My little sound port test

[Alan Carruth]

Fitness1 wrote:
"Ports change the air resonance, and if they are done correctly, can change a lot of the instruments tonal qualities and volume."

Opening a port will always raise the pitch of the 'main air' resonance, and also makes it more powerful, based on a lot of measurements I've made over several years. This is the lowest resonance of the guitar that can produce useful sound, so the port can change the timbre, particularly in the low end. Whether this sounds more 'bassy' or more 'treble' depends on a whole lot of variables, such as just where the port is, how big it is, and where everything was before hand.

A port you can see into as you play sends some high frequency sound out to you that you may not hear otherwise. High frequencies tend to 'beam' off the top and out of the hole toward the audience, and the player may not get much of that in a large, dead, or noisy room. Even a small port, such as the one in your upper bout, can make a useful difference in a restaurant gig.

I did some blind tests using a classical guitar at two Montreal festivals, a local folk festival, and a luthier convention. Blind folded players who could not see the state of the port were quite able to hear a difference when it as changed, but listeners out in front (who were not blind folded) said they heard very little, if any, difference. Again, this was in large and fairly 'dead' rooms, with an average of 72 dB-A of 'white' noise from conversation, etc.

[zmf]

Originally Posted by Alan Carruth

Opening a port will always raise the pitch of the 'main air' resonance, and also makes it more powerful.....

Alan -- does this have anything to do with sound directionality increasing with frequency? Low-frequency energy can "leak" in all directions?

[Kaydee]

I have always loved sound ports,for the player they make the whole experience better and for the listener it either does or doesn’t but it’s my guitar and I want to hear it at its best so Sound ports all the way 😀

[Alan Carruth]

Opening a port any place is the same as enlarging the main sound hole. It's easier for the air to get in and out, so the pitch of the Helmholtz mode rises and a larger volume of air is moved.

If you stick a microphone into a guitar you can measure the sound pressure level in different places inside at the resonant pitches. The Helmholtz mode shows little pressure change at the hole; all of the energy is in flow. As you move further from the hole the SPL goes up, and tends to be highest at the tail block, and somewhat lower at the base of the neck. A small hole in either of those locations will raise the Helmholtz pitch and power more than a larger one near the main hole. Even removing a 'strap jack' and leaving the hole in the lower block can make an audible difference out in front.

The lowest notes the guitar makes have fundamental pitches with wave lengths far longer then the body of the guitar. The sound is radiated as from a 'point source', about equally in all directions, so the player hears those low tones as well as anybody else. As you go up in pitch the wave lengths of the sounds get shorter, and the areas producing them become relatively larger. In the high range, say above 2000 Hz, the top will be divided up into a bunch of small areas that are similar in diameter to the wavelengths of the sound in air. Each area is out of phase with the ones around it, so they tend to cancel each other out. But they are also different sizes, and vibrate with different amplitudes, so the cancelation is not total: some sound is produced. It tends to go out in one or more 'beams' in particular directions, out of the hole and off the top, toward the audience. The player doesn't hear these sounds directly; you only pick up reflections of them from the room. If the room is big or dead, or it's noisy, you might not hear them at all.

I have seen examples of this. In one case, in a listening test at a luthier convention, people in the back of a typical narrow and deep college class room felt the guitar in question was a 'cannon'. I was off to one side, and did not find it remarkable. The player could barely hears it, and the person conducting the workshop, standing about three feet from the player's right knee, said he could not hear it at all. This sort of thing seems to be more common with 'better' guitars.

Hence, the use of ports. A small port the player can see into as they play 'hears' high frequency sounds inside the box that may not get out of the main sound hole, and 'beams' them to the player. The closer the port is to the normal hole the more alike the sound from the two will be, and the smaller the effect of a given size port on the sound out in front.