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The lowest audible frequency is in the ballpark of 20 Hz. I don't think any of us make guitars that go anywhere near that low, but even if we did, that little post-it note would be flapping at least 20 times per second. Unless someone has a high-speed video camera, I don't think anyone could accurately tell whether pieces of paper fluttering on our guitars are in phase.

[QUOTE=WaddyT] It makes me wonder - Are soundhole sizes "scientifically" sized? If so, there might not be an advantage to a sound port. If not, then the sound port might correct for the sizing error relative to the box size, and air movement created by the top. If scientifically sized, it would have to be reduced by a ratio to the port.???????
[/QUOTE]
A week or two ago, Alan Carruth referred to some work on soundhole position (and size?) from the GAL- I've misplaced the reference just now.
I think that AC is also the person who built the 'guitar with corks' to test soundhole position?? Again, I stand to be corrected on this.
Also, Siminoff (in his 'Tap Tuning' book) talks about 'tuning' the soundbox by making small adjustments in soundhole size for both mandolins and guitars.
I believe that Kasha talked about the soundhole edge being important- his designs usually had 'rings' at the soundhole. Sergei deJonge doubles the top thickness at the soundhole and he told me that it improves the sound.
So, whether it's scientific or not, there has been some work done on this.
It's interesting, since guitar soundhole sizes seem to be often decided using cosmetic criteria- what looks right, and what's 'traditional' (aka Martin).

Probably fodder for a separate thread...

Cheers
John

This stuff makes my head ache sometimes, too.

In the High-C can you were out to maximize flow, and did it by punching another hole that prevented the formation of a vacuum that would supply a force that prevented the juice from flowing. Without that vent you still got some flow, but it became less and less as the vacuum built up. Usually the momentum of the stream of juice would carry it out even after there was enough vacuum inside to stop the flow, and in that case, when the flow actually did manage to stop, there would be a low enough pressure in the can to pull in some more air, reducing the pressure enough to allow for some more flow, and, you get the 'glug-glug'.

That's a mechanical resonance: it has a mass that can be set into motion, so that there is energy represented as momentum, and something that can work as a spring, such as air pressure or bending, to supply a restoring or retarding force, with a way to trade the energy back and forth between them. Unlike the High-C can, on the guitar we're not interested in extracting all of the fluid from the inside: the flow is not all in one direction. We want resonance.

Start by thinking of the guitar as a bass reflex enclosure. There is an air resonance in which the air moves in and out of the hole, just like it does when you blow across the top of a bottle: this is called a 'Helmholtz resonance'. The pressure inside the box rises and falls everywhere in the box in step, making a sort of 'air spring' on which the mass of the air in the soundhole can bounce. Note, however, that although the pressure change is in phase everywhere for this mode, is not of the same magnitude everywhere. The pressure changes most at the tailblock and least at the soundhole, and the level of change is in between at the upper block.

The top goes in and out like a loudspeaker cone, and pumps air through the soundhole as well as radiating sound off the top.

Unlike a well designed bass reflex speaker cabinet the pitches of the 'Helmholtz' resonance and the 'main top' resonance on the guitar are usually pretty far apart in isolation; often around a fifth. The interaction of the two pushes them even further apart. At low frequencies the two will be 'in phase', with the air moving in at the hole while top moves out. You can think of the mass of the top in this case as being added to the mass of the air moving through the hole, and this slows the motion down, dropping the pitch. Nearer the pitch of the 'main top' resonance the air and top motions are out of phase, and the air pressure change inside the box acts to 'stiffen' top, raising its pitch. Thus the 'main air' and 'main wood' resonaces on most guitars end up being close to an octave apart.

If you're paying attention, you'll see that at the 'main air' pitch the sound at the soundhole, and the sound coming off the top are out of phase, and tend to cancel out. Usually there is so much air being pumped through the hole, partly from the flexibility of the sides and back, that it dominates. At the 'main top' pitch the two are in phase, although usually there's more sound coming off the top.

The larger the soundhole is, the easier it is to move air through it, and the higher the pitch of the 'Helmholtz' mode will be. This holds true even if the soundhole is a plural, with holes in different places. Thus punching a port in the guitar tends to raise the pitch of the 'Helmholtz' resonance. The greater the pressure change in the given place for the basic Helmholtz resonance, the more effect a port there will have on the pitch, since there is more pressure for the hole to relieve.

Does porting a guitar 'relieve back pressure' on the top? Certainly it can: I've measured more motion of the top on the 'corker' with ports open than with them closed. Is this a good thing? Well....

Remember that the the top is one of the things that's trading energy back and forth with the air inside the box: it's part of the resonant system. If you reduce the pressure changes inside the box, you reduce the strength of the coupling between the top and the air: it's less of a 'system' and more 'a top and a box'. This may or may not be a good thing; only your ears can say for sure.

There are a lot more complications to this, of course. This is, for example, at least one other 'Helmholtz' type air mode. There _shouldn't_ be, but because of the shape of the body, and the location of the hole, and fact that the top and the air are coupled, there is. Depending on where your port is you could reinforce or weaken that, alter it's pitch, and change the timbre of the guitar.

There are a bunch of other air resonances inside the box too. Usually you don't hear them; they don't 'talk' to the main soundhole, and any energy that goes into them tends to be 'wasted'. A port will probably 'hear' one or more of those, and thus boost the output at that particular frequency. Because these are usually at higher pitches than the 'main air' and 'main top' modes you can hear them better: your ears are more sensitive. Thus they often make the guitar sound louder, even if there is only a little more power going out.

If the 'Helmholtz' type air mode were all that was happening, the air flow through the port and the main hole would always be in phase: in or out in both places at any given time. Becuase of the fact that there are those higher air modes you can have more than one resonance moving air through the hole, at different pitches, and with different strengths. Yuo could set up a couple of little mics at the two holes, and run them into a 'scope or some such, to see what the phase relationship was like at different pitches. Now there's something that would generate some headaches!

This gets real complicated, real fast, and that's why there aren't any quick and definitive answers. I've spent 'way too much time (just ask my customers!) measuring this stuff as best I can, and other people have done some measurements as well. We don't all agree, yet, so there's something wrong with somebody's measurements, or maybe we're looking at systems that are too much different from each other. It's going to take a while to sort it all out.

As for the OP:

I think the key to your feeling something you haven't before lies not in the port, but in the lighter top. It's moving more air; enough to reach the threshold of your feeling. It's also likely to be moving it at a lower pitch: touch is generally more sensitive to low frequency vibrations than high.    

Here's a thought experiment for you. Suppose you pushed in on the soundboard of a guitar to its maximum inward deflection without breaking (remember thought experiment) and hold it there. The volume (V) inside the guitar decreases. PV=NkT, The pressure (P), and temperature(T) inside the guitar will quickly reach equilibrium with outside pressure and temperature by reducing the amount of gas (N) inside via sound ports. Air is expelled, both holes. Release, the opposite will occur.

Now what happens at 400Hz? Are the ports large enough to allow the expulsion of air quickly enough to reach equilibrium? What about modes where one part of the guitar squeezes near one sound port while another portion expands with no net volume change?

Likely many different scenarios are possible at high frequencies. I imagine the air is sloshing in and out of the guitar in a complicated way and the addition of an extra port of different size further complicates an already very complicated thermodynamic system.

   With the infinite palette of variables that will be presented, first, by the
size of the two openings and their proportion to one another, then, their
proportionate relationship to the actual physical volume of the body
cavity, there is no way to nail the phase characteristics and the cycles at
which they will match and not during any reading effort.

    Then there are the infinitely variable parameters that will be presented
by the woods used for the top, back and sides and braces and all of their
dimensional and mass/weight relationships to one another and effects on
one another. The fundamental content and harmonic blankets presented
by different woods will create a new environment for data and results with
every guitar...even when they are built from the same woodsand
dimensions all of the way around since every set will create its own
unique environment for study.

    Then there are the variables that are presented by humidty and
temperature conditions during the whole process.

    I've watched the same things happen as i've built with ports in different
locations on the guitars over the years as Tim. I always place those flap
like covers over both holes and one or the other to see the effects of the
air's movement on them. At times, I've used very light weight papers and
even tissuelike material to present as little resistence to be able to see
movement under the very lightest activation of the body cavity's air
content.

    The ports have consistently shown a draft toward the inside of the
guitar during playing while the volume of the guitar has never suffered,
leading me to believe that the monitor like effect of the port may be
generated more by reflective sound release than the physical pumping of
the top that is typical in any guitar that is built correctly. The location of
the soundhole has much to do with how efficiently it operates as the
release point for the volume and projection of the instrument.

    The exception to that inward draft direction in all of the ports has been
during the actual attack portion of the players' efforts to drive the guitar
with as much dynamic variation as possible. Whether playing with a light
or heavy technique, the attack of each note or chord saw both the
soundhole and port presenting an exhaust only to have the port begin to
pull its cover or draft direction indicator in toward the body again while
the soundhole continued to work in an exhaust fashion.

    Don't get caught up in formulas and numbers that will be nothing more
than a one time, one guitar result, but experiment and try soundport of
different sizes, shapes and in different locations on the side until you find
the port that gives the best result for your guitars.

    These are not speaker enclosures and, even though we can learn
something from speaker technology, whether of an acoustic suspension
or ported nature, the technology and application of it are going to be
completely different. The tensions present in guitars and the vibration
source being external and determined by the respective cycling of the
strings and the pitch at which they are tuned present and very complexed
environment that will continue to push us for as many years as there will
be luthiers trying to improve their skill andunderstanding of these
wonderful things we build.

    I've heard a few guys who have claimed to have nailed some of the
most illusive things in lutherie down and have chuckled as they rattle off
their scientific formulas and mathematical equations that always end up
with countless holes in them. Given the organic nature and infinite
variability of our materials and amount of human input that will always (or
should always) be present, I believe that our approach needs to be just as
flexible...just as variable...just as...well...infinitely variable.

    When I hear these guys, my mind wanders to the many people who
have found the secret to miracle of the quality of Stradivari's violins...the
finish...the treatment of the wood....whatever. I tend to like to give a little
more credit to the skill of the builder and his willingness to be flexible
enough to address each set of components that landed on his bench to
be fashioned and assembled into one of his instruments as a unique
platform from which to work.

   These are just my opinions and some of my findings so please feel free
to take or leave them but don't fall for the smoke and mirrors that show
up so regularly either.


Regards,
Kevin Gallagher/Omega Guitars

Alan,

Thank you as always for sharing your wealth of information and research with the OLF. I do hope you realize that you are a highly respected member of our community. Your ability to communicate difficult material to a variety of readers with a style that is always congenial and educational is amazing. Much of it is over my head but I enjoy and appreciate it anyway.

John,

Funny you should mention Sergei de Jonge and Dr. Michael Kasha. Mike Chock who was my instructor at Hana Lima ‘Ia studied with Sergei de Jonge and with Richard Schneider of Kasha fame. He has taken what he has learned and has applied it to the ukulele. His shop is a hot bed of new and exciting ukulele design.

Philip

_________________
aka konacat

If you think my playing is bad you should hear me sing!
Practice breeds confidence and confidence breeds competence. Unfortunately, I'm stuck in practice.

Is it possible for a person's brain to bleed? 'cause I'm pretty sure I've got a pretty good size hemorage going right about now.

...So we can change the resonant frequency of the air (Helmholtz resonance) by making a soundhole larger or smaller. We can do that while not changing the main top resonances if we use a side soundport because, given a very stiff rim, we're not changing the mass or stiffness of the top.

...It follows that if changing the Helmholtz frequency makes a significant difference to the tone of the guitar, then we have an excellent tool to fine tune and color the final instrument to the player's preference. Ideally we would string the guitar up, have the player play, and add or remove material from the soundport until it sounds best.

So does changing the helmholtz resonance color the sound enough where this might be worthwhile?

[QUOTE=Alan Carruth] ........As for the OP:

I think the key to your feeling something you haven't before lies not in the port, but in the lighter top.........[/QUOTE]
Alan
That's what I was thinking.

Thank you for your time and informative post.

_________________
"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Paul, allow me to simplify this for you...

Picture the top as being a sheet of flexible rubber.
If you push it in, what happens?
The air goes out wherever it can, right?
That's what happens to both the soundhole and the port...they both move the air in the same direction at the same time during that piston action, which is one way a guitar top moves. Now if you let the rubber top back out, the air will then get sucked back in through both the soundhole and the port at the same time.
Make sense?

That said...
Since there are many different phases and signals going on at the same time inside a guitar, and since it's happening so fast, some things are working in opposition to each other, and there can be cases where air is moving in one while moving out another. It all depends on the physical arrangement, size of the two openings, and the type of sound waves that are moving. Don't ask me any more, beyond that, only Al or Ervin or someone could explain it. But then, it's all Martian to me.

_________________
"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

But if the ambient air pressure is different from one day to the next, how will this affect the way the guitars push air?
The same way they dial carbs in with racing, would you need to change the size of the ports?
Lance

[QUOTE=McCollum] But if the ambient air pressure is different from one day to the next, how will this affect the way the guitars push air?
The same way they dial carbs in with racing, would you need to change the size of the ports?
Lance [/QUOTE]

Or how about if you tuned a guitar's air resonance below sea level in New Orleans and took it to a gig in Denver? It would certainly push more of the lighter air in Denver, so the resonant frequency would rise. Enough to make a significant difference in tone? I surely don't know.

[QUOTE=JohnAbercrombie]
A week or two ago, Alan Carruth referred to some work on soundhole position (and size?) from the GAL- I've misplaced the reference just now.
[/QUOTE]

That was "Basics of Air Resonances" by W.D.Allen, GAL RedBook #1, pg 8.

Thanks, Al!

John

To me thinking of an air pump is confusing. In my mind(and I certainly do not think I fully grasp everything that is happening)I came to my understanding by thinking first about the soundboard vibrating with an open back, and no sound hole. When the soundboard moved in one direction it would increase the air pressure about the board on one side and decrease it on the opposite side. Then as it moved the opposite direction it would have the opposite effect. Much like what would happen if you pushed an oar back and forth in water.

If you seal the back (but still no hole in the soundboard) you would do the same thing, but this changes the dynamic of air movement and resistance on the soundboard (quantifying that effect starts to open a can of worms). What it still would do is increase and decrease pressure in a "similar" fashion.

When you open a hole in the soundboard, you allow air to press out as the sound board compresses the air behind the soundboard. If this was all instantaneous you would have the top of the soundboard trying to drop the air pressure, while the air from the hole tries to increase the pressure (that would not seem to be ideal, but then again a guitars shape is not that simple and the waves of pressure are very rapid and complex, as is the way the soundboard is moving not allowing for a simple plus and minus pressure situation). Of course the opposite would happen as the soundboards direction changes. We also have the box which will expand and contract/ distort(some I suppose think of reflection). This will effect the phase relation of the pressure changes developed in the box by the soundboard, and the soundboards movement.

The changes in pressure are similar to the action created by a Helmholtz resonator. A Helmholtz resonator resonates when a pressure on one side of a hole is slightly higher or lower than the opposite, with one side having a limited volume. The limited volume builds pressure if air is added and drops if air is removed. You need to set up that source of differential on the side opposite the sealed volume to create the oscillation. The frequency of oscillation will be a product of the volume of sealed space, the length of the hole and size of the hole. I hope I am not too far off in my description(a quick web search would probably give a better definition of a Helmholtz resonator).

So the action of changing pressure is pretty much the same, but the source of the oscillation is a bit different. This brings me to the conclusion that fine tuning would not have the same direct effect as a classic Helmholtz resonator. However, I could see how the function is so close that similar rules may apply to volume/length of hole/size of hole as it relates to a frequency that is favored(I don't know if favored is a good description). Size of the sound hole should certainly effect the shift of pressure as it would resist flow to an extent. That may be a tool that could allow for some control of the phase relationship between the pressure changes created by the soundboards surface, and the changes in pressure developed by the box through the soundhole. The size of the hole at a glance seems like it would create little restriction to such a small volume of air moving in and out, but that air is moving pretty quick even at low frequencies when a larger volume of air would be moving(for all I know the size of the hole is not even close to being small enough to effect or regulate flow ).

Just some of my thoughts for what they are worth . Hopefully they are not too wacky, and this is a subject I have changed my thinking on several times. So I am sure I don't have a solid grasp yet. I enjoy reading all the responses to this topic(lots of ideas to chew on ).

Had to flip to post or not to post this

Peace,Rich

Ok, how many of you discussing this thread have built more than 50 guitars? If so it really doesn't matter! Believe me it changes!
Lance

Careful, Lance...
You'll get the same reputation I'm building here!

Don't forget, folks, that air is a compressible medium. It acts as a spring and thus the volume itself has a resonant frequency. In the classic Helmholz resonator, you're not dealing with the surface of the enclosure moving, too. It's not coupled.   In a guitar body you've got coupled resonances making things weird.

You know why I put side ports in guitars and now in some ukes? Because I like them, and more importantly, my customers really like them. My customers like paying me to cut holes in the sides of the guitars we build for them. I like getting that little bit more sound in my face, and I like getting that little bit more money that lets my customers get a little bit more sound in their faces, too. I like side ports because it's win, win, win all the way to the bank.

Lance and Rick,
I've only built 8 guitars, I cannot pronounce Helmholz, I don't have clue what it means or why my sound port sucks.
I played guitars for over 30 years before I ever though about making one. I know what I like in a guitar, and my latest is very good to my ears. A large part of the credit is due to guys like you that freely share here.

Thank you for your help!

_________________
"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Lance, I am a little confused by your comment. You say it "changes", but if a person has not built 50 guitars they should not try to understand what is happening?

If you meant shut up and ignor how this works because there are more important things to understand(walk before you run kinda thing). I guess I could understand where you are coming from.

I am not taking issue with what you are saying, I just don't understand the point you are getting at.

Rick- What you say makes sense to me.

"Don't forget, folks, that air is a compressible medium. It acts as a spring and thus the volume itself has a resonant frequency. In the classic Helmholz resonator, you're not dealing with the surface of the enclosure moving, too. It's not coupled.   In a guitar body you've got coupled resonances making things weird."

When I am thinking about the body itself. Should I be thinking it is like a second "spring"(air being the first). What makes really makes things tricky is that spring on a spring with different rates of compression(I hope that made sense). This making the effective volume of the box a moving target depending on how the box is vibrating at the time?

Peace,Rich

   The Hiller Lockwood Pulse Jet which is essentially a u shaped tube with fuel and ignition attached uses the principle of pressure waves to create thrust out both ends of the tube. It works reasonably well but creates an incredible racket and uses huge amounts of fuel!

     Here's a link to a REALLY crazy Kiwi, that built one and put it on a 90mph Go-kart!

      Crazy Kiwi

     I think these southern hemisphere types go a little wacky because they're upside down and the blood must run to they're heads! Sorry Guys! Ozzies and Kiwis are the best!!

     The pressure through disimilar openings can and does produce a pump if phase is considered in all it's variables. The phase is effected by size and distance of the openings/ports, and God knows how many other things... meaning very complicated if all are considered together!

     Yes a guitar(standard porting) is quite a bit like a "tuned port" speaker box in principle, the difference being mainly energy source and degree.

    In the Hiller/Lockwood one has to be very careful to get porting and distance exact or you get a dissimilar pump and the engine doesn't work. The care in balancing the ports let alone knowing how to do it to the correct spec's hasn't even been studied well let alone how one would approach studying it.

     The only person that I have seen even get close to the work necessary is, of course, Al.

     In a guitar there are so many more variables involved, adding ports is going to do nothing but make it more of an art as oppose to science. With, of course, our resource available.

     I trust enough of the guys here to believe that if they say it's sucking, then he is definitely experiencing something even if none of use understand totally what it may be.

    Discussion is excellent, as hypothesis is the very first and necessary step to scienific understanding.

    Whether this pumping is good, bad, complication, effective, or no, would be very interesting to find out.

Here's a link to the full site!

     No Regard For Personally Safety

_________________
Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

[QUOTE=Pwoolson] Is it possible for a person's brain to bleed? 'cause I'm pretty sure I've got a pretty good size hemorage going right about now. [/QUOTE]

Paul,

You'll be OK, just put a couple of postits over your ears

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

[QUOTE=Rick Turner] Careful, Lance...
You'll get the same reputation I'm building here!

Don't forget, folks, that air is a compressible medium. It acts as a spring and thus the volume itself has a resonant frequency. In the classic Helmholz resonator, you're not dealing with the surface of the enclosure moving, too. It's not coupled.   In a guitar body you've got coupled resonances making things weird.

You know why I put side ports in guitars and now in some ukes? Because I like them, and more importantly, my customers really like them. My customers like paying me to cut holes in the sides of the guitars we build for them. I like getting that little bit more sound in my face, and I like getting that little bit more money that lets my customers get a little bit more sound in their faces, too. I like side ports because it's win, win, win all the way to the bank.[/QUOTE]

Rick that's a great post start to finish...when it comes down to it, all that matters is customers like them, and are willing to pay more to get them.

My wife wants me to build her a guitar with a bunch of ports along the side...shaped like cat's paws walking up the side. Go figure. Hey, if she's willing to pay for it...

_________________
"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."