Sound Port Tuning

RobT · #1 11-12-2015, 04:32 PM

After some deliberation, I decided to add a sound port to one of my classical guitars. It is a nice instrument made by a luthier from San Francisco named Alastair Fordyce. It is a pretty typical classical guitar in size, with cocobolo back and sides, European spruce top and Torres style bracing. I added a backing plate to prevent side cracking. The port turned out to be a bit larger than I had initially planned because of a slip with the dremel router. So instead of a sharper oval I ended up with a more rounded uniform oval that is 1 5/16" wide and 2 1/8" long.

I strung the guitar up with new nylon strings of the same gauge and after the obligatory couple days of settling in, I have some observations regarding this guitar. The volume to me while in the playing position has been noticeably increased without any decrease in tone at least to my ears. I don't know if it is psychoacoustic effect or not but the guitar with the soundport open seems to be stronger in the mid range. The bass strings and the treble string seem to be unaffected except volume wise.

My questions to the builders/luthiers or anyone experienced with soundports are the following: Is there a rule of thumb, formula etc. for determining the size of the sound port in relation to the body of the guitar e.g. sound hole size, body depth or size? Are there any thoughts as to the shape of the hole e.g. oval, round and any impact it might have on volume or tone?

Alan Carruth · #2 11-13-2015, 12:09 PM

I've done a lot more work on sound ports than I meant to. As usual with the guitar, things are more complicated than they 'should' be. Basically, putting in a port raises the pitch of the 'main air' resonance. It affects other 'air' resonances as well, but it's harder to say just how in any general way. A port you can see into puts out some sound toward the player that they would not normally hear.

There are two variables (at least) that figure into the pitch change of the 'main air' resonance: the size of the port and it's position relative to the 'main' soundhole. The larger the port is, and the further from the main sound hole, the larger the pitch change in general. This makes it hard to predict precisely what's going to happen.

So far as I can tell, the exact shape of the port does not seem to matter too much. OTOH,I have not really spent any time experimenting with this either.

RobT · #3 11-13-2015, 04:16 PM

Alan, Thank you for your reply. I have read some of your postings on the Delchamp (classical guitar) forum so I am somewhat familiar with some of your experimentation. As the crow flies if you will, the center of my sound port was placed at the farthest point from the edge of the sound hole (roughly 5 inches). I was pleasantly surprised by the increase in volume to my ears as the player but I was also struck by the apparent increase in the mid range from a tonal point of view i.e. the bass strings (E,A) and the treble strings (B,E) seemed to be just louder but the D and G strings seemed more pronounced. Not in a bad way but just seemingly thicker if you will.

It is a shame that with today's technology we don't seem to empirically understand how a guitar works and the impact that bracing, soundholes, top thickness have on volume and tone.

Hence my question regarding the size of the port. I'm going to put a port in CFox small jumbo steel string but I'm going to just do a simple round hole about 1 1/2" in diameter. I'm now primarily a jazz fingerstyle player and I like this guitar (Cfox) for that style of music. This guitar has Charles Fox's double X bracing. Again, thank you for contributions to our understanding of the guitar. rob

Alan Carruth · #4 11-14-2015, 05:41 PM

RobT wrote:
"It is a shame that with today's technology we don't seem to empirically understand how a guitar works and the impact that bracing, soundholes, top thickness have on volume and tone."

Basically we understand a lot about how the guitar works. The problem is that it's not the basic stuff that makes the difference between a 'good' one and a 'great' one. A lot of that is up in the higher frequency range where its hard to get any level of control 'up front'. Add in the variability of wood and it gets pretty complicated, and that just covers the 'objective' stuff. When it comes to subjective aspects, like what constitutes 'good' tone, things get really complicated.

Ned Milburn · #5 11-14-2015, 06:54 PM

Quote:

Originally Posted by RobT

It is a shame that with today's technology we don't seem to empirically understand how a guitar works and the impact that bracing, soundholes, top thickness have on volume and tone.

Further to what Alan stated, we know how it works and we generally know the impact that certain common variables will give, but the guitar soundboard is bombarded simultaneously with different waves and since it is the interference of these wave patterns that matters more than any single wave, and since wave interference patterns can vary greatly with only a slight input change, the "science" behind it cannot be precise and definitively quantitative.

poltad · #6 06-12-2022, 01:13 PM

Do you still have the Fordyce? I am considering buying one, seems well priced, and curious to hear your thoughts.

fitness1 · #7 06-12-2022, 03:06 PM

Quote:

Originally Posted by poltad

Do you still have the Fordyce? I am considering buying one, seems well priced, and curious to hear your thoughts.

The OP hasn't been online here for over a year (7 year old thread) Not likely you'll get a reply.