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A sound port question
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=54541		 Page 1 of 1
Author:  Dave Higham [ Mon Nov 15, 2021 9:32 am ]
Post subject:  A sound port question
Yes, sound ports again!
I'm drawing up plans for an OM sized guitar with a sound hole in the bass side upper bout. (Don't ask!)
For various reasons to do with the rest of the construction, whatever shape I make the sound hole, it can't be as big as the conventional round hole would be.
So I thought that adding a sound port in the side, of a size which when added to the size of my unconventional sound hole is the equivalent of the conventional one, might be the answer.
This sounds logical to me - does it to anyone else?
I don't seem to recall people saying that when they add a sound port, they reduce the diameter of the sound hole.
Complicated answers involving Helmholtz and main air resonances or whatever, may tend to go over my head. Just saying ;)
Author:  SteveSmith [ Mon Nov 15, 2021 9:54 am ]
Post subject:  Re: A sound port question
I don’t change the size of my sound hole. My ports are in the upper bout and are relatively small. No idea what effect a port in the lower bout would have and I would be concerned about making it very large in any case.
Author:  jfmckenna [ Mon Nov 15, 2021 10:30 am ]
Post subject:  Re: A sound port question
Sounds logical to me. In fact I am soon to build a classical guitar and was going to ask the same question of the luthiers over on Delacamp.
Author:  Dave Higham [ Mon Nov 15, 2021 10:33 am ]
Post subject:  Re: A sound port question
Thanks Steve.
It might look something like this.
The upper left sound hole and the ellptical sound port are the equivalent area of the round hole.
Image
Author:  SteveSmith [ Mon Nov 15, 2021 10:40 am ]
Post subject:  A sound port question
That's different enough from mine that I don't think my experience would be useful. I do know that this subject has come up before and Allen Caruth has done some research into this. Hopefully he will see this and jump in or perhaps someone will have some links.

My ports are not that large so I ignore the size and have not had any issues.
Image
Author:  Clay S. [ Mon Nov 15, 2021 11:18 am ]
Post subject:  Re: A sound port question
I've read that moving the soundhole into the upper bout and making it smaller will emphasize the bass response more (or perhaps de-emphasize the treble idunno ) and from some experiments I've done that seems to be the case. The soundport may ameliorate this effect some and might be a good idea.
If an OM winds up with a little more "Bass" I don't think that would be a bad thing, but again, the soundport might bring it back to the more "traditional" sound.
I wouldn't worry about being too precise with the soundhole/soundport dimensions, just use what looks right for the design - it will still sound like a guitar.
Author:  SteveSmith [ Mon Nov 15, 2021 2:25 pm ]
Post subject:  Re: A sound port question
Ok, I misread the original question. My first question would be 'should a sound hole in the upper bout be the same size as a sound hole in the standard central location'? If that answer is yes then I would probably try to make up the missing area with an appropriately-sized sound port. All a guess on my part, I've only done experiments with sound hole size in the standard position.
Author:  DennisK [ Mon Nov 15, 2021 2:59 pm ]
Post subject:  Re: A sound port question
Due to the proximity, I'd say the hole+port will function more like a single hole than usual. Sort of like a large hole with a bar through the middle of it. Perhaps even better if you make the port a half circle, with the flat side as close to the soundboard as you can get without structural issues. John How posted a photo of a guitar-in-progress on the AGF back in 2010 that has fascinated me ever since, which was styled to look like a violin, with the spaces left by the C-bouts in the soundboard becoming soundholes for the guitar-shaped body, and the bass side one wrapping down into the side as well. Not sure if he ever finished it, but someday I would like to build one like it.
Author:  Alan Carruth [ Mon Nov 15, 2021 3:06 pm ]
Post subject:  Re: A sound port question
The sound hole in the upper bout in the corner will indeed tend to bring up the bass. The 'main air' resonance tends to drop in pitch as the hole is moved from the center of the top out toward the end, assuming the hole is the same size. A smaller hole will drop the 'air' pitch even further. There is other stuff going on with 'air' resonances up in the corner as well that will alter the timbre of the guitar, Finally, from what I can tell, the 'normal' sound hole position works in ways that are fairly complicated to increase the 'complexity' of the sound, which is why it has been retained for so long as a feature of guitar design.

Opening a 'port' in the side will raise the 'air' pitch and make it stronger. This usually comes across as being 'less bassy', or maybe more 'forward' or 'open' sounding. The combination of the hole in the corner and a port could well end up placing the 'air' resonance pretty close to it's usual spot, at about G on the low E string. The main utility of a port is as a 'monitor', and it doesn't need to be too big for that. If the port and the corner sound hole have about the same total area as a normal round hole the 'air' pitch might well work out pretty nicely; the usual range is somewhere between F# and A, so you've got a pretty big target to aim for.

It would be nice to be able to talk about this stuff in simple terms, but acoustics is almost never simple, and the guitar seems to be deliberately complex. Sorry.
Author:  Dave Higham [ Mon Nov 15, 2021 5:25 pm ]
Post subject:  Re: A sound port question
DennisK wrote:
I'd say the hole+port will function more like a single hole than usual. Sort of like a large hole with a bar through the middle of it. Perhaps even better if you make the port a half circle, with the flat side as close to the soundboard as you can get without structural issues.

Ken Parker seems to have had the same idea as he does something similar on his archtops.

Thank you all for your contrbutions. The consensus seems to be that my idea might be valid. I'll let you know how it works out, but don't hold your breaths, it won't be soon.
Author:  Dave Livermore [ Mon Nov 15, 2021 6:35 pm ]
Post subject:  Re: A sound port question
Dave you just described one I’m working on.
I put it together and strung it up and played it for a while. (Finish is messed up so I took it apart agai)
All things considered, it sounds a lot like a guitar.
Balance is good. Bass is good. Treble is good.

I’m happy to help if you have any questions.


Using advice from Al Carruth which you have already gotten, it is a good guitar.

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