Official Luthiers Forum!

Old joke...Scott van Linge39038.8248958333

Scott asked:
"Al, you there, dude? Get back when you can... "

Somehow everybody seems to have missed my post, 'way back on the 16th. For a quick recap: as far as I can tell the sides don't transmit much energy to the back from the top: most of it goes through the air in the box. The back seems to contribute most to the low range sound, and adds to the 'color' above about the pitch of the 'main top' resonant mode, around the open G string.

There is a way the sides can effect the tone on some guitars, though, that I just remmebered today. On a wide body with an X braced top it's possible for the top 'cross dipole' resonant mode to be close enough in pitch to the 'crosswise sloshing' air mode in the lower bout for them to couple. When this happens the top will respond at two frequencies, in phase and out of phase with the air mode. This air mode normally does not radiate any power outside of the guitar, but the coupling with the top can extract a bit of energy. The more or less flat area of the sides just below the waist can also move along with the motion of the air. The sides are too small (usually) to radiate a lot of power at this frequency (around 400-500 Hz) but their flexibility can effect the pitch of the air mode.

When I 'tracked' Dana Bourgeois through his tap tuning process once at a GAL convention, I noted that this was one thing he was doing on a Dread. By thinning the edges of the top outside of the bridge wings he dropped the top pitch so that it coupled more effectively with the air mode. An 'extra' cross dipole top mode showed up, and you could feel the 'kick' when tapping on the bridge ends. Although he doesn't do it (or didn't, at the time), I'll note that thinning the sides could give the same result in the case where the top cross dipole was too low in pitch to couple well with the air mode. Much of the battle, of course, lies in knowing just where you are in the first place.   

Well, how about one more opinion?
My take on the sides is that their principal duty is to define the air cavity. I know respected builders that are laminating their sides in an effort to stiffen them up considerably. (Which has the added benefit of reducing their tendancy to crack to near zero.) Their thinking is that they don't want the sides to vibrate excessively, like one doesn't want the peghead to vibrate excessively. They are thinking like the engineers who build loud speakers--rigid frame supporting a flexing diaphragm. And I tend to lean this way as well...though I don't laminate my sides.
Well, usually. I did one early on, and it turned out like an early-on guitar....

Steve

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From Nacogdoches...the oldest town in Texas.

http://www.stephenkinnaird.com

Interesting question only because I am way too analytical for my own good.

If I had to guess (and that’s what I am doing) I think I would more likely subscribe to the top being the driver and the back having the potential to be a passive radiator if it is designed optimally to work with the top resonance, internal volume and possibly tuned to work with the sound hole size.

Taking the guitar top as a speaker cone or driver concept and extrapolating what we know about how the various designs of speaker housing can significantly alter which frequencies are enhanced or cut and also affect the efficiency and volume of the speaker.

Each different raw speaker cone could have a box designed to make very different speakers sound or perform very similar by changing the volume of air, changining size of ports or the lack of one and perhaps changing some internal baffling or angles of interior reflective surfaces.

In subwoofer or bass guitar cabinets a variety of variables are used to optimize the bass frequencies that come with associated strenghts and draw backs.

1. Sealed box designs with a good stiff speaker are known to provide tighter and punchier bass frequencies, have quicker transient responses and are also thought to be more accurate. These are also extremely inefficient in producing volume compared to ported designs so you have to supply much more powerful amps and spend more money.

Pure acoustic guitars don't have the otpion of upgrading with a more powerful added component other than hitting the strings a little harder to a diminishing point.

2. Ported enclosures can provide twice the volume of a sealed design especially at lower frequencies but they can suffer from a less punchy and less focused (less detailed) response, are less accurate with a slower transient response. Poorly designed ports can add a "whooshing" kind of artifact as air rushes in and out a narrow tube or opening.

3. Passive radiator designs - the ones I have seen have primarily been in sealed subs or bass amp cabinets and claim to offer the benefits of both. The radiator is said to increase volume by increasing efficiency but don't drop the transient response as much as ported designs.

All the above methods in the hands of a skilled and knowlegable audio designer along with good material can deliver stunning euphoric audiophile repsonses but I'm sure most designers favor one of the three in most circumstances based on their experience, ears and goals of each design. The passive radiators in these designs are usually larger in diameter than the powered speaker and can take up the majority of surface area of one wall.

In this analogy I see acoustic guitars obviously needing a "ported" design. A passive radiator in the back and possibly to a lesser extent the sides as well could easily offer benefits when it works with the overall design and could also detract from the top when it does not work well with the other variables.

I'm not sure how accurate this analogy is when looking at side designs but the general consensus in speaker design is to use thicker non flexible walls so sound wave energy is focussed as much as possible in one axis and not lost to vibrating a cabinet wall that does not project a desirable frequency response or direction of transmission.

From what I have learned about speaker design and sound wave transfer from my utrasound physics I would be incline to believe the sides function would be to be the frame of the speaker and that very little if any energy once transmitted either directly through to the back from the top or indirectly after traveling through the air and then into the sides would constitute less than 1-5% of the audible sound energy ultimately produced.

Then again different combinations and shapes of wood in solid body electric guitars do impact tone in part by repsonding and interacting with the string vibrations even when the wood is not realy moving any air.

So I may be completely off base but I think there is every reason to believe that some of us have found good results with backs that work like a passive radiator and others noticed either no difference or discovered better results with a stiff non resonating back if the attributes of the back did not work well with the existing variables present in the top, sound port, insert your own variable here.....