Official Luthiers Forum!

Well, with all of the talk of sound ports lately I decided to do some experimenting on my own. I cut three oval shaped ports in three bodies. The oval is 1" wide x 1-3/8" long. One was cut above the waist on the bass side, the next one was cut on the apex of the upper bout shoulder and the last one was cut close to the neck block. The one closest to the waist is the easiest to hear in the playing position, while the neck block is the hardest to hear. All of them raised the (Helmholtz resonance) or pitch of the sound box equally and seemed to slightly dimish the sustain (I think). All of these are just boxes and are not strung up yet so I don't know what the exact effect on the sound will be. All my observations are just from tapping on the top at the bridge location. All of ports will easily blow out a lit match!

Al Carruth or other experts out there: If the Helmholtz frequency is raised will this result in a loss of bass timber once the box is strung up? While covering the port the bass comes back but it seems to be quite a bit of bass loss with the port open.

_________________
tim...
http://www.mcknightguitars.com

Tim...I have been wanting to start experimenting in this area for some time. I believe Ervin Somogyi has done some research in this area. You may want to email him. Please keep me posted. Thanks!

_________________
Dave Bland

remember...

"If it doesn't play in tune...it's just pretty wood"

I guess it all depends on what you mean by 'loss of bass timbre'. There will most likely be more power output in the range of the A-0 resonance, but it will certainly be at a higher pitch.

The A-0 mode is the lowest frequency one that can radiate sound effectively. Any lower resonance, like the 'neck mode', will be moving a surface so small relative to the wave length of the sound that it won't put out any appreciable amount of power. The air resonance pumps a lot of air, even though the hole is small relative to the wave lenght, so it gets by.

Any note you play that is below the 'main air' resonance will tend to lack power in the fundamental of the tone. This can make it sound 'nasal', and, although it might 'cut' well, the timbre can be a little 'thin' for many tastes.

Of course, the ability of the A-0 mode to support radiation of sound also decreases rapidly as you get above its pitch as well. The next resonance that can help out is the 'main top' mode, frequently about an octave higher. If the 'main air' or 'main top' mmodes have particularly narrow effective bands there can be quite a 'hole' in between, and in that case moving the 'main air' resonance up a little can help fill things in. This is usuallly not a probloem on flat top guitars, but might be on an archtop. most archtops have thier 'main air' resonance 4 to 6 semitones higher than the usual flatop. It's one of the big differences in the way they sound.

IMO it really doesn't pay to have the main air resonance end up as low as the fundamental of lowest note you're playing, and usually a second or third higher works out best. Much higher than a fourth above the lowest note also seems to be less desireable: I've measured some good Classical and Flat top guitars with the air resonances around A=110, but much higher than that seems the province of really cheap and tinny boxes. That's just my 'take'.