Increasing Acoustic Guitar Body Depth

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WJ Guitars
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Increasing Acoustic Guitar Body Depth

Post by WJ Guitars » Wed Dec 19, 2018 2:00 pm

Hi All

I am interested in knowing whether increasing the standard guitar body depth of an acoustic guitar be it for example an 'OM' or 'GS' size model in the range of 2mm to 4mm would have benefits. Keeping in mind that adjustments may be necessary to the size of the sound hole to suit the change in proportions.

I noted that 'Kevin Ryan's' guitars body depths are generally around 4mm deeper than most of the standard size proportions.

Maybe the volume would increase or decrease, or also cause unwanted tonal issues. Any thoughts and comments would be appreciated.

Wayne
https://wjguitars.wixsite.com/mysite-1

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kiwigeo
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Re: Increasing Acoustic Guitar Body Depth

Post by kiwigeo » Wed Dec 19, 2018 2:28 pm

The way I see it the important thing is not to detrimentally change the Helmholtz frequency of the instrument.
WJ Guitars wrote:
Wed Dec 19, 2018 2:00 pm
Hi All

I am interested in knowing whether increasing the standard guitar body depth of an acoustic guitar be it for example an 'OM' or 'GS' size model in the range of 2mm to 4mm would have benefits. Keeping in mind that adjustments may be necessary to the size of the sound hole to suit the change in proportions.

I noted that 'Kevin Ryan's' guitars body depths are generally around 4mm deeper than most of the standard size proportions.

Maybe the volume would increase or decrease, or also cause unwanted tonal issues. Any thoughts and comments would be appreciated.

Wayne
https://wjguitars.wixsite.com/mysite-1
Martin

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Re: Increasing Acoustic Guitar Body Depth

Post by WJ Guitars » Wed Dec 19, 2018 4:10 pm

Thanks Martin for the remaindering me about the "Helmholtz Frequency" factor that you mentioned being very important and probably difficult to determine accurately the size adjustment required depending on how much the side depth was increased by (eg 4mm). I'd expect that Trevor would know the pros and cons.

Wayne
https://wjguitars.wixsite.com/mysite-1

blackalex1952
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Re: Increasing Acoustic Guitar Body Depth

Post by blackalex1952 » Wed Dec 19, 2018 5:13 pm

My opinion as an amateur builder is this: The extra air means a little extra mass which usually equates to a lower resonant frequency along with more inertia in the air mass as well. The misnamed 'Helmholz" resonance acts as a spring, coupling the movement of the soundboard with the efficient movement of the back, in order for the guitar to move air as an efficient "air pump"at lower frequencies. Resonances in a complex system repel each other as well, as the back and soundboard resonance coupling in an assembled guitar. Changing one frequency will in one way or another affect the other resonances. Efficiency is the key to this thinking. There are ideal resonances for guitars, they are in the Gore Gilet books, on the forum here in the discussions and on the internet as posted by various luthiers. Referred to as "target Frequencies". An interesting question I have been contemplating posting here I will now ask the more experienced who will be watching this thread. I have been shown this by a couple of experienced luthiers who build this way, it is simple and practical when tuning the resonances in a guitar once the body has been assembled.....
I have a current build going at present and noticed that my target frequencies are slightly off the targets for a guitar of this nature, and the guitar is coming out too strongly at around a low string G. Confirmed by using Visual Analyser and Chladni tests. I want to try lowering the air resonance but don't want to do the obvious and make the soundhole smaller or use a tournavoz. But I was shown that a balanced instrument has the tone frequencies in between notes and not on them so that the "admittance" of the string energy is not so fast at the frequency in question as to form a classic "wolf note".
The luthier who showed me got me to put my hand just over the soundhole so as not to lower the air resonance but just enough to "feel" the air kick out of the guitar. On the build in question this kick isn't quite happening, but when I lower the air resonance slightly by blocking the soundhole, thus making it smaller, the kick begins to occur..it just so happens that the frequency of the soundboard and the back and air (you called it Helmholz) fall exactly on the recommended target frequencies for a guitar of this size when the air resonance is lowered. ie, the guitar seems to be doing what is demonstrated in the following video from Irvin Somoygi: So in simple terms the question is: what do the more experienced builders think of this concept and approach....?https://www.youtube.com/watch?v=fAEQKmyB5Nk
I'm referring to the weights and the elastic band demo within the vid. -a nd also to the "echo" he demonstrates...I think that is the back activating just after the system responds...there are some clues in this vid.
Really hoping for opinions on the video!!!
Re the question:
Maybe the volume would increase or decrease, or also cause unwanted tonal issues.
I make Selmer style jazz guitars as well, and the projection of these guitars is excellent-they are one of the loudest guitars around because they throw the sound, - the long dipole caused by the design helps in this regard, but tricky to get a good strong bass out of them. Particularly from the players listening spot, ie close up...but these guitars will cut the mustard with loud instruments un amplified and good ones can be heard above the sound of a noisy bar, at the back of the room! My point is, horses for courses, guitars are a perceptive thing really and different guitars, different approaches to building need to be assessed with the following caution-"the proof of the pudding is in the eating"!
ps: a Helmholz resonator has solid back, sides and top, so only the air is excited within it, and the diameter of the soundhole and the air volume within are the only adjustable parameters.
There is a guitar that Gibson made for a bloke called Nick Lucas which was built with a deeper than normal body in a few incarnations. Bob Dylan played a thirteen fret neck join version of this guitar. They have become highly collectable models for a few good reasons which I won't go into here...except to speculate on the added body depth relative to the sound of the guitar..I would expect that a deeper body guitar would sound, if the plates and air are tuned correctly, more bassy to the player at least but there may be a trade off with the projection of the instrument at low frequencies when compared to a "normal"? body depth, along with a decrease in the attack as the extra mass lowers the efficiency of the system. But, these days, with mics pickups etc this probably wouldn't pose a huge problem for most players. But don't take my word for that please, it's only speculation. The current build, BTW, is a normal depth L-OO, and the next build will likely be a Nick Lucas Special build with the deeper body. I'd like to figure that one out for myself. Also I have varied the bracing slightly to forward shift the X and made the 13th fret body join so that the bridge falls where most luthiers feel is a better position on the soundboard.
Cheers! Ross
"Everything I say on the topic is based solely upon inexperience and assumption!"

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Trevor Gore
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Re: Increasing Acoustic Guitar Body Depth

Post by Trevor Gore » Wed Dec 19, 2018 6:50 pm

WJ Guitars wrote:
Wed Dec 19, 2018 2:00 pm
Hi All

I am interested in knowing whether increasing the standard guitar body depth of an acoustic guitar be it for example an 'OM' or 'GS' size model in the range of 2mm to 4mm would have benefits. Keeping in mind that adjustments may be necessary to the size of the sound hole to suit the change in proportions.

I noted that 'Kevin Ryan's' guitars body depths are generally around 4mm deeper than most of the standard size proportions.

Maybe the volume would increase or decrease, or also cause unwanted tonal issues. Any thoughts and comments would be appreciated.

Wayne
https://wjguitars.wixsite.com/mysite-1
It depends, of course, on what you see as benefits.

Generally, increasing the body depth has much less impact than you might think. The increase in volume of the box and corresponding reduction in the frequency of the main air resonance is mostly negated by the drop in coupling between the top and back which increases the frequency of the main air resonance. So the effects largely cancel. The greater the air volume of the box, the lower the Q of the resonance; so the effect of the resonance is less in amplitude but spread over a greater frequency range. This may give a "fuller" sounding timbre. The effect of 2-4mm change in body depth would very likely be obscured by variations (controlled or otherwise) in other parameters.

It's generally more fruitful to focus on getting the resonant frequency right, so as to avoid wolf tones.

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WJ Guitars
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Re: Increasing Acoustic Guitar Body Depth

Post by WJ Guitars » Wed Dec 19, 2018 8:15 pm

Thanks all for the helpful comments regarding the aspects of increasing guitar side body depth.

I agree as Trevor has advised:

"
It's generally more fruitful to focus on getting the resonant frequency right, so as to avoid wolf tones.
"

I am not an expect on these matters and appreciate the technical explanation provided. I was considering undertaking an experimental guitar build. However, I rather lean on the advise give and continue on guitar builds that focus on "getting the resonant frequency right".

Wayne
https://wjguitars.wixsite.com/mysite-1

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