Acoustic feedback is caused by the guitar resonating with the sound waves from a speaker, creating a loop between the guitar and speaker. To prevent feedback, the loop must be interrupted. Try using a soundhole dampener, changing the distance and direction of the speakers in relation to the guitar, and reducing the offending frequencies by controlling EQ.
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.
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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.
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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.
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.
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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).
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.
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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.
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.
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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.
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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.
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.
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If your guitar uses a pickup or onboard mic a soundhole cover can help reduce feedback, and as an added bonus prevent picks from disappearing in your guitar!
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.
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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.
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.
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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.
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.
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.
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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.
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).
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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.
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.
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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.
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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
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