Understanding feedback in hollow body guitars

[cwilliamrose]

Why do hollow bodies and semi-hollow bodies have feedback problems that require a solid chunk of wood down the center to control? Magnetic pickups only respond the vibrations of the strings, or do they? How does the wood isolate the pickups? Are HB pickups worse than SC pickups for feedback? Gibson ES guitars seem to use HB as do many other types.

Another way to ask this would be;

If you were designing a hollow body guitar could you control feedback without a big chunk of lumber inside to mount the pickups in? If so, how?

.............Bill

[jerryjg]

Wjhats to understand? Stuff it with foam. J/K, but not entirely.

[cwilliamrose]

Stuffing it with foam wouldn't be my first choice. I have to understand the mechanism so I can try to avoid bandaids like foam. I'm talking a clean sheet of paper here, not fixing or modding an existing existing guitar......Bill

[jerryjg]

Stuffing it with foam wouldn't be my first choice. I have to understand the mechanism so I can try to avoid bandaids like foam. I'm talking a clean sheet of paper here, not fixing or modding an existing existing guitar......Bill

Understood. Perhaps you might begin by studying the approach of some of the masters of Hollow/Semihollow legend; Nugent, Petty, Beatles, just to name a few. Read articles and stuff, like.
Even a study of someone like EVH would work; how the hell can you get that kind of volume on a Strat without insane uncontrolled feedback??
One cannot just pickup an electric Hollowbody guitar an automatically crank it through a wall of Fenders like Nugent,The Beatles at Shea, or even a solidbody through a wall of Marshalls Like Van Halen, without being a complete wizard of the art , and a thorrough preoffesional, cause otherwise all you'll get is a deafening(literally) squeal.
Now, I realize your ambition may be quite a bit more modest, and that I've taken it to the extreme, however some good knowledge may be found by takeing a look at the masters..otherwise, lets here what the boys here have to say about it.

[TommyJ]

The resonance of the hollowbody,resonates off the speaker
of your amp,hell what do I know ask brian setzer,go youtube
him..

[Mesotech]

A hollowbody or semi-hollowbody works much like an acoustic with a pickup. True the strings cut through the electromagnetic flux field of the pickup producing current that is then amplified (as is also done in solidbody guitars with pickups), but the acoustic, hollowbody and semi's gain extra sympathetic vibrations from the wood which adds to the character of the sound. When amplified sound enters the body of the guitar at the same frequencies that the strings are already vibrating at, a feedback loop is generated. The body continues to vibrate, transferring that mechanical energy back to the strings which continue to vibrate generating more current to be amplified which in turn feeds back into the body of the guitar.

The center block in a semi is added to minimize how the body resonates without adversely detracting from the natural tone of the acoustic properties of the guitar. Thus, it helps reduce, but does not eleminate the natural feedback. Players also can control that feedback with arm and/or hand pressure on the resonating portion of the body.

Other techniques used to minimie feedback is the use of wax potting inside of the pickups. Some pickups can become microphonic as the windings of copper wire relax over time, or the wax melts or otherwise breaks down. This microphonic characteristic within the pickup can also cause feedback, but not of a musical style. Even solidbody guitars with poorly designed or operating pickups can feed back in this manner. Controling this type of feedback is nearly impossible without a reduction in volume. The better solution is to re-pot the pickups or replace them.

Most people who play amplified acoustics (acoustic electrics) use a "feedback buster" inserted into the sound hole. While this technique does not drasticly detract from the natural tone of the guitar, it helps minimize sound from re-entering the body, thus interrupting the feedback loop.

In a hollowbody guitar without a center block of wood, a similar technique as a feedback buster can be used. Most who are bothered enough to attempt finding a solution opt for stuffing the body with some material (foam) to assist in keeping sound out and reducing the natural resonance of the wood by applying constant pressure against it. Typical F-hole designs are difficult to block with a form fitted piece of rubber such as a feedback buster. However, F-holes are also designed and placed on the body in a particular place so as to minimize just how much air pressure (sound) is allowed to re-enter the body. Different designs have been attempted by different manufacturers over time (Ovation's peacock pattern, Taylor's sound slits, diamond shaped holes of Yamaha and/or Ibanez, ect...).

Suffice it to say, this is a well known and throughly researched issue with guitars, and as such has driven designers in the direction of current development practices of today. Either get a guitar that meets your volume needs or learn how to mechanically control the feedback with playing technique. Or... Become creative and design something new that works better. LOL

[Wilko]

It's a common misconception that solid guitars don't have resonance that equates to changes in the way the guitar sounds.
Once you get past it, there are whole new avenues of tone and understanding of guitar construction and improvements open to you.

[Ryan Givhan]

if pickups only picked up the vibrations of the strings you could have a stick with a pickup on it and it would sound like a les paul, think about that one.

[cwilliamrose]

.......but the acoustic, hollowbody and semi's gain extra sympathetic vibrations from the wood which adds to the character of the sound. When amplified sound enters the body of the guitar at the same frequencies that the strings are already vibrating at, a feedback loop is generated......

Let's see if I understand this.

The amplified sound does two things, it excites the strings directly when it finds a matching frequency or harmonic and it also is picked up by the body and ends up exciting the strings indirectly. The natural frequency of the strings depends on what notes are being played. What is being amplified is that guitar's contribution and other instruments too. If the natural frequency of some part of the guitar matches the amplified sound's frequency or a harmonic of that frequency, then it can add to the feedback.

........The center block in a semi is added to minimize how the body resonates without adversely detracting from the natural tone of the acoustic properties of the guitar. Thus, it helps reduce, but does not eliminate the natural feedback......

The center block raises the natural frequency of the body by making it stiffer. The top and back plates are stiffer because they are supported in the center and effectively divided into two smaller plates. Does the center block also isolate the pickups acoustically from what's going on inside the body?

...Other techniques used to minimie feedback is the use of wax potting inside of the pickups. Some pickups can become microphonic as the windings of copper wire relax over time, or the wax melts or otherwise breaks down. This microphonic characteristic within the pickup can also cause feedback, but not of a musical style.......

Would all pickups designed for hollowbodies be potted? How about other pickup design features to control feedback, are there others things that can be done?

....Most people who play amplified acoustics (acoustic electrics) use a "feedback buster" inserted into the sound hole. While this technique does not drasticly detract from the natural tone of the guitar, it helps minimize sound from re-entering the body, thus interrupting the feedback loop.......

The sound holes in the body allow amplified sound to enter the body. Why do they put sound holes in hollowbodied guitars if that contributes to the feedback issue?

....Most who are bothered enough to attempt finding a solution opt for stuffing the body with some material (foam) to assist in keeping sound out and reducing the natural resonance of the wood by applying constant pressure against it. Typical F-hole designs are difficult to block with a form fitted piece of rubber such as a feedback buster....... Different designs have been attempted by different manufacturers over time (Ovation's peacock pattern, Taylor's sound slits, diamond shaped holes of Yamaha and/or Ibanez, etc...)

Is it fair to say that stuffing the body damps the amplitude of those frequencies present due to the design and structure of the guitar. It doesn't/can't change those frequencies.

....Suffice it to say, this is a well known and throughly researched issue with guitars, and as such has driven designers in the direction of current development practices of today. Either get a guitar that meets your volume needs or learn how to mechanically control the feedback with playing technique. Or... Become creative and design something new that works better. LOL

You bring up a good point about volume levels. A guitar might be perfectly fine in a living room setting but be a beast to control on stage.

I've had this crazy design floating around in my head for a couple of months. It won't go away so I've decided to pursue it long enough to figure out if it's real or not. I have two goals for this; First to build a unique and hopefully beautiful instrument. Second, to showcase my design and fabrication skills so this unemployed person might attract some for-hire type work. I'm not trying to re-invent the hollowbody guitar but I'm not sure I want to do this project if it results in a severely limited instrument. I saw an interview with Monte Montgomery where he was asked about to problems of doing what he does with an acoustic guitar and he said he uses every trick in the book to control feedback. That scares me because there are very few players that have the skills to pull that off.

The first thing I'm looking at is whether or not I can actually build it. If I can, then I need to be comfortable that the result won't be hopeless as an instrument. I want to do the body in an ES-335 shape and thickness but using a non-conventional material. The neck will be a wood bolt-on sourced from someone like Warmouth. The volume inside will be greater because I won't have a center block (or I should say I don't want to have one) and the plates will be thinner than one done in wood. I can't know for sure but I expect the stiffness of the body's parts to be as high or higher than a wood structure. It might even be a bit bright sounding compared to a conventional guitar.

I wasn't really ready to discuss the details of this crazy idea but it's hard not to if I'm going to get help understanding the pitfalls. I'll be more forthcoming if it gets real. Thanks everyone for the help! I need all I can get.............Bill

[Mesotech]

You seem to have understood what I was trying to say pretty well.

Why do they put sound holes in hollowbodied guitars if that contributes to the feedback issue?

Most often the reason is to enhance the acoustic properties of the instrument. Not all hollowbodies or semi-hollow's have soundholes though (B.B. King's Lucille for instance, or the Gibson ES-336 for another) and they still sound great. But not every player who owns a hollowbody or semi-hollowbody plays amplified. They tend to prefer the natural tone of an acoustic instrument and the option to plug in to get louder when necessary. My boss tends to only play his Gibson L5, and rarely ever plugs it in except to record what he's playing. Even then, he wants to get the sound he's hearing from the guitar acoustically instead of what a typical guitar player might want when recording. He plays jazz exclusively.

Would all pickups designed for hollowbodies be potted? How about other pickup design features to control feedback, are there others things that can be done?

No, they wouldn't. Most are, but not all. A majority of the less expensive pickups are not potted, and a few of the most expensive are not. Potting the pickup is a way to make up for the inhierent abnormalities of pickup construction. To be fair, I'm not a pickup winder and my experience on this matter is based upon what I've seen for sale in the marketplace. To get a proper technical explination, you might want to direct this type of question towards someone who makes pickups. There are several of these fellows on this forum who would be more than happy to discuss these things with you in detail if you created a new thread with this question, as it would attract their attention faster than this thread might.

Does the center block also isolate the pickups acoustically from what's going on inside the body?

No it doesn't, or shouldn't. The center block should serve two purposes. It should devide the cavity into two acoustic chamers that are of less volume, thereby reducing the air pressure inside of either chamber that would cause the top and back to resonate less drastically. Secondly, it should help stiffen the top and back so that they don't resonate as much as they would without the centerblock. Many acoustic instruments are braced on the top (and sometimes the back as well) in various patterns to stiffen the wood. It seems that most companies have their own method of doing this, which contributes or detracts from the tone of their instruments.


Good luck with your project. It sounds like it would be something fun to do. You might also want to check out a company (ironically from Lafayette La.) called CA Guitars (the "CA" stands for Carbon Acoustic). Their quest is to make the perfect acoustic guitar, and from the ones I've played they do a grand job. It is extremely interesting (to me) how they can achieve so many tonal variations in different models that look identical just by the bracing techniques they use. A fellow guitar player friend of mine works for them, and would talk my ear off about how they are made if I'd let him Ironically, he's a heavy metal player and looks SO out of place in his workplace.

[NeoFauve]

Reverend Guitars is making a new Pete Anderson signature model.
It's a rockabilly looking hollowbody.
It employs something they call the Uni-Brace.
I'd like to see a cutaway view of the inards, but it sounds like it's a 1/2" thick wall the runs under the bass side of the strings and pickups, from the neck to the tail of the body.

[cwilliamrose]

Thanks Mesotech,

Very helpful stuff.

I'm thinking the F-holes don't need to be there initially and can be added at any time. There are enough hollowbodies out there without them that mine wouldn't look strange because of their absence. Given the way it will be finished, it might actually look better without them.

I'm not too worried about the straight acoustic performance, there's not a lot of volume in the body to use as an amplifier. I play my CV Tele unplugged a lot and I'm sure this one will be louder than that.

I'll start a thread in the Pickups section when it becomes clear that this thing is a 'go'.

I saw on a guitar manufacture's site somewhere that they were using some changeable insert blocks to mount the pickups and they seemed to play a part in the guitar's tone. I'm wondering if feedback is part of why they are using these? This is why I asked about the center block and acoustic isolation. I'll see if I can find a link to that site....

The CA guitars are interesting. I wonder how thick the bodies are in carbon, I'll bet they're pretty rigid and not very thick. My material of choice is a bit less high-tech.

My wife and I were in your town and the surrounding area just about exactly one year ago, we went to the Breaux Bridge Crawfish Festival. The weather was not good, this year was better but we couldn't justify the expense. The couple that went with us last year called us on Saturday from the festival. I hate missing out!!

NeoFauve,
That's a very good link for me and my (possible) project. They talk about dividing the interior volume to reduce feedback. Looking at that guitar and my design, I'm at maybe half their volume already. I could divide my interior into two or three chambers but I thought initially to join them. I don't have a Uni-brace in mind -- more like a Duo-brace. I could actually assemble my guitar design without the body and play it, maybe only adding a little gusset to act as a shear web between the braces.

But I haven't gotten that far yet with my model. I'm still trying to get a decent top and back contour (both parts are the same and symmetrical about the centerline). I may not be able to make them exactly as modeled but it will give me some idea if the thing can be assembled using the build sequence dictated by my choice of materials.

One problem is that my braces may need to be four (or more) parts becoming two inside the guitar. Kinda' like building a model ship in a bottle! They'll carry the neck and string loads all the way to the tailpiece and will mount the pickups and bridge. I had thought about running them to the strap button mount but figured I already have enough to deal with getting them to the tailpiece..........Bill

[Doc W]

You can solve the feedback problem the same way that acoustic guitarists with pickups solve the problem. Don't stand near the speaker. In the case of an electric hollow body, it means not playing too loud and getting away from the direct path of your amp.

I have played an ES-345 for years and I personally love the feedback I get. What is wrong with the block of wood running through it? It helps me exploit the feedback in musical ways. What IS the problem you are trying to solve? What am I missing here?

Long live feedback (of the controlled variety)

[cwilliamrose]

What is wrong with the block of wood running through it? What IS the problem you are trying to solve? What am I missing here?

I'm not trying to solve a problem with hollowbodies, I'm considering building a guitar body out of a non-traditional material and looking at the potential pitfalls of my approach. Feedback is one of those. If I felt I had to use a wooden center block, I'm not sure how I'd get it in there. I'm also not sure I'd bother with this if that were the only option since doing that would compromise the design's appeal, at least to me......Bill

[cwilliamrose]

Here's the link to Alumisonic's custom page. It shows and talks about adding chambers around the pickups to effect tone and control feedback. Also on their site is a mention of removable chambering for their hollowbody offering.

I could something along those lines, most likely as a permanent feature. A removable chamber might be possible depending on how I mount the pickups and how I provide for access to the pots, jack and switch.

...........Bill

[yankeerob]

I'm not trying to solve a problem with hollowbodies, I'm considering building a guitar body out of a non-traditional material and looking at the potential pitfalls of my approach.

I admire your trying to think outside the box for starters... g'won...

Feedback is one of those. If I felt I had to use a wooden center block, I'm not sure how I'd get it in there..

The centre block is a device that's 50 yrs old if it's a day - either because it's still the best solution or a time honored easiest way out. Probably a little bit of both... I'd imagine that a construction that inherently inhibited standing waves would be a great start... if air - by some clever internal porting - as opposed to using dampening materials which to my mind moots the point of what you're trying to acheive - can't build up in the chambers then it can't vibrate the top from the inside... another huge mistake that people make with hollow bodies is putting anything more powerful that a standard output PAF in them... but yer asking some interesting questions...

[cwilliamrose]

Your comments make me want to do a development prototype but I'm going to resist that and risk building an elaborate wall ornament.

I'm kinda' stuck with doing the body 'shell' complete with no guts in order to have it look the way I want it to look. That may be the deal breaker for this project. I'm doing a 3D CAD model so I can figure out my options before I get in deep trouble. I'd love to play with baffles and ports and isolators but this will likely be a one time only build -- if it happens. The more I think about and talk about it, the more I want to do it. I've got a great spot picked out on the wall to hang it.

Thanks for the comments Rob..........Bill

[ES350]

Stiff tops, heavy bass side braces, laminate tops instead of carved...look at an old middle-level Epiphone; they are less prone to feedback than most other archtops for some of these reasons. I am talking about full hollowbody archtops, not 335 or 225 type construction. OTOH, I never quite understood why Gibson/Guild/etc hadn't used sound posts since they are used in most other stringed instruments...maybe because nobody ever even considered trying to play them through a stack of Twin Reverbs.

[cwilliamrose]

Mine will be much more hollow than a 335 even though it will have the same outside dimensions. I'm not at all sure how stiff the body will be but the structure underneath will be pretty stiff. The body should be pretty rigid since there are no flat areas -- just about every surface is curved or compound curved. I'm planning on a symmetrical structure -- I don't see any reason to make it purposely weak or flexible on one side.

This image is very early in the design process. I made the top 60% transparent so you can see the brace. The seams visible running through the center of the top, back and sides are the result of mirroring the part after creating the first half. Each of those parts will be made from a single piece of aluminum and welded at the center of the corner radius.

Anyway, it shows a brace designed to carry the string loads from the neck to the tailpiece. It also provides hardpoints for the bridge and pickup rings. There will be a mirrored version on the other side. There will be additional braces going to the strap button and picking up the back of the body -- if I can get the parts in there. There's a nice big hole created by the neck pickup cutout and the neck (which will bolt-on) so there's a chance this will work.



So, I can anticipate some feedback issues (I think) but will it be a huge problem or will it be manageable?.......Bill

[ES350]

One thing I have noticed with thinline hollowbodies (ES 330, 350T, 225, etc) is that the resonant frequency shifts up toward the middle range of the instrument (somewhere along the D and G strings, all maple construction can shift it even higher) rather than the lower range. The only partial solution I have found for this is to add mass, whether it's hardware, a solid center block, a plate under the top (ala 225), or the thickness of the top.

[cwilliamrose]

What's an acceptable weight for an ES-335 style guitar? I'm trying to keep the body under 3.5 lbs, ready to mount the parts (neck, hardware, electrics, etc). I have reduced the thickness of the body panels to .050" (from .0625") to save about 0.6 lbs. I'd rather use the thicker material for a number of reasons but this will be different enough that it may not share much with a wood ES-style guitar.

Does anyone have a ballpark weight for a couple of HB pickups with controls and rings, a neck with tuners, bridge and tailpiece, pick guard and all the screws/strap buttons, etc? I need a weight budget for the body.

......Bill

[yankeerob]

Hey Bill... thought I'd wait until you got a bit further along to comment... I think you will wind up with something - despite it's being hollow - that will have a very 'linear' response - if you recall the likes of Travis Bean and early Kramers - they had great sustain but there was 'something not quite right' with regard to the interaction of string and body resonance which resulted in the ultimate failure of those guitars to sell... I had one of the first production Kramers and despite the great sustain there were far too many other aspects that made it sound very sterile... even when they beefed up the pups on later models they just couldn't get it right. The basses were a little bit better but as you're probably aware they are no longer made - plus the neck took a long time to warm up on a cold day and it just never felt quite right... regards a weight budget for the body you're just going to have to accept there's a minimum thickness of top and back that will be practical and you'll most definitely be governed by that... good luck with it though...

[Toneseeker]

What's an acceptable weight for an ES-335 style guitar?

I recently sold an Epiphone Sheraton on Ebay and had to ship it to France. It went with a Gator hard shell case and I only weighed the entire package for shipping. That weighed in at 8 Kg. 5 Kg is probably not unreasonable estimate for the guitar.
I have a Telecaster now which feels as light as a feather compared to the Sheraton.
Incidentally, I swapped the stock pickups on the Sheraton for Bare Knuckles and the guys at Bare Knuckle recommended alnico 2's as anything more beefy would cause problems with feedback due to the semi-hollow body.

[cwilliamrose]

I think you will wind up with something - despite it's being hollow - that will have a very 'linear' response - if you recall the likes of Travis Bean and early Kramers - they had great sustain but there was 'something not quite right' with regard to the interaction of string and body resonance

Those two guitars were aluminum necks on wood bodies, correct? My project is more like a Normandy or Alumisonic were the body is metal and the neck is wood. Are those guitars selling? I don't know, you don't commonly see them but there are a lot of guitars from small shops you don't commonly see.

As to the linear characteristics, that may well be the case with my design, I'm not experienced enough to have any intuition here. What you're saying (I think) is that wood acts as an EQ device that colors the tone in a familar way that makes a more linear instrument sound 'funny'. I wonder how much of that is the neck and how much is the body. Again, no intuition here but logic says the neck is a bigger part of the system that supports the strings so it might have the larger effect. I'd love to hear comments from anyone that has heard a Normandy or Alumisonic in a live setting, or better yet, played one.

Does a pedal steel have a linear tone response? Those are very rigid with tons of sustain and some designs feature non-wood construction. Or does a pedal (or lap) steel guitar's tone sound OK because it's a different instrument with it's own character?

.......plus the neck took a long time to warm up on a cold day and it just never felt quite right...

Another aspect I had not considered -- differential thermal expansion. The structure in my design's body may take a while to stabilize because it's inside a shell and somewhat isolated from the shell by the adhesive I intend to use in lieu of visible fasteners. The bond joints will be thin and have a lot of area so there might be decent heat transfer. Until it all finds a happy temperature the tuning may want to wander a bit. That's probably the case with any instrument, it will be a little worse with aluminum.

As far as feel goes, the player interacts primarily with the neck so I'd expect my guitar to feel somewhat familiar.

regards a weight budget for the body you're just going to have to accept there's a minimum thickness of top and back that will be practical and you'll most definitely be governed by that...

Yes, I think .050" (1.25mm) is getting pretty thin. I can weld thinner metal successfully but the result would be more prone to dents where the metal is soft (near the welds). What I can do is lose weight in the braces to allow thicker panels. With CAD, I can apply loads to the brace parts and see what I can remove without losing much rigidity. At some point you'll be hurting the structure by removing too much material. I have not optimized the structure yet but it looks like my 3.5 lb initial target is going to be too low. I'm a little over 4 lbs now and I'll be fighting to get that below 4 lbs, I just don't see it getting much below that number. There's not much structural contribution from the body's shells so I don't get anything back by shifting the material to the panels.

It may be time to start a new thread since the details of the build are being discussed and not all of those details relate to feedback.

Thanks Rob.........Bill