The Steel Guitar Forum

Alan,
You've given me an idea! Why not incorporate a 2nd back that is isolated from the real back, allowing the inner back to resonate without interference?

You mean... Like this?

Darrell, Is that outer back isolated from the larger inner back? If so, that's what I was thinking

Mandolin players can get this little wire rack thing that holds the mando of the body and gives an enormous sound boost. When I play my Dobro I always stand up to get the thing of my lap and get a little more ring, I angle it about 45 degrees down to project the sound into the mic. uh,,Not in my pic tho
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Loni Specter wrote:Darrell, Is that outer back isolated from the larger inner back? If so, that's what I was thinking

Yep. It's floating off the back, attached only at three points. I believe this one is owned by Ben Harper. A Norwegian Tone Guard! Or I suppose maybe a Tonegaard?

Read more about it at the Knutsen Archives

The OP wondered about the differences shape make in sound production... Well, Knutsen tried just about every shape imaginable! I've never heard any of the Harp Steels with the very large upper bass arm harp chambers, so I don't know if they're much louder/bassier than other steels. But I have heard Knutsen Harp Guitars, and the difference in air volume makes for a definite difference in six string tone, IMHO.

Loni, that's already been done by a Spanish classical guitar maker many years ago.

I'm not a builder, but just from the physics involved, I don't see why resonance in the back and sides is a good thing. In speaker cabinet design, resonance in the cabinet is a bad thing. It causes a boxy sound, and emphasizes certain frequencies, thereby interfering with flat speaker response. The goal is to have the box itself be perfectly rigid, so that all the sound is produced by the speaker, or is reflected out the reflex port.

By analogy, the guitar top, sound board, or resonator is like the speaker, and the sides and back are like the cabinet. The goal would seem to be that all the sound be produced by the guitar top or resonator, and be simply reflected by the back and sides, with as little abosorption (resonance) as possible. All the sound is then projected out the top or front of the instrument.

I realize many players like to feel the vibrations through the back of the guitar against their body, and judge a guitar on that basis. I always thought that was a bit of misguided self-indulgence that detracts from the volume of sound projected out the front of the instrument.

In the above examples, we see people making instruments with resonant backs, then putting feet or a second layer on to keep that resonance from being deadened by the body or surface the instrument is rested on. Wouldn't it make more sense to just make a back that is rigid, non-resonant, and that absorbs as little sound energy as possible?

IF the back and sides are non-rigid and do resonate, their material and the way they resonate (their size and shape) will strongly affect the sound. I suppose, once you have that situation, you can then design them to emphasize certain frequencies, and thereby manipulate the tone. But this would seem to be introducing a needless complication. Why not have all the tone manipulation be designed into the top or resonator, and have the back and sides rigid, so as to affect the tone as little as possible. I thought that was why tops are made of thin softwood (and resonators are made of thin light metal), whereas, backs and sides are made of thicker hardwoods.

David -

I agree that the tops and backs can be made to resonate and then incorporated into the overall sound, or conversely made to simply reflect the sound back out for further projection. It seems most folks feel that the third option, absorbing the sound, is not cool, at least in an acoustic instrument. Yet absorption happens and is just a part of the resulatant sound coloration.

Even in electric instruments, the vibration transmission and absorption through the neck, body, etc, makes for discerneable differences. The difference between maple necks and mahogany necks is noticeable.

Its kind of like the amplifier cabinet makers, who are of varied schools of thought. There are those who make the finger-jointed wood boxes, and say that the amp acts as a whole in establishing tone. And of course, there are the MDF box guys who coat everything with asphalt and padded vinyl to completely isolate the box from the sound. I've heard great amps produced both ways, and they sound great, just different.

The cool thing is that the instrument or speaker box sound energy can only go three directions with respect to the wood (or metal, or formica) - reflected, transmitted, or absorbed. Designing around this principle can yield a whole spectrum of cool tones from the same basic set of stretched steel strings. Its why we are all constantly sniffing around these thousands of instrument permutations!

Its kind of like beer, in a way. All from the same humble barley, water, and hops, but the way the ingredients get treated yield a wide variety of results.

Kurt

David Doggett wrote:I'm not a builder, but just from the physics involved, I don't see why resonance in the back and sides is a good thing. In speaker cabinet design, resonance in the cabinet is a bad thing. It causes a boxy sound, and emphasizes certain frequencies, thereby interfering with flat speaker response. The goal is to have the box itself be perfectly rigid, so that all the sound is produced by the speaker, or is reflected out the reflex port.

This is only true for PA or hifi/stereo-speakers, though - with many guitar amp/cabinet designs, the resonance of the cabinet (while maybe not intended by their builders) is a big part of the signature sound - eg. a Fender tweed-style amp just doesn't sound right" without a resonating open-back finger-jointed pine cabinet with a floating speaker baffle, while for a vintage Marshall sound you'll need a less resonant, much more rigid birch plywood cab.

Same thing with acoustic guitars: not only the top makes a difference for their tone - wood choice for the sides and back (mahogany vs. rosewood, mostly) is a big part of the "signature sound" of certain models...

Loni Specter wrote:Alan,
You've given me an idea! Why not incorporate a 2nd back that is isolated from the real back, allowing the inner back to resonate without interference?

Loni: Not wishing to dampen your enthusiasm, but this idea comes to every luthier eventually. People have been building second backs in guitars as long as guitars have been around and they've never achieved anything. The theory is great, that you can prevent the muffling of the back, but everyone who has tried it, and there have been hundreds, have always come up against the same problem.... you effectively then have two sound chambers, one between the table and the first back, and the other between the first and second back. The air pocket between the first and second backs creates a deadening effect. As the back tries to vibrate forwards it is pulled back by air pressure, and as it tries to vibrate backwards it is pushed back by compressed air pressure. The soundhole in a guitar is there as much to equalise the air pressure on both sides of the table as it is to let the sound out. Without a soundhole, the captured air inside the guitar acts as a cushion for the sound. So you need a second soundhole between the first and second sound chambers. This effectively negates the effect of isolating the back. With the back open to the outside air it is expected to vibrate, yet cannot because it is resting against the player, the same way as if there were only one back. I gave up experiments on this about 1971. As an organiser of the Northern California Association of Luthiers I know most of the luthiers on the West Coast. They've almost all tried false backs at some stage, and have all given up because it doesn't work.

David Doggett wrote: The goal would seem to be that all the sound be produced by the guitar top or resonator, and be simply reflected by the back and sides, with as little abosorption (resonance) as possible. All the sound is then projected out the top or front of the instrument.

I realize many players like to feel the vibrations through the back of the guitar against their body, and judge a guitar on that basis. I always thought that was a bit of misguided self-indulgence that detracts from the volume of sound projected out the front of the instrument.

Absolutely. David has expressed this very well. If you want to get the best sound out of an acoustic instrument you hold it so that you make as little contact with the back of the instrument as possible. Classical guitarists know this all too well, and in fact will use this to good effect by holding the guitar more rigidly against themselves when they want more staccato. Instruments played horizontally are best played by supporting them in a frame with as little contact as possible. I put small rubber feet around the outside of dulcimers/hummels/kneeharps that I build so that they can be played on a table with no contact on the back, the "feet" being attached to the sides. This at least doubles the volume of those instruments. Try resting a Dobro on a table, with two or three small spacers around the edge. The improvement in tone and volume is very noticeable.

That having been said, be aware that there are two different body shapes in the lute family. Instruments such as a guitar and cittern have a flat back, the purpose of which is to vibrate along with the table and create a bellows effect of air coming out of the soundhole, while allowing the top and back to freely resonate. Instruments such as the lute, theorbo, bouzuki and mandoline have round backs. The purpose of the round back is totally different: it is to reflect the sound back on the table of the instrument, which is built light and expected to vibrate more than the table of a guitar.

Alan, were all your experiments with "second backs" enclosed within the guitar body? It would seem like the Knutsen pictured above (along with the "tone guards" being used by mandolinists like David Grisman nowadays) are complete add-ons to the regular instrument and not inside/part of it. I would think that thing on the Knutsen would act just like your 3-feet-on-a-table, no? I suppose I could rig up a wire frame similar to the Tone Guard and try it on my Weissenclone to see if there's any difference.

I believe that projection toward the player is critical to the performance - projection to the aft of the instrument is key to knowing what the listeners are hearing from the front. And even for less of a technical reason than that, if the overall resonance thrumming inside the chest of the artist makes her play more passionately, then the music becomes all the greater.

The extreme directional nature of resophonic instruments is one of the things players must learn to account for and control- just because you have the power, does not mean you should (something like that once said by Bob B.)

Lap style players probably don't transgress as severely as Spanish-style players, as the sounding areas project up into their face a bit more, rather than out and away.

Darrell Urbien wrote:Alan, were all your experiments with "second backs" enclosed within the guitar body? It would seem like the Knutsen pictured above (along with the "tone guards" being used by mandolinists like David Grisman nowadays) are complete add-ons to the regular instrument and not inside/part of it.....

Darrell, I'm not just reporting on my experiments; I gave up on the idea many years ago; I'm reporting on the many experiments that I have seen produced by other luthiers over the years, none of which has been successful.
I can't comment on the guitar pictured earlier, to which you refer, until I see a schematic of the design, or some photographs from other angles.
Every time you add additional wood you deaden the sound because the only energy to set the body into vibration comes from the strings, and there's only so much the strings can set in motion.

Kurt Schoen wrote:I believe that projection toward the player is critical to the performance - projection to the aft of the instrument is key to knowing what the listeners are hearing from the front...

Now this is a completely different problem, and one that a lot of work needs to be done on. A resonator guitar projects its sound upwards, and you can't turn it towards the audience because it puts you into an impossible playing position.
The Melobar was an attempt to solve this problem, and others, but, of course, it was an electric instrument, and not entirely successful.
You would have to entirely redesign the cone so that it pointed towards the audience, yet the strings would have to be at right angles to this....

...what a problem !

Alan F. Brookes wrote:I can't comment on the guitar pictured earlier, to which you refer, until I see a schematic of the design, or some photographs from other angles.

Well, I have a link to more pix in my post...

Every time you add additional wood you deaden the sound because the only energy to set the body into vibration comes from the strings, and there's only so much the strings can set in motion.

The Knutsen seems to just be a plank of wood pinned to the back in three points. When played on your lap, it would seem to function like a portable "table with three feet" that allowed the back to resonate more freely.

There haven't been any other examples of this construction to surface, which might imply it didn't really make a difference. But with Knutsen they were ALL one-offs, so that doesn't necessarily mean anything!

I see this as being very close to the Tone Guard idea, except it's horizontal rather than vertical. Now I understand that lots of people see the Tone Guards as being snake oil too. But as I said, easy enough to try and see for myself...

Darrell Urbien wrote:...I have a link to more pix in my post...

I can't see a link. Which post is it in ?

click--->Knutsen Archives listing

Loni Specter wrote:Alan,
You've given me an idea! Why not incorporate a 2nd back that is isolated from the real back, allowing the inner back to resonate without interference?

Like this Grimshaw from my collection :-

Weren't the original Gibson flattop Hawaiians double backed as well? The ones with the little f holes around the edge? I guess the Selmacs with internal soundboxes would count as well. So it would seem a lot of people tried this concept... Once.

Darrell Urbien wrote:click--->Knutsen Archives listing

Since the second back acts solely as a stand, to keep the instrument from contact at the back, and plays no part in the tone, it would probably work.

This is curious, because I posted this response yesterday and it seems to have disappeared.