Why are Acoustic Guitar Bodies Tapered?
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- Опубліковано 10 січ 2019
- In this episode of DIY Guitar Making, I talk about the radius of the soundboard and back plate, and the body depth taper from tail block to neck block as a design feature of acoustic guitars.
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I suspect the taper may have a structural aspect as well as an acoustic one. The string tension creates a bending moment that wants to curl the neck toward the front. The guitar structure spreads the static load between the front (compression), back (tension) and sides (bending). On the front a flat plate handles compression well and can be voiced to a lower tone while maintaining the strength needed for the string loading. On the bottom the dishing (which adds stiffness) can be more pronounced as the voicing is to a higher tone. The taper tends to align the back with the tensile stress so the structure can be more weight efficient. I think it produces the optimal structure for the acoustic air volume contained in the body with good ergonomics for the player.
Love your videos!
Keith Short
Thanks, Keith! Great Response! I think you might be on to something there with the structural aspect.
Now we’re talking...engineering...moments...static and dynamic loads...maybe a few Fourier transforms...Square Root Sum of the Squares..,🤓🤓
In theory if the top and back are parallel that would reinforce a tone with a wave length that's four times that distance. If that happens to fall on an overtone of a played note it would alter the timbre and power of that note, and probably the sustain as well. In other words, you'd tend to end up with a 'wolf' note at some pitch(s). On most guitars this would be a tone at around 1000-1500 Hz. Tapering the depth from one end to the other 'spreads out' the effect: instead of a big peak in the spectrum on a single pitch you get a lower peak that's spread out, so it's not as much of a problem. Mark Blanchard, who generally knows what he's talking about, stated that guitars he's built with more taper tend to sound more 'even', which is about what I'd expect.
Of course, that may or may not be the reason people started to taper the boxes in the first place. The ergonomic explanation makes as much sense as any, and they may have started doing it for that reason, and decided they liked the sound.
There is the accepted theory that the tapered upper bout deadens the sound. Sounds counterintuitive, as our goal is sustain and projection but sustain can and will "muddy" the sound. Early Luthiers explored these variations to achieve, what they perceived, as the best sound.
The science and technology of instrument construction is certainly interesting, with all the latest sound equipment, metering and whatnot but I think we have to be careful to keep technology in balance and not let it overwhelm the art, the skill and craftsmanship...the love and passion of building. The personal relationship and satisfaction of building an instrument is the goal...all else is just business.
@Eric Schaefer Maybe I can answer your question.The sound hole purpose.
This was explained to me during a guitar building class with David Wren, himself a student of luthier Jean Larrivee. It's not for sound. It's to allow free movement of air when the top vibrates. Most of the sound comes right off the top, like a speaker cone. This can be verified with a shotgun mic. The hole stops air pressure building up and dampening the top vibrations. It allows for more sustain and louder sound. Think of an acoustic instrument as a speaker box. Heavy stiff support to increase the efficiency off a vibrating cone/top.
Sound is not made of particles that can be sped up or constrained. It's waves at Xfrequency. Sound waves (other than certain boundary effects) travel in straight lines from source and are reflected, diffused or absorbed. What comes out of the hole is mostly bassier tones, since the waves inside the guitar have bounced back and forth a number of times and the highs have been attenuated and diffused by the wood and bracing inside. That said. Low bass does spread out if it goes through a hole in the solid wall. The difference is that low bass waves are actually pressure waves and are hard to stop.. That's why your pant legs will flutter when standing next to a cranked bass bin at a concert.
This leads to why the back is curved. Again from David Wrens class. We want a stiff box for the top to sit on to control dampening the sustain of the top. A compound curve is the best way to stiffen a flat plane. You can hear a tonal difference even then, between holding the guitar back against your body and holding it away. Also how much of the forearm touches the top while picking will also dampen out the top.
David also explained that the top is only curved in one dimension. From the lower bout to the top bout it is flat, a-la flat-top guitar This allows maximum flexibility and less tension on the top to vibrate from the strings. On his guitars most of the curve is from the waist to neck joint to allow proper string height at the bridge. Then again it's actually fairly straight where the fingerboard is glued over the body. I still have my templates from that build. Check out David Wren on YT at Redguitar12. Here's a link to one of his stunning one-offs. ua-cam.com/video/ERpf8iX_qHU/v-deo.html
I was thinking along the same lines. I watched a video recently (though it was made about 20 years ago) on voicing with presentations by Tim White, Michael Hornick, and Richard Schneider. I believe it was Tim White that stated that only about 5% of the sound you hear comes from the sound hole and as you point out, is mostly low frequencies (bass notes). If you have 2 1/2 hours free sometime and want to understand some of the science behind these concepts, here is the link ua-cam.com/video/0qIjxNjMxxw/v-deo.html
Sorry, I'm going to apply here the same principles we do when building speaker cabinets. Now the resonant top of the guitar acts like the speaker membrane, which makes that the guitar top creates a back wave acting in the opposite cycle has the top of the guitar which would in effect cancel the frequencies and create distortion when reflected by the back. So IMHO clearly the sound whole is not there to create sound but to somehow prevent the top waves and the bottom waves to mix and cancel each other, allowing also the top membrane to vibrate freely. For example drums don't have backs nor sound holes. Cellos and double basses have very small sound holes. We know that air chambers are very good sound isolators...
A truly parallel body would have standing waves that would cancel themselves out quickly.
Eric buddy, your videos are so enlightening. I am encouraged to keep working by your work, thanks.....
Thank you so so much for this video Eric! You've explained what I have never found in any lutherie book 🙏🙏🙏 thanks from France
Eric, you explain technical term very clear , thanks
I appreciate your references to Bernoulli's Principle and the Venturi Effect. Btw these are also factors in how a wing lifts aircraft and have analogies in how fluids in general behave in other pressurized systems. "Pressure flow is High to Low." I believe your intuition on these principles as they apply to design and construction of acoustic instruments (not just guitars) is right on. The tapers of the top and back plates toward the sound hole constriction increase the velocity of exiting sound waves while locally reducing the pressure as pumped air accelerates through the hole, therefore reducing the net vibrational energy needed to drive that sound at higher amplitude or what we perceive as 'volume.' I think that on that theory, more air volume in the lower bout resulting from greater taper geometry, without changing anything else, would produce correspondingly higher exit velocity through the sound hole and therefore more sound amplification. I guess this has been thoroughly researched through the centuries but it would be fun to experiment with in the shop just to prove to ourselves. For example, try a 110mm body height at the lower bout, tapering to your typical 90mm at the upper bout/neck, just to hear what happens. Intuitively we'd expect the guitar to be louder if nothing else, do you agree? Maybe it would alter the frequency response slightly too- I would guess more bass. Refer to "dreadnoughts" and the huge bodies seen on some Mariachi guitars- all about pumping more air for a given playing energy input. The acoustic guitar is a vibration-driven 'sound pump.' The body taper increases the output efficiency for a given energy input (provided by playing) and results in amplification. And of course curved structures are naturally stronger and more resistant to deformation under stress. Acoustic instruments, being somewhat delicate and fragile, need all the help they can get to be strong and little things add up. Please, anyone feel free to correct or enlighten me. Enjoyed this show!
Maybe it started out as not a "tapering-in", but a "belling-out" in the lower bout, to increase volume and bass response-in a sort of let's-see-how-deep-we-can-comfortably-get-it experiment? And they liked the results. It worked.
04/26/2021: Wow, such detail and well explained ... Thank You!
You rock man. Thanks!
thank you Eric. the taper i would think is to funnel the sound makes sense to me . love the work you do
Thanks, Walter. As always, I'm glad you like the videos!
Thank you for explaining this. It is making the concept easier to understand. So if my side layout in the plans indicate this taper is the taper cut into the sides prior to using the 15’ radius dish or is the taper achieved by using the radius dish? My sides show 81mm (lower) & 71mm (upper) on the back side only. No radius in the top. Thank you
You were right about the Bernouli law and the venturi effect. But you said the half of it. As the speed rises the pressure reduces. For instance, in airplanes the pressure becomes negative and that is what keeps the airplane in the air. The negative pressure that is applied on the top of the wing, "sucks" the airplane higher. The venturi effect seems to apply in guitar bodies, that follow this kind of tapering and in a way, you can say that they manage to "suck" the sound out of the body, having the result of better projection of the sound.
I studied a little bit about the acoustic reason for not having flat and parallel surfaces, the reason these radii exist in a guitar, uke, etc. is to avoid "flutter echo" which is a condition that occurs in acoustic spaces when two parallel surfaces reflecting sound between one another are far enough apart that a listener hears the reflections between them as distinct echoes. The audible effect is in many cases a sort of “fluttering” sound as the echoes occur in rapid succession. Also, the curvature helps the sound vibrations to "bounce" at an angle towards the soundhole instead of just up and down, generating flutter echo and cancelling each other.
I’m an ultrasound tech, and sound physics are the same, in or out of the audible range and on something as complex as a guitar. Torres believed the back was for reflection only.( hence his paper Mache’ experiment) so the curvature was probably used to focus toward the spin hole but also would cut down on frequency cancellation along a segment of the guitars response. All waves being reflected back and forth between the top and back that correlated to a wavelength along of say 4” would bounce off the back and soundboard and destructive interference would cancel the frequencies out. This is mitigated by changing the distance and the angle of the boards to each other. Plus the arch adds a huge amount of structural integrity to the box. A win - win
Hi Eric! I think the guitar looks better with that taper, as well.
Hey Eric, where did you get those radius measure tools shown at 2:26 into this video?
Are your back braces at a 60 degree angle all the way to the lower bout or just near the soundhole due to the cutaway?
As Ervin Somogyi says it is an Air pump created by the vibration of the strings.
Thanks. I like the scientific explanations. Firstly, I do wonder if the Venturi effect even applies at all to sound? I can’t really find much info on that. Secondly, the up and down vibration of the spruce is what largely makes the air move to create good sound. So would it not make sense that the air is being pushed out the sound hole then immediately being pulled back in, all very fast and ultimately resulting in zero net flow of air? If that’s the case then Venturi effect would not apply here either? I wonder what the effect would be if you were able to flow air in one end of the guitar body (larger end) and have it come out the tapered area at the sound hole. Fun to think about but I don’t have the ability to test it haha.
What size radii would you recommend for a beginner?
I am a rank amateur, not even that really. But the maker I have read most instructs the reader to build a work/clamp board where there is a slight rise on one end. Thus, when building the face of the instrument, there is a modest decline toward the bridge though the portion under the fretboard is more flat. His explanation is that this feature counteracts the string tension somewhat and thus makes bowing (over time) less likely.
I've responded to your comment in this Q&A video. Check it out! ua-cam.com/video/_8qkMgJgJWE/v-deo.html
Nice. I especially benefited from your "Benelli"? Principle explanation. Sound compressing at the waist and speeding up... Maybe that's why my musician friends say my first build has very long sustainability...? I dunno, intuition told me to tighten up the waist. Now I have a very feasible explanation. Cool!
"Feasible explanation". No, you have a hypothesis, which requires rigorous testing to come to any reasonable conclusion. It's not "Benelli", it's Bernoulli's Principle. Not knocking your spelling,
hell, Nobel physicist Richard Feynman called it the Branalloy principle as a student.
Is there any sound difference between a body with vs without a cutout ?
What's the difference in Archtop and Flatop Guitar Builts? Are the Bracing different or the same from each other??? Do tell.
Great video Eric. Sound is in the Ear of the hearer (pre scientific measuring). So, its kinda subjective. An instrument maker can make all sorts of claims for a particular shape or stiffness or whatever. If that maker is really good at marketing and can convince a lot of people that it sounds better, then, all other makers are basically forced to incorporate something they know is BS into their builds. Psychologically a person who wants to be a great player will insist on certain elements be present in the guitar even if they suspect its BS. Why, because it might no be BS, for the player, they just cant take that chance. Years later we try to understand why that thing exists. My rule of thumb is, if the reason is not obvious, its probably BS, so, leave it out and see what happens. Once you have built it you know the answer.
Also the top is angled more on the back half behind the sound hole .. its something you pay attention to when "driving the bus " theres usually about a 1/8" drop where the bridge is going to be ...which of course allows for a thinner , thus , lighter bridge...
Which allows the top to vibrate a slight better..
All the little things add up and when you got enough of them , you have a great sounding guitar instead of a good one....
Better to concentrate on those than all the inlay , tuning keys , binding and other furniture that has no effect on the tonal quality .. IMHO .. Once you conquer that , then spend the time on the others...
Also, the top radius is less so the strings needed ing to be somewhat equidistant off the fret board, and if had more.top radius the strings would touch the fret board above the sound hole.
The taper, if there because of the ergodynamics of holding the guitar, can be discarded. Why? The guitar can be played by putting it on a stand, solves a LOT of ergodynamics problems. Dome top and bottom are easy methods to reinforce the structure of the guitar, and extend the life of the guitar. A guitar can also be built for high performance, like a race car, but have a very short lifespan.
There's a guy on UA-cam building box guitars he Compares 1 too a a d28 its no d28 but it does sound remarkably well for a rectangular box tapered from end to neck and he built one out of a beautiful piece of reclaimed salvage yard redwood
Can you give me the c40 guitar measurements with many thanks
Hi Eric..love the videos....did you make the radius rulers yourself or are they commercially available?
I've made a few myself, and i've bought some. Lmi sells them: www.lmii.com/measuring-tools/2874-back-radius-template-15-foot.html
Eric Schaefer Thanks very much. Might make some myself as I need a 28 foot radius. I usually build Weissenborn so this is my first guitar that has had a radius.
taper is increasing volume of a sound box without hampering the playability, so it is still comfortable to hold
Hello, it a pleasure to watch your videos and listen every advice you give. Right now I am workin on a proyect making a acuoustic guitar very similar to a J200. Do you know the depth of the body?, it is a flat top but I couldn't find expecifications about the back radius and depth of a body. Could you give me some advice about it?.
I don't know what Gibson's specs would be on that. If you can get your hands on one you could, of course, measure it. Barring that, I would look at specs for other Gibson Jumbo models and use that. Radii usually vary very little if at all between models for factory guitars. It is simply easier and more cost effective not to do so. Which is actually a good reason to not use their specs if you are so inclined to break with tradition. It's not going to make or break anything if you use a different back radius and you could actually decide on a radius based on sound acoustic principles rather than production line economics.
Of the two, the depth is more important, so if you want a sound approximating what you would get from a J200 I would make more of an effort to get that right. But even still, in the end, you could make that up as well.
@@EricSchaeferGuitars thank you Eric, you are a genius. And for real thanks for taking your time to answer me. I will apply what you said, crossing fingers I hope to make a good instrument.
Patricio - Lmi sells a J-200 plan.
About $16- US.
I wonder if you had thought of the violin family of instruments, that do not have tapered bodies or indeed radiused bodies but have a heavily carved front and back with no tapering from neck to lower bout. I watched an interesting video on Stradivarius violins that suggested an interesting reason for this. Unfortunately I have forgotten the author of the video, so if you are interested then I am afraid you will have to do some research.
Actually, these instruments are built with a slight taper, though it's only about a millimeter for the violin for example. The reason might be only esthetic, to make the proportions of the body more natural looking. Or could it also be acoustic? I don't know. On the other hand on flat back double basses, the top is tapered heavily, but really I think it is for better ergonomy...
I think a discussion of Bill Kaman's Ovation guitars might be at home here. I'm not sure how much science he did, but those rounded backs seem awfully physics oriented. Anyway, I loved the sound of the original Adams guitars. Very different and very uniform dynamics.
I think the rounded back has more to do with the fact that it was made in a mold. The reason conventional guitar backs are not rounded is because it is much easier to construct with wood plates. I build guitars and mandolins, the latter sometimes are roundbacks. A roundback mandolin is much more time consuming.
Takamine ef341sc pro series dreadnought
7:55 well, in that case the curvature should be a parabola with focus on the sound hole.
If you become rich with this idea, don't forget to send me a guitar. At least.
I think I heard somewhere that the taper helps to avoid standing waves within the soundbox.
I've heard that as well. My understanding of the mechanics of that sort of thing ends there, though. Lol
@@EricSchaeferGuitars yeah, mine too!
:)
Hearing something, somewhere, is useless. Have you never heard of science?
design of acoustic spaces, theaters, recording studios, the same taper theory
how are the sides prepared for the taper prior to bending? Is the taper cut the full length of the unbent side?
I've responded to your comment in this Q&A video. Check it out! ua-cam.com/video/KVPnz4Gb3LU/v-deo.html
I've responded to your comment in this Q&A video. Check it out! ua-cam.com/video/KVPnz4Gb3LU/v-deo.html
The air molecules are not flowing through the guitar. They are vibrating and creating waves as they vibrate the molecules near them. I don’t honestly think the instrument makers were doing wave theory physics in the 17th century. They were experimenting with different designs and this one sounded the best. I am sure they tried totally flat and parallel because it would be easier to make, and those designs probably sounded okay but these much more complex and difficult to make ones sounded better. Also, there’s the structural component. Violins were carved into pretty tight arches because it made them far stronger and able to hold up under string tension. Truss rods and bracing with superior adhesives help in modern guitars but there’s an arch in almost every one.
Throw a ball at a wall ,then at a curved surface .The curve will diflect the ball .Towards the sound hole .
Tempanis bonggos snare drums scotch bass drums have reasons why there thin elongated flat deep and shallow that s why they have different siunds sane goes with both brasswind and woodwind instruments. A real flat and thin guitar naturally will sound like a thin can because sound frequecies cannot reverberate widely and distributed fully.
Torres incorporated it into his early 19th Century designs. As I'm sure you know, Torres designs blew everyone out of the water and we have been copying him ever since. And with good reason, the man knew what he was doing. Torres is the Man
Many things Torres did where already invented earlier. He was not a God!
How can i make a hard real hard finish in a ceder top? For a acustic.
Polyurethane.
But why do you want a real hard finish?
If the neck isn’t shorter (90 in your example), then the guitar body will be thicker/taller in the middle. If both ends are 100, the middle would be around 105-110 due to the radius. Not sure if it would matter or not though...just an observation.
This is one of the best reasons I have heard.
Hi!
The main aspects of your effort in this video, absorbed by me is just this:
1. Dome - for structural integrity.
2. Sound transmission and volume.
3. Sound projection using area constrictions.
It seems you have already made some guitars which have held together and their sounds were close to your expectations, which is an achievement.
Early or ancient builders of instruments or monuments did not attend colleges or have engineering handbooks.
Success claimed by some, highlight the expenses and losses sustained in a development.
If I could afford it I would ask N A S A to build me a guitar.
How did Edison know about electrons without seeing one.
Do what you want to do so long as you have rationalised your attempts and observed the results.
Hope you find what you seek. There is a lot on Utube, just filter out junk if any.
You absorbed folklore and speculation, not repeatable science. 1. Dome is not needed for "structural integrity." 2. How do you conclude "sound transmission and volume.?" Nothing of the sort was actually demonstrated here. 3. Sound projection using area constrictions. What is sound projection, and how do you measure it? The luthier here seems to be a capable builder, but knows only buzzwords and very basic knowledge of some science principles, we must discard such folklore in science.
Edison probably knew about electrons by spying on Tesla.
If braces are tapered away from the center there will be radial bulges in both sides
Did you ever see a perfectly flat guitar? It's just ugly. I find this aspect, besides ergonomic and acoustic issues, even so important. The old instrument makers knew very well what they did. I am working on my #80 now (lutes, guitars and traditional instruments, besides repairs and restaurations) and I never used any 'scientific' calculations, hollow sanding plates and other expensive stuff to reach my goal. Just do it on intuition. Made a lot of mistakes too, but I believe this (hard) way you learn to really feel what happens, to know what you are doing. Great channel and thanks Eric!
That's kind of a stupid thing to say, you can't see the curve on any guitar unless you look really close or more likely you have to put a straight edge on it to see the curve.
If the top is flat... string pull pulls up the soundboard.... if the top has a radius, string pull is parallel to the soundboard.... a radiused soundboard therefore has a less contorted pull on the body as well as a parallel pull on the glue joint under the bridge. In English, it redistributes string pull to make the soundbox stronger. stevetheguitarguy.com
All tops have bulge and a little bit is built in. The rest is string pull. Not flat.
EL34BluGlo yes, most guitars have a radius built into the top... it angles the bridge glue joint back a bit to make it more stable. That's what I'm trying to say here. stevetheguitarguy.com
I think there is a design aspect too. I recently saw a crudely made guitar and looked like the body is deeper at the neckend then the tailend. I measured it and it turned out that it was actually the same. So by tapering the sides we create a simetrical looking guitar.
All your reasons and some of the comments make also sense but not tappering for sure ends up in an really ugly guitar...
Trial and error.