http://cgi.ebay.co.uk/RARE-1930S-DOBRO-RADIO-TONE-RESONATOR-GUITAR_W0QQitemZ260232622985QQihZ016QQcategoryZ2385QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Slide On!

Ian.

www.diamondbottlenecks.com

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Originally posted by Paul Norman:
Looks like a very bad idea. Those cones need to be able to breathe. Wall up a quarter of the surface and you have a lot more missing than a quarter of the sound.

--
Forbidden Fruit Guitars

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Originally posted by bluesriff:
Woah! That swell Dobro was originally sold at HESSEY MUSIC in LIVERPOOL!!! That's where The Beatles purchased all their fab gear in the very, very early days. That store sticker alone must surely add to the collectors value!
Hey Ian!!!

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Well.. it sold for £500 (a thousand dollars) and I would have liked it very much.

I surprised at your result - of course, real experimentation works better than theory most of the time anyway. I'd like to hear the difference in sound (trivial as it may be). One can never have to much data.;-}

I suspect that quite a few guitar builders don't really grasp the significance of the size of the sound hole and its effect on the main body air resonance. One of the things that may seem counter intuitive at first is that making the sound hole smaller lowers the main body resonance.
I'm particularly suspicious of some of the new boutique guitars that have an ordinary sound hole and then some additional ports that can be opened in the sides of the guitar. It looks like this would just kill bass response.
Looking at the Dopyeras designs with either basket weave or fiddle slot cut outs leads me to suspect they may not have been trying to use the body as a Helmholtz resonator. With those complex shapes its a little hard to determine or tune the resonance. The earliest prototype tri-cone even had extra diamond holes punched around the edge, as though to let the sound out. Perhaps they felt the guitar sounded louder with more cut through if they made the grilles as big as possible.

Thanks for the info.

Our New England Luthiers (NEL) group have been doing a lot of experiments with this porting phenomenon. The instigator is our own Alan Carruth. The guy has done a huge amount of work on guitar acoustics. He made what we affectionately call "the corker." It''s a strange looking beast he built in a week that sports multiple side ports - each stopped up with a removable cork. We are currently doing blindfold experiments to determine whether people can really hear any difference when a side port is opened in a specific place on the side. I will keep you in the loop on this.

Most Luthiers never really mention the possibility of adjusting the sound of a guitar by making the soundhole a different size, preferring to cite bracing, thicknessing/tuning of the soundboard, tonewood choice etc.
Perhaps they go for a standard Helmholtz-equation-derived diameter & just stick to that, or the size specified in a plan they're working from, or maybe even what "looks right".
There's plenty of guitars out there with "technically wrong" soundhole sizes, that's for sure!

I read of a prominent acoustic player ( I forget who ) that preferred one of his guitars due to it's overall sound being "less bassy", he reckoned it recorded better, without having to resort to EQ to even it out...

You could perhaps "calm down" a boomy guitar by opening up the soundhole or cutting a port in the side?

I'd be interested to know what this sounds like...

quote:

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Originally posted by Paul Norman:
Tark,

Thanks for the info.

Our New England Luthiers (NEL) group have been doing a lot of experiments with this porting phenomenon. The instigator is our own Alan Carruth. The guy has done a huge amount of work on guitar acoustics. He made what we affectionately call "the corker." It''s a strange looking beast he built in a week that sports multiple side ports - each stopped up with a removable cork. We are currently doing blindfold experiments to determine whether people can really hear any difference when a side port is opened in a specific place on the side. I will keep you in the loop on this.

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lap steel wise it seems side ports would work for projecting to the audience.DONMO guitars offer a side port option that can be open or closed

The experimental guitar with lots of ports seems fun. I'd love to hear what conclusions you guys come up with.

The Helmholtz body resonance is of course just one factor that affects the guitars sound, but I believe quite important because, and it seems like an such an obvious thing to say, but having more or less bass relative to the mids and trebles affects the guitars tonal balance.

Helmholtz resonators are very well understood and the basic equation to calculates the resonant frequence is pretty simple. The classic Helmholtz resonator consists of a sphere pierced by an open ended tube. There is a mechanical analogy which treats the air in the tube as a mass and the air in the sphere as a spring. The mass combined with the resistance of the spring have a resonant frequency at which the mass will bob up and down on the spring. However the classic Helmholtz design and equation assumes that the walls of the resonator are rigid. The top and to some extent the back of a guitar are not rigid and this affects the resonant frequency obtained (the more flexible the more the resonant frequency is lowered - If I remember rightly). This may explain why the Helmholtz equation does not figure more strongly in guitar design. Choosing and calculating body volume and adjusting sound hole size are one thing, but it all gets much more complicated if the flexibility of the guitars back and top have to be considered.

For the guitar I think I'm right in saying that as the soundhole size is increased for a fixed body size the body resonance not only rises but the sharpness (the Q in engineering terms) of the resonance flattens out. So if you wanted to try out the effect of moving the body resonance around but keep Q the same you would have to change the body size relative to the soundhole size. The implication for the 'corker' is that the more ports you open up the less dramatic the change in sound will be.

Interestingly the man often called the father of the modern guitar - Torres, experimented with a metal tube fixed inside the body below the soundhole. This corresponds to the tubular port in a Helmholtz resonator. There has been much speculation about this device - what its purpose was and how it worked. The name given by Torres - Tornavoz - is derived from an architectural feature found in some Spanish churches of the time. A reflecting hemispherical recess behind the pulpit. I'm not sure pulpit is accurate for the Spanish churches of the time, anyway the purpose of the Tornavoz was to reflect and project the voice. I think this has caused people to draw the wrong conclusions about the Tornavoz - it was not intended to project the sound, to me it was very clearly intended to help tune the bass response of Torres guitars.