Official Luthiers Forum!

Ok, so I've been admiring so many beautiful guitars from all of you on this site. One thing I've been noticing is that some of you who build acoustics put an extra sound port on the upper side. So, I was curious as to how this influences the sound and if there's some good book/article references for me to learn more about the ports.

I've read Siminoff's "Luthier's Handbook" and am half way through Cumpiano's book at the moment. I also have Kinkead's book that I've only thumbed through at this point, but I think I'll be following the Cumpiano book for my first build. That being said, none of them mention the ports so I was just curious to read up and understand them.

Thanks in advance!

thanks guys - I searched this site, but all I could find was posts about how they were either added to a build or how it was reinforced, but I'm having trouble finding anything about the design, placement, or reasons why they were added - which is why I asked. Thanks for the Acoustic Guitar Forum suggestion. that helped.

I think you did not search hard enough.
Try searching for Alan Carruth's post.
He wrote quite a few post on the placement of the port and
how to adjust the main sound hole etc.
He even have a test mule guitar with lots of ports and all stuffed with corks to test the effects of the placement of sound ports.

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Sen CL Goh
http://senguitar.blogspot.com

Yeah: I can't seem to get away from ports! I'm just trying to write up another article on them. There is one on my web site that you could look at, too.

There's a lot of hype out there about ports; often from folks who have some sort of patent or copyright interest in them. I've been trying to do real science to figure out what's up, but, as with everything related to guitars, it's not easy.

To cut to the bottom line, adding in a port does two things:
1) it alters the pitches and strengths of some or all of the internal 'air' resonances in the box, and,
2) it alters the directionality of the sound output.

On most guitars the low end sound goes out about equally in all directions, and as you go up in pitch the sound becomes more directional. Usually the high pitched sounds are coming off the top or out of the hole, and the audience hears a lot more of that than the player does. In some cases; in a 'dead' room with some background noise, the player may not get very much feedback of those high frequencies, since they are not bouncing back off the walls or the floor and are masked at any rate by the noise.

A port puts some sound out in the direction it faces, so having one pointed at the player can give you some high frequency sound directly, without having to rely on room reflections. It doesn't take a lot of this to be useful, so a small port can be a help.

The port 'vents' at least some of the internal 'air' resonancs, and this effects their pitches and how much you hear of them. The 'main air' resonance, the lowest pitched one, is effected most. If you don't alter the main hole the larger the port is and the further it is from the main soundhole the greater the effect on the 'main air' resonance. My more recent measurements show that a port of the size that a lot of folks use, say about 2"x 3", in the wide part of the upper bout, can more than double the output of the guitar at the 'main air' pitch, and if there are a couple of holes at the root of the neck that add up to the same area, the enhancement is even greater. I'll note that this is _one note_ that is much stronger. There can be some added output at the 'main top' pitch as well, but it's much less. In between those pitches the effect is not very great.

Adding the port also raises the pitch of the 'main air' mode, and does so more or less proportionally to the amount that it adds to the output. Iirc, that 2x3 port in th wide part of the upper bout raised the 'air' mode pitch about a whole tone. Normally the 'main air' mode is down around G on the low E string, and enhances the power of all of the lower notes on the guitar. Raising it in pitch can leave the lower notes 'high and dry' so to speak. You can get the pitch back down by making the main soundhole smaller, but that, of course, cuts the strength of the 'main air' mode back down too. There's no free lunch.

Since the effect of the port depends so much on it's size and location it's hard to give any iron clad rules about how to compensate the main hole's size when adding a port.

I general the effect of the port is greatest at the low frequency end and drops off as you go up. Certain pitches can be enhanced by quite a bit if there's an inside air mode that the port 'hears' which doesn't normally get heard. Again, these are gains to single notes. I believe that adding a port actually tends to reuce the output slightly above the 'main top' pitch (roughly, the open G string), but it's hard to say. Again, I think the 'no free lunch' principle applies: if you get more output in the low range, you're likely to see less in the high range. Overall, I don't think the port increases the efficiency of the instrument to any great extent.

Most of the unbiased opinion I've seen suggests that opening a port does not do a lot for the sound the audience hears. This is the subject of a lot of ongoing debate, and will be difficult to settle, since there are 'churches' of folks who have already made up their minds. I do think the port can make a useful 'monitor' for the player, but, again, it does not take a vey big port to make a useful difference.

I could, of course, be wrong: it's been known to happen. I'm open to disscussion that is based on DATA: everybody has an opinion.

Thanks Alan, your post are always enlightening. Always based on data

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Say what you do, Do what you say.

I've followed Alan's and others posts on Soundports. I like the idea of letting the player hear more of
what the audience hears, but I don't want to affect the instrument in other ways. So I go with a small
1" diameter, or so, port. Minimizes affects on the internal resonances, but helps the player hear more
what the audience hears. At least that's the way I summarize the issue...As always, anything concerning
the sound of the guitar re: the listener, is subject to any number of interpretations. I really appreciate the
way Alan attempts to put some science into his conclusions. As an old engineer, I understand that approach.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

I was under the impression that such a person did not exist anyway......

Ha .
Yep - sane builder, an oxymoron if ever there was one.

So, what about the direction of the soundport? I have seen some that are located pointing toward the neck. Does placing the soundport in a position more on the side, pointing up toward the players face, give more sound to the player or is it too harsh?
Steve

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

The ones at the base of the neck, pointing up toward the head, make more of a difference in the output. However, as you say, you can't hear them as well. I put the port near the widest part of the upper bout. As goziert says, even a 1" port can make a real difference.

My daily player has a sound port on the upper bout facing upward toward the player. It is not circular but is probably the equivalent of a 1" port. After almost 2 yrs, I still like the way it sounds when I am practicing or playing solo. I don't see any advantage or disadvantage when playing onstage with a group, too much ambient noise.

Purely my opinion, of course.

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Steve Smith
"Music is what feelings sound like"

Alan,

Have you tried placing a floating baffle ( at least limited contact) in front of a sound port for deflection. Just interested in what effect it would have on air pressure and sound.

Kevin

thanks everyone for your comments, and especially to Alan for that incredibly thorough explanation that I'm still trying to get my head around I have do do more reading just to understand all of what he was talking about....

I just checked out your blog, Alan. Fantastic articles, thanks so much for offering them. would you suggest any books to bring me up to speed on acoustics?