The Causes of Acoustic Feedback in Semi and Full Hollow Body Guitars

The key reason why Tim Armstrong is able to play

punk rock on his signature Gretch G5191BK axe 

without feedback is because it's never turned on!

Throughout my years in the pro audio business as well as all things music in general, I find that there exists common problems that many players out there might not even realize. There also exists common problems that many musicians do know about; they just don’t know the solution, let alone the cause. One such prevalent problem – especially with newer guitar players – is that of the relationship between the body type of their instrument and the prevalence of feedback. The veterans out there probably can already see where I’m going with this; yes, the very real and very common association between the hollow body/semi hollow body guitars and signal feedback.


Get Back (From That Amp)

I remember going to a local bar by my house one night and decided to take a quick look inside. Being the huge Beatle fan that I am, I couldn’t help but be enticed by the admittedly John Lennon looking frontman of the particular group on stage along with that signature Epiphone Casino hollowbody. Even the bass player and second guitarist had either a hollow or a semi. These guys really like F-holes, I thought. Everything seemed in order; the band looked professional, the equipment was pretty top notch. Even the microphones they were using – Shure SM58’s – pointed towards a group that knew what they were doing. Then came the first song. The feedback was simply harrowing. As the Lennon figure went over to his amplifier to fix what he thought was the cause of the problem, the feedback grew much worse. As the bassist went along to help the frontman with his noise issue, the feedback grew louder. By the time the third guy went over to his own amp to make sure it wasn’t coming from his own end, all you could hear was the very specific screech signal feedback. As I inched my way close to the front of the room, I noticed that not only did each of them have their volume settings on their amps all the way up, they had a significant amount of gain rolled in there as well. To make matters worse, the tiny stage didn’t allow them to be more than maybe five feet away from their amps – at best. I wanted to tell them that it wasn’t their amp's fault… it was their guitars'… more specifically – the body of their guitars.

Unfortunately, what these guys didn’t learn from guitar 101 is that the body style of a guitar IS NOT for looks alone. The body style of a guitar affects tone, timbre, coloration and yes – the propensity of feedback. Ever wondered why hard rock groups that use plenty of overdrive (such as metal) tend to stick with solid body guitars? That’s because they can handle it better, or rather, are immune to a specific type of feedback, but more on that later. Now, think about the typical style of music that sees a lot of hollow body guitars – jazz, country, pop – pretty much styles that don’t use too much distortion and overdrive. Semi and full hollows just weren’t created for big distorted guitar sounds of today. Simply put, hollow body means more propensities for feedback – and if you don’t like it, too bad, it’s simply in the nature of the beast. Before any of you out there scream and point towards certain guitarists that break that rule, I can bet you right now that they have added plenty of refinements to their axe to combat feedback and this is


Acoustic Feedback

All guitars experience feedback, some just more than others. The reason why hollow body guitars tend to be affected more is because they are highly susceptible to acoustic feedback. Solid body guitars are completely immune to this while semi hollows are somewhere in between. Some people out there might think that switching over to humbuckers might help alleviate this problem. Well, that’s not going to do you any good actually as humbuckers battle outside electronic noise interference (such as from a strong transformer coming from an electronic device nearby) and not acoustic feedback – two different kinds of noise.

To put it as simply as I can, acoustic feedback happens when a soundwave strikes the hollowed inside of the guitar’s body, causing it to resonate. The resonating body then gets the strings vibrating, which are picked up by the pickups, which is amplified by the amplifier, which causes more soundwaves coming from the amp to strike the body again, causing it to resonate... again. This creates a weird sort of perpetual loop that we hear as feedback. Since the root cause of this type of acoustic feedback is too much actual sound hitting the guitar, the easiest way to combat it is as simple keeping an appropriate amount of distance between the guitar and your amplifier. How much is appropriate? That depends on how loud your amp is, for one. If you’re working off of something like a small 20 watt practice amp, you can technically crank it as loud as you want and will probably have to get very close to it for some feedback (which can be pretty fun and a common technique actually). The bigger and louder the amp, the farther away you have to get. Also, the more overdriven the tone of your guitar is, the more susceptible it will be to feedback, further extending the “feedback zone,” if you will. You can still get feedback using a completely clean tone in case you’re wondering – your guitar just won’t be as sensitive to it as if it were overdriven.

So just to backtrack for a bit, lets laydown what we just learned about the nature of acoustic feedback in hollow and semi hollow guitars:

  1. 1. The bigger and louder the amp, the more susceptible you’ll be
  2. 2. The more overdriven your tone is, the more susceptible you’ll be
  3. 3. The closer you are to your amp, the more susceptible you’ll be


Remember that these aren’t mutually inclusive – meaning that passing the acceptable “noise free” boundary of any one of these 3 listed scenarios alone will cause feedback. Alright, so now we can talk about combating it.

We already spoke about getting a good distance away from your amplifier in order to avoid feedback, but much like the band I checked out that night at the bar, sometimes you just don’t have enough room! Well, now that we know the causes, it should be fairly easy to find some creative work-arounds, even when getting farther away from the amp is not an option. One thing though; this might just be a personal peeve of mine but if you are in a band and planning on playing with overdrive and distortion, please do yourself a favor and just invest in a solid body. Why make it harder for yourself? WHY?!

Although giving yourself a good amount of space between the amp and guitar is by far the best way in combating acoustic feedback in hollows and semi hollows (outside of getting a solid body), there are plenty of good little tips and tricks that can be applied on both the guitar and the amp that actually work. Some of these things are as simple as covering up the F-holes on your guitar or using a noise gate with your amp. Unfortunately, we don’t have enough time today to get too deep into in so we’ll have to wait until tomorrow in order for me to properly explain all of these pretty interesting work-arounds. Hopefully, we’ll see you then!

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