Ported upper bout....?

[Alan Carruth]

High frequency sounds tend to be more 'directional' 'than the lows. That's why the port makes an effective monitor in the first place. The lowest notes the guitar produces have wave lengths that are far longer than the size of the box, so those sounds go out about equally in all directions: the player hears them as well as anybody. As you go up in pitch (starting, say, around the open G string pitch or a bit above) things start to get more directional.

In the high frequency end (2000-4000 Hz and above) the sound can actually go out in fairly narrow 'beams'. The top in particular vibrates as a number of small areas that are out of phase with each other, and, along with the sound coming out of the hole and and 'ports', it all works as a 'phased array'; adding up in some directions and cancelling out in others.The exact geometry of all of this varies from one instrument to another and with changes in pitch, so it's probably impossible to predict who might hear the effect of any change in that range. The player tends to get some benefit from any port they can see into as they play. How it all works for the audience is another issue, and harder to determine except by experiment.

I once made a guitar that had two sound holes in the top out near the edges of the upper bout. The idea was to move the hole out of the middle to get more soundboard area. As it turns out, there's a fairly strong resonance across the upper bout at around 700 Hz, iirc; the air 'sloshes' across the width of the box, so the sound pressure level is high at the edges, and out of phase from one side to the other. With the two holes there that mode was radiating sound strongly, and out of phase from the holes. That pitch tended to reinforce right about where the player's ears were: any note that had much power there was stronger than it 'should' have been to the player. The folks out front didn't usually hear it that way. The player tended to back off on those notes, which made the sound to the audience 'uneven'. It wasn't the guitar's fault. As I say, this stuff gets complicated.

[PTL]

Absolutely fascinating thread. Been thinking for years to experiment with porting, especially my favorite carbon fiber guitar or a 000 12th fret.

If I may, two questions for conjecture or informed input....

1. If a guitar is too bassy, could porting be a solution by raising the Helmholtz frequency?

2. I am intrigued by the idea of two smaller ports - one to each side of the ear instead one large port. Would the impact on the Helmholtz frequency be the same if the two holes add up to the exact same area as the one larger hole?

Thanks.

[fetellier]

If a guitar is too bassy, could porting be a solution by raising the Helmholtz frequency

The port does indeed increase the Helmholtz frequency of the instrument but as far as I have observed by covering and uncovering it while a person is playing the bass does nor change by very much. A port of approximately 2 square inches will increase the box frequency by about 4 hertz and increasing beyond that causes very little change.

Fred

[Alan Carruth]

Adding a port to an existing guitar does raise the Helmholtz pitch. It also makes the resonance stronger. Whether it sounds more 'bassy' or more treble depends on the balance between the rise in pitch and the increase in power.

How much the port raises the Helmholtz pitch depends on it's size and where it is. A port of a given size at the base of the neck, or beside the tail block, with raise the pitch more than one at the wide point of the upper bout. The further the port is from the main hole the larger the pressure change it 'sees' at the Helmholtz pitch. The output is greater and so is the pitch shift.

Since the primary utility of a port in most cases is as a monitor, having one you can't see as you're playing sort of defeats the purpose.

Any port also tends to 'hear' internal air resonances that are normally not heard through the main hole. That changes the timbre, particularly in the high frequency range. Two ports can 'hear' two different internal resonances, or the same one out of phase to a greater or less degree. You might well end up having the sound from the ports cancel at your ear at a particular pitch, which could make the sound uneven to the player, even though the audience would thing it was fine. These things get complicated.

[ChalkLitIScream]

Fitness, in your experience, how does having the ports on the upper and lower bout differ from having a single, larger one on the upper bout (most common) or even a larger one on the lower bout (Ryan guitars makes a variant of this idea)? Assume the area of the holes adds up the same.

Purely from a non-scientific observation, I feel like the larger single port would project more towards me. I would like to be corrected!

[fitness1]

Originally Posted by ChalkLitIScream

Fitness, in your experience, how does having the ports on the upper and lower bout differ from having a single, larger one on the upper bout (most common) or even a larger one on the lower bout (Ryan guitars makes a variant of this idea)? Assume the area of the holes adds up the same.

Purely from a non-scientific observation, I feel like the larger single port would project more towards me. I would like to be corrected!

My assumption is the results will be very relevant to your playing position.

Mine is classical style with my left foot on a stool and waist on my left leg.

When determining the placement of the ports on both bouts - I get into my playing position and as closely as possible plot out spots that are directly at each ear.

This creates an enveloping sound that is hard to articulate into words - it's not intense, more subtle. It's definitely there, though.

Unless you play in this position and keep the instrument away from your body, you may not hear much difference - you REALLY have to be doing everything right and listening!

[Guitars44me]

My Kinnaird Customs have a good size oval on the bend of the upper Bout.

I play in a lot of different positions, but for the most challenging songs tend to put the Lower Bout on my left leg. I don't raise my foot but tilt the axe to get close to Classical position.

This puts my ports up by my left ear. I get a nice stereo effect, with plenty of sound from the top closer to my right ear.

I also stand and stroll, and can really hear my guitars better no matter the positions.

Try it! You'll like it

Paul