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I have some random thoughts and questions regarding live back design and reflective back design. Perhaps this will generate some discussion that I (and others) will benefit from.

- I assume that in order to accommodate a player, the luthier needs to know what style the player is going to use the guitar for. That is, to be able to handle bluegrass style of play, such as strumming and need to cut through competing instruments and therefore require a reflective back. …or a finger stylist or singer/song writer that would perhaps have a lighter touch and benefit from the live back design?

- Perhaps the reason players gravitate to a particular shape or style of guitar reflects the style they play. For instance, bluegrass players prefer dreadnaughts, rag players like L-00s, many country players like jumbos, and fingerstylists like OMs and OOOs. It appears that generally, the various styles of guitars are designed with either a live or reflective back depending on their style of play.

- So, this would imply that maybe there are only a few styles (or shapes) of guitar that the luthier could choose to design with a live back or reflective back, i.e, OM, OOO, grand auditorium, grand concert, or small jumbo.

- I play finger style pretty aggressively sometimes. So, would I benefit from a reflective back?

- Does a live back limit the head room and projection? ...or does a reflective back limit overtones?

- Is it possible to design a back somewhere in the middle between live and reflective to attempt to achieve an overall versatile instrument or will this effort simply render the design neither live or reflective?

- Which one (live back or reflective back) do you prefer?

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Steve, I like your thinking. You ask some really good questions. I too look forward to the responses of the folk here.

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Joe Beaver
Maker of Sawdust

I prefer a responsive back because I'm also a fingerstyle player. I also like more resonance and overtones (live back) vs. a more snappy sound (reflective back) that a lead guitar player would be looking for. I don't see why you couldn't compromise with something in the middle for a versatile instrument and maybe offer it as a specific model fit for both uses.

One important thing to consider when making a live back is to make sure that it and the top are not harmonically tuned to each other in octave, third or fifths as that will create a woof note when those particular notes/chords are played. To further avoid that I try to tune my top and back to be off pitch to whatever note they are closest to by good amount. For example if my top tapped to an A tone and the back tapped to a F tone I've got the makings of an F chord built into the box and that would be much louder when played than any other chords. I would remove a bit more material to get the tones off those pitches. This is something that I try to inch up to, not swing for the fences on. That way I can find a balance where the proper amount of strength is there and the top and back still ring like bells.

I'm interested to see what others have to say about shape as I've approached it more as an aesthetic thing thus far. I do under brace my OM's compared to most plans. I don't find two tone bars necessary and depending on the top, one pair of finger braces has worked fine as well. I've only built one dread (which I decided I'd likely not build again as I don't care for the look) and the rest of my builds have been OM's since then. Next round I'm going to add a Jumbo though and the differences should be interesting.

sdsollod asked:
"- Is it possible to design a back somewhere in the middle between live and reflective to attempt to achieve an overall versatile instrument or will this effort simply render the design neither live or reflective?"

Im starting with this one because it's really the central question. As far as I've been able to figure out from testing a lot of guitars, the only truely pure 'reflective' back is on an Ovation, although some of the Renaissence 'chittarras battante' might qualify. Any normal guitar you're likely to run into has a back that both 'reacts' and 'reflects'. It's really a matter of degree.

If you think about it, when the back vibrates it has to be getting the energy from someplace, and the only place it can come from is the strings. Since the back is heavier than the top, facing in the wrong direction, and is usually up against somebody's pudgy avoirdopois, it's not going to be nearly as effective a sound producer as the top. This suggests that making the back as 'dead' and 'relfective' as possible might be the way to go. I think that's probably pretty close to right, but not totally.

There is one range where the back can actually help the output power of the guitar, and that's in the 'bass reflex' range down at the low end. The top and back get into a sort of complicated dance as you go up in pitch from the lowest notes to about the open G pitch, and a back that can move even a little in that range will pump some air through the sound hole. It's complicated in part because the back is responding to two forces from the top that are out of phase with each orther: the air pressure in the box rises as the top moves 'in' and pushes the back away, but, at the same time the edge of the top pulls the side up, and pulls the back along with it. These things don't exactly cancel out because the lowest resonance of each plate is at a different pitch. If the top were the only thing working the bellows the air moving through the hole would be pretty much cancelled out by air moving off the top in the opposite direction ,and thus not much sound would be produced in that range as you move away from the guitar. With the dance the back is doing you can end up with significantly more power output, and an 'interesting' spectrum too.

Above roughly the open G string pitch back movement does pretty much 'steal' power, but mostly at fairly specific pitches. In other words, the back is mostly reflective anyway, except when it's not. The resulting 'dips' in the output of the guitar account for a lot of the 'tone color' that allows the player to 'shape' the sound. What probably counts here is how deep and narrow the dips in the spectrum caused by back motion are: if they're shallow and/or narrow they won't coost much power, but can still contribute to 'color'. A heavy and/or stiff back will tend to be harder to move, and keep the dips shallow. A back with low losses makes narrower dips. Note that we're describing woods like Brazilian rosewood, that is dense, stiff, and rings a long time when you tap it because it has such low losses.

"- Does a live back limit the head room and projection? ...or does a reflective back limit overtones?"

Let's see: given the above we'd expect a mahogany back to be 'live' and an IRW one to be 'reflective'. Does a D-18 have more or less 'projection' than a D-28? Overtones?

In my experience as a maker, 'headroom' comes from the top more than the back. It's mostly a matter of top density and stiffness (which go together), the top bracing profiles, and bridge mass, in order of importance, IMO.

"- Perhaps the reason players gravitate to a particular shape or style of guitar reflects the style they play. For instance, bluegrass players prefer dreadnaughts, rag players like L-00s, many country players like jumbos, and fingerstylists like OMs and OOOs. It appears that generally, the various styles of guitars are designed with either a live or reflective back depending on their style of play."

Again, it seems to me that it's not so much the 'live' or 'reflective' back that comes in here, as a combination of other design elements. Dreads have a particular timbre, in part because of the shallow waist, that suits the rythm style called for in Blugrass playing. Small bodies, like the L-00, are more 'treble balanced' than bigger ones, and can also actually produce more power, so they have a tendency to 'cut' and 'project' more, as well as the kind of 'clarity' that helps bring out the ragtime structure. OMs, and 000s have a 'warmer' sound than the smaller boxes, and the waisted shape gives them more mid-range clarity than the Dreads, which suits the usages of fingerstyle playing. And so on.

So, yes, a maker can and does mess with the proportions of 'active' and 'reflective' in the back as part of the way they shape the tone. There's no 'purity' though, and the proportions that will work depend on each maker's methods of work, design philosophy, and choice of wood for a particular guitar, as well as the desired outcome, of course.

Alan - I always learn something from you, including "avoirdupois" this time... I've read over your comments several times. As expected, its quite complicated and there are no easy answers. It appears from your comments, that the back wood species plays a large roll... "we'd expect a mahogany back to be 'live' and an IRW one to be 'reflective'". I would assume that bracing also contributes to a large degree. As you said, " a sort of complicated dance". Which I assume has to do with size and dimension of the bracing. It stands to reason that you could make a mahogany back more reflective with stuffer bracing and a IRW back more live with less stuff bracing...

Your comments on the various body sizes certainly makes sense. I guess there are also other things that come into play, such as, short vs. long scale, 14 vs. 12 fret neck, etc. I'm wondering how a soundport effects the air movement. It seems that the guitars I've put soundports in have more low end... Perhaps that's a topic for another thread...

So, I'm wondering if there are some practical applications for back bracing based on your assumptions... I have noticed that sometimes (probably on smaller guitars) thin braces are used in the upper AND lower bout and other times heavier braces are used in the lower bout. For instance, when plans (for the J-185 I'm getting ready to build) call for lower bout braces of 3/4" x 3/4", when do you say, "That's too heavy"? Is the reason these large braces are prescribed is because this type of guitar is typically used for strumming cowboy chords and call for a reflective back? I like this shape, but my intension it to scale down the lower bout width by 0.5", install an arm bevel and a Florentine cutaway, with the intension of making a versatile guitar but perhaps leaning towards fingerstyle. For this reason, should I also scale down the braces? ...and by how much?

I am in the intuitive camp regarding plate tuning. Typically, when carving back braces I tap and listen for the plate to have a musical tone. I don't go so far as to measure the note.

I think you summed it up when you said, "There's no 'purity' though, and the proportions that will work depend on each maker's methods of work, design philosophy, and choice of wood for a particular guitar, as well as the desired outcome, of course."

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Well, the characteristics of the piece of wood in the back make a difference. Keep in mind that the properties of any species of wood can vary quite a lot. I've worked with mahogany that was as hard and dense as some Indian rosewood, and vice versa. And you can always leave a soft and light piece of wood a bit thicker, and brace it up more. Most importantly, though, keep in mind that most of the sound does come from the top. It's been shown that in 'blind' tests very few people can tell the difference between guitars with IRW backs and other woods such as walnut. Look up the 'Leonardo Project'.

I've done 'way more work on 'ports' than I planned on. Basically, any time you add another hole in the guitar the low Helmholtz-type 'main air' resonance becomes stronger and rises in pitch a bit, and the two effects go together. The 'main air' resonance is the lowest pitched on that can produce usable sound, so the effect on the tone will depend on the balance of the two effects: does it sound 'bassier' because the mode got stronger, or more 'treble' because the pitch go higher? The main thing the port does, though, is that it can act as a 'monitor'. Any port you can see into as you play sends some high frequency sound toward you that you would only hear otherwise by reflections from the room. In a large, dead, or noisy room (think 'restaurant gig') this can be a help.

That all sounds simple. You would not believe how much work it was to get that, though: lots of careful measurements. 'Intuitive' only takes you so far.

I know when Trevor Gore talks about the subject he always stresses the notion of "live" equating to "tone" and "dead" relating to "volume" as being generalisiatons. This one word, which he always stresses, most of us ignore. It's a generalisation. Often we don't agree about what a good sounding guitar is. Oddly enough, in my experience not everyone even agrees what a loud guitar is.

When making an instrument with a live back, keep in mind (with Classical guitars especially, apparently) it's quite possible that if you make your live back "wrong" the result can actually take you away from your intended modal target, rather than towards. Again, Trevor can explain this better than I.

What I do know is, every instrument I've made in the last 30 years has probably had a "dead" back. Regardless of who it was for and how they played. Now, the sound of my work isn't for everyone, but then, whose work is? But no one has ever complained about the quality or quantity of the sound they produced.



Here is a recent cedar top guitar, with a "dead" back. Not lacking in "tone." And it is rather "loud."

To answer your (very sensible) questions - my feeling is you may be making things more complex than they need be. That said, your questions do remind me of conversations some makers and clients have prior to a guitar being built. Such talk has its uses, but how much these ideas are based in reality - how much these ideas have any real positive bearing on the end result when carried out by those of us who don't have Trevor or Alan's understanding is another matter.

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nigel

http://www.theluthierblog.com

Nice sounding guitar, Nigel.

Yeah, great sounding guitar Nigel. And classy video!

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Pat