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Soundport AFTER guitar made?
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HolyEvil
Posts: 1240
Joined: Nov. 6 2008
From: Sydney, Australia
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 RE: Soundport AFTER guitar made? (in reply to HolyEvil)
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This is from the classical guitar forum that I posted the same question to
It's replied by a luthier called Alan Carruth..
I think it's quite informative.
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I've done a lot of experiments with ports of various sizes in different locations. The conclusions I've reached are that the outcome from adding a port is fairly easy to understand, if you know enough about how the guitar works. However, since the guitar is actually a pretty complicated beast, it's not necessarily 'simple'.
Opening a port primarily alters 'air' resonant modes. Most of these involve air 'sloshing' around inside the box in one way or another: for example, there's one called the 'A-1' resonance where the air sloshes the length of the box. This is sometimes called the 'lengthwise bathtub' mode, because it works just like the water sloshing the length of the tub. In the tub the depth of the water at the ends changes a lot, and that's what happens to the pressure of the air inside the box of the guitar. There's a pressure 'null' in the center, where there is a lot of flow. Since what we hear is pressure changes, and the pressure null for this mode in the guitar is pretty close to the soundhole, we tend not to hear as much of this, at least directly, on a 'normal' guitar. Opening a port at either end of the box will tap into this pressure change, and allow you to hear the mode. The way this stuff works, that hole near the end will also raise the pitch of the mode, and that's a general rule: opening a port where there's a large pressure change makes the pitch rise, while opening a hole where the flow is high, and there's no pressure change, can drop the pitch.
Most of the internal sloshing modes in the box have nulls pretty close to the normal soundhole, so you don't usually hear them directly. This doesn't mean they don't effect the timbre of the guitar: that 'A-1' mode, for example, couples with a top resonant mode called the 'long dipole', and the interaction, combined with the waist and soundhole location, gives rise to some 'new' resonances that can be important in the sound of the guitar. Depending on where you put a port, and how big it is, you will alter a number of the internal air resonances, and change the way they interact with the top, so the overall spectrum of the sound will be changed.
There's one 'air' resonant mode that does not involve the air sloshing around inside the box, or, at least, doesn't _only_ involve the box: the 'A-0' or 'Helmholtz' mode. The zero in the number refers to the fact that the pressure inside the box changes in the same way everywhere at any given time: there's no pressure 'null' inside the guitar. For the A-1 mode, as the pressure rises at one end of the box, its falling in the other end, but for the Helmholtz mode, it's either rising or falling everywhere at the same time. This pressure change pushes air in and out of the soundhole, and that's where the 'null' is. That's why I say it's not 'only' the box: the air in the room is involved as well, although moving to a different room won't change the resonance. If you plot the pressure change/sound level inside the box, it's high at the tail block, a little lower at the neck block, and lowest as the hole. Putting a port anywhere in the box will vent the Helmholtz mode, making it louder, and shifting the pitch upward. The further the port is from the normal hole, the greater the change in pitch and the more power it will vent for a given hole size.
This, in fact, is one of the major changes you see when you put in a port. The change can be quite large: a pair of holes about 30mm diameter up at the base of the neck will raise the 'main air/Helmholtz' pitch by more than five Hz (around a semitone) and can increase the output in front of the guitar at that pitch by as much as 7dB. A six dB rise is four times as much output, and it's definitely audible. Because of the way the 'main top' and 'main air' modes work together, there will also be an increase in the output at the 'top' frequency, but much less of one: on the order of 2dB for that same pair of holes. There's also an increase in output at the (raised) A-1 pitch, as you'd expect.
Generally speaking, as you go higher in pitch, the difference become less and less. Its even possible that the overall output of the guitar drops of a bit in the higher frequency range with a port: when I sum over the power in the spectrum it doesn't seem to be much different with a port than without. Remember, too, that the increases, particularly at the 'main air' pitch are only over a narrow range; usually just one note.
This explains why at least some people report an increase in bass response with an open port. The 'main air' resonance is the lowest pitched on on the guitar, and if that becomes more powerful, it can make the guitar sound more 'bassy'. OTOH, raising the main air pitch, without changing the output, tends to give a more 'treble' sound, so it can be hard to predict just what the combination of more power at a higher pitch will sound like.
With the port 'listening in' on higher frequency modes inside the box there will be changes is the overall spectral response, as I've said. Often these internal modes show up as 'dips' in the spectrum, since they take energy out of the top and don't produce any sound you can hear. When you open a port some of those 'dips' become 'peaks', so even though they may still be at a fairly low level, they change the response audibly. Since our senses are set up to detect changes this sort of thing tends to jump out, and we often interpret this ad 'more power', when there may, in fact, not be much more, with the exception of the 'main air' mode as I've said. In other words, opening a port probably does not increase the overall efficiency of the guitar much, if at all.
Finally, opening a port can alter the directionality of the sound output of the guitar. In particular, low frequecy sounds tend to go out in all directions, while high frequencies are more likely to go off the top and out of the hole toward the audience. Usually the player can hear these high frequencies as they reflect off the walls, ceiling, and floor of the room. In that case, a port might not help, as was shown in one experiment recently. However, in a large, 'dead', or noisy room you might not have the high frequency feedback. A port that the player can see, say in the wide part of the upper bout on the right side, will direct some high frequency sound toward the player, and can thus make a useful 'monitor' in those restaurant gigs, or when you're playing in a group. Some blindfold playing tests I've done in noisy rooms show that in that case opening a port is quite audible to most players. IMO, this is the best reason to use a port.
Adding a port in an existing guitar, particularly one that has a nice sound, is something of a crap shoot. A lot of the low- and mid-range balance has to do with the way the 'main air' and 'main top' modes work together, both in terms of pitch and power. Adding a port changes those balances. As has been said, in new construction we can plan for this, and make the normal soundhole smaller. When you add a port, you may find that you'l need to reduce the size of the main hole a bit, or add a sleeve to make it 'deeper', to get the 'main air' pitch down to where it should be. This might be a worthwhile endeavor if you really need that monitor, and don't mind changing things around a bit. I would _not_ do this on a Fleta or Hauser, but it might be justifiable on even a pretty nice guitar.
Most of the 'affordable' guitars, in the $200 range, will have plywood sides. These won't need any cross grain side reinforcement for a port, since the center ply takes care of that. On one of my experimental boxes I put cloth tapes on either side of the hole locations, and had no troubles with cracking. These days I use a cross grain patch of some soft wood, like willow when I put in a port.
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REPORT THIS POST AS INAPPROPRIATE |
Date Aug. 21 2012 3:21:55
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Florian
Posts: 9282
Joined: Jul. 14 2003
From: Adelaide/Australia
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RE: Soundport AFTER guitar made? (in reply to estebanana)
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hundreds of guitarists love soundports (and some guitar makers) and swear by them ..some of which include jason m, grisha, and others....oh and me and Deniz and about at least 20 other guitarists here on this forum...I LOVE the results of my soundport..and a speech about balances or whatever technical side of it or some lame attempt to insult because i dont see it your way dosent change that...you are talking about it..i am here playing it and experiencing it...its my guitar would why would i purposely do something to it i didn't think it made it for a better experience, or if i did and didn't think it was why would i so openly encourage anyone that's thinking of getting one to get it...why would i put myself on the line like that if i didn't believe 100% in it...its easier to say dont do it, nothing lost nothing gained...no risk
as Deniz and everyone else of 99.9 % of members with a soundport will tell you...i dont know the technical words, i dont have some analyzes about air travel...they just like it better...feels better and sounds better to the players ears...now i dont know whats more important than that to a guitarist...certainly not weather the maker of the guitar approves of it or not.(and actually the maker of my guitar made me aware of it and encouraged me to do it and i am grateful to him for it)
Even if its not real...its just an illusion ...i mean i dont know what else is there when it comes to sound, whats the alternative...give me the illusion of having a sound i like and il take it everyday
..personally i am grateful to be made aware of it...made my experience that much better...and it was good before...absolute no negative here or any reason to regret anything, nothing but positives...and the port was done after...i am a musician not a collector, i didn't buy the guitar to sell it or keep it in a display window, or to marry it, i got it to play it..if i need to sell it again one day ...to anyone that tries it..the guitar will speak for itself..no doubt it will sell...my worst nightmare ...is getting another new guitar and wait 2 days without it while its in the shop getting the sound port put in...so 2 days without a guitar...because soundport would be the first thing i would do to a new guitar or any other guitar i owned from now on...theres no going back, it would not have reached its full potential without it...without even the need to debate about it.....I like it that much 
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REPORT THIS POST AS INAPPROPRIATE |
Date Aug. 22 2012 5:12:28
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