One of the worst noises known to the ears of a guitarist and fans is to hear the high-pitched scream of feedback emanating from the amplifier’s driver – unexpectedly and undesirable. The remedy for this is a fast spin of the volume knob or the sound guy’s hand slamming the faders down as fast as possible (with a shade of crimson rising in their face, embarrassed that it happened at all). That’s great – feedback eliminated, but HOW does feedback happen at all? And how can you eliminate it from occurring – ever! 

When I started I had no idea how feedback happened or that it had a term, but there is a hint in the proper name for the phenomena’s cause – a feedback loop. Pictures don’t do this justice, so imagine you’re a sound wave travelling through the air and being magnetically captured by the microphone or pickup, running to the amp at light speed. Sound is slower, as we know from elementary science, and as the speakers push the greatly-amplified sound back out into the air, some of those sound waves re-enter the mic. That same tone gets amplified again and again, causing the runaway effect called feedback – a specific tone that is resonating is reinforced time and time again in very quick succession, including the harmonics of the original tone. The only quick fix is to break the cycle by pulling the originating signal back (like turning the volume down).

Since the goal is to avoid feedback (unless you’re prone to walking up to a Marshall stack at full volume and causing feedback on purpose), how would you accomplish this? I’m going to take the beginner’s perspective in describing these options as you might do to put it all in one place, in plain language. Each has a role to play in feedback control and its own set of benefits and pitfalls.

Compressors – Do they make a difference?

Noise Gates – Should I run a noise gate?

Gain Reduction – Turn it down – seriously?

Sound Hole Plug – Put a cork in it?

Bass Reduction – Huh – what does that have to do with anything?

Position/Location change – Seriously?

Compressors
My first introduction to compressors was through a bandmate’s pedal setup, and I misunderstood what the purpose of the miracle device was. First, what compressors are not: they are not a different kind of distortion or effect pedal (a common misconception). Compressors work through balancing the signal coming from the guitar and treating that signal by ‘compressing’ the wave signal leaving the compressor. While compressors are fantastic at limiting stronger signals (leveling everything out; setting a threshold), their impact to feedback control is limited to maintaining your ‘Volume’ signal to a constant level. Because of this, it would not be my first choice if you are struggling with a feedback problem (but do learn more about compressors if you are not familiar with them).

Noise Gates
A noise gate is an interesting addition to a guitarist’s pedalboard primarily because it’s designed to do its job when you are not playing. A noise gate works by monitoring the input signal, and if you aren’t actively playing/making noise (above a certain level) it blocks the signal completely. If you’re familiar with the hybrid cars that stop the engine at a stop light and restart it when you touch the accelerator, it’s the same thing. A noise-gate is the first stop you should make if you are experiencing feedback when you have both hands off the strings and any small vibration turns into a squeal-fest (assuming you’ve tried all the free stuff). There are several great solutions that generally are placed at the end of your effects chain to eliminate any potential issues. By setting the gate level you can have the noise gate stop any signal until it goes over a certain threshold. You may still need to palm-mute between riffs and balance how high you set the gate so you can avoid the choppy smack in the face of a signal that comes in hard or not hearing softer passages because the gate is too high.

Jimi Hendrix – master of the GOOD kind of feedback.

Gain Reduction
Sometimes it’s all about ‘user error’ and not understanding how our equipment works – I’m guilty of this on a daily basis. Thankfully it only happens once and I learn, most times! Many of the folks using amplifiers and effects think, “If some is good then more MUST be better” and crank the Gain knob to ‘10’ using the Volume knob alone to control how much ‘oomphf’ leaves the speakers. The same goes for pedals – distortion pedals for many folks I have met could just as soon not have a Gain control – just set it to ‘10, rip off the knob and leave it be. This is okay for some uses, but for most players it’s rarely an absolute choice – especially if you experience feedback problems.

Modern amplifiers have a minimum of two separate ‘amps’ built inside – One to control the signal coming from your 1/4” cable and amplify that ‘before’ it reaches the main amplifier that boosts the signal immensely and sends it to the speaker. If you have a ‘Gain’ knob, it could just as easily be called a “pre-amp” (but Gain is catchy on the control panel, and sounds friendlier). Turn down your gain a bit and notice the change. You can increase the volume to get the same amount of ‘grunt’ coming from the speaker, but the signal is less likely to loopback if the pre-amp is lowered. The lower Gain setting will be less sensitive to looping back and starting or allowing feedback.

Sound Hole Plug
My Taylor acoustic guitar is a wonderful instrument, but if I’m recording and use a microphone for the guitar instead of using the electronics onboard, I experience terrific feedback and it’s really hard to control. Astute concertgoers will note that the vast majority of acoustic guitars played on stage have a plug (or a baffle) blocking off the sound hole. The scenario is simple – if you block the sound hole and cut-off the big echo chamber designed to naturally boost and amplify the sound, you can better control the signal as it’s amplified. This also ties into the next item on our list of theories, but we’ll get there soon.

If you must perform or record without a sound-hole plug (because the sound is adversely affected, etc.) control the Gain and/or microphone pre-amp signal to  eliminate feedback from happening as frequently. The inside of a guitar is an incubator for lower frequencies (the lower frequency sound waves are long – they need space to bounce around in order to be heard) and the acoustic and hollow body guitars are a perfect chambers to let them stretch out. If you can live with or can’t tell the difference with a sound plug installed, use them – they’re simple, cheap and battery free.

The Dirty Deed Distortion gives you plenty of ways to keep your feedback in check when you need to – or help to generate awesome walls of feedback when that’s what you want.

Bass Reduction
In researching this article I ran across many forum posts of jazz players who swore by the practice of reducing the bass EQ level just enough to quash any feedback issues they experienced. Where you turn down the bass response in your effects chain isn’t necessarily important: whether at a pedal, sound desk or an amplifier control panel, turn down the bass level just to the place where you aren’t experiencing feedback.  For bassists using active basses, reduce your booster on the bass itself first if you happen to experience this problem of feedback.

Position/Location change
Earlier we talked about how the feedback loop ran away between the amp/speaker and microphone – something as simple as moving a monitor to the side, placing yourself between any amp/stack and microphone or just moving away from the amp a little to see if you experience the feedback again. You may find that our salt-water bodies (horrible at reflecting any sound) are enough to block the signal just enough to eliminate your feedback issue.

Sound checks, home practicing and recording studios are the most frequent situations I’ve experienced feedback. With a little care and some cheap solutions, you can keep those situations from happening at all. Venues that are crammed with people (what we all want, right?) can limit the feedback opportunities further because the patrons/fans act as a sound buffers, filling the space. The most reliable feedback eliminator product will be the noise gate pedal/rack unit or software control (once you’ve learned its eccentricities) along with a good sound dude at the ready controlling the faders for the PA/desk. Remember – it’s all physics and there’s no magic or voodoo to getting rid of feedback. The trick is remembering what triggers feedback and staying on top of avoiding it. Happy feedback-free strumming!