributing factors to feedback is sound reflection e.g. sound waves bouncing or reflecting from objects back into the direction of the microphone, creating a feedback loop.
Recording environments usually require sound treatment to increase the clarity of a recording. This is because as sound waves reflect from other surfaces the microphone will pick up a lot of unwanted ambient sound. The same principle applies with regard to feedback. The less reflective the environment, the less likely a feedback loop will occur.
Think of how sound may bounce around the room and look for ways to absorb sound reflection. In my case, early on, I hung blank canvases on my walls to absorb reflective sound. This had an immediate impact on my recordings and reduced some of the issues I was experiencing with feedback.
EQ and Mixing
Reducing the offending frequencies that are causing the feedback is the first thing a sound engineer will do to prevent feedback.
If playing larger venues, the sound will be the responsibility of the audio engineer, and your job will be solely to perform. But if you are playing a smaller venue and are responsible for the sound yourself there are some things you can do with regard to EQ to reduce feedback.
The first thing you should do is consider the dominant frequencies of your guitar e.g. is it mid-range dominant? Or is it particularly bright sounding or does it accentuate the low-end frequencies more?
Next, consider the room you are playing in. Compare the sound of the room to other venues you have played and create a sonic snapshot in your mind. What frequencies are most dominant?
If you are using a D.I (direct input) or have EQ on your amplifier you can reduce the offending frequencies by doing the following:
- Raise the volume of the speaker until you hear the first instance of feedback
- Using trial and error, reduce the frequency that is causing the feedback
- Raise the volume of the speaker again until you hear feedback again
- Repeat step 2 as many times as required
Remember EQ is more or less adjusting the volume of particular frequencies. By reducing the volume of the frequencies responsible for feedback you are essentially ‘turning down’ feedback.
As you can see, acoustic guitar feedback is a complex issue. The majority of the time, if playing larger stages controlling feedback will be outside of your control but there are ways you can assist the sound engineer by adjusting the eq on your guitar or using a soundhole cover.
If on the other hand, you have a more hands-on role with regard to sound try some of the tips above, starting firstly with changing the direction and proximity of the output source in relation to the input as this is likely to be the most effective measure you can take.