thereshopeyet wrote:
Wrench Wrote:
I tune the top and back by altering the mass (by either adding mass or shaving the braces) to obtain the nearest natural frequencies that are prime numbers.
Wrench that sounds involved ! It would be interesting if you would elaborate a little.
How do you go about accessing the braces on a built guitar to carry out such work.
Also, how do you determine wether to ad or remove mass from the braces.
If you need to add mass what do you add and how?
Dermot
Hi Dermot,
The heavy lifting of this whole process is a working knowledge of spectrum analysis. The modern developments that make it possible for anyone to play with this stuff at home or in a small shop are cheaper computers, Audacity (free software), and cheap microphones. When analyzing guitars for sound, you need to watch the mic you use. Most vocal mics have a frequency range beginning at about 100Hz. To analyze guitars, you need a mic that can go down to about 50 Hz or even lower. The mic I use has a cheap Radio Shack capsule that has flat response from 30Hz to 30KHz.
Audacity has an "Analyze" function, which performs FFT (Fast Fourier Transform) on a sound sample over a time domain. At the end of all this, you get the frequencies and amplitudes of the component sounds in a sample. I started doing this about four years ago to measure the effect on sound of doing things to guitars like saddle changes. It yielded so much information, I kept studying and collecting data.
So with a minimal investment in free software and a cheap mic, You can apply some of this technique when:
a guitar warbles or beats in response to an open strum
a guitar exhibits a Wolf Note
when you want to know the fundamental sound signature of a guitar
The technique of collecting the sound for analysis is the tap test. This is a controlled impact to the guitar in these specific locations (with strings muted):
the center of the bridge with mic at the bridge
the center of the bridge with mic at the soundhole
the center of the top at the lower bout with the mic adjacent
the center of back at the lower bout with the mic adjacent
the center of the back at the upper bout with the mic adjacent
I make the impact with a small rubber ball on a twelve inch string at a six inch distance. The lengths are arbitrary, and identified only for repeatability. I analyze the sound samples in Audacity, and I now know what are the natural frequencies of this guitar body, and how responsive it is. From here, I try to coax all of that potential out of the guitar body with materials, joins and fits, etc.
So to your question, how do I determine which way to go with mass, and how much mass? The other relevant question which you didn't ask but I answered above is why address the mass at all? I will tell you that from the information I collected over the last four years, a guitar is born with a voice, and changing that voice has a very low percentage of success, so I discourage thinking you can significantly change a guitar's voice for the better. But is likely that somewhere in its life, a guitar may get a sore throat, and it is possible to clean up its tone a little. As I previously noted, a warble in response to an open strum may indicate a trim is in order. It may also indicate the strings need tuning, too, so check that first! The other problem that can be improved is the wolf note, which often happens on dreadnoughts around the low G. This is where it thuds instead of rings.
Now this is where I part ways with some very famous luthiers. I have had discussions with and read the writings of these luthiers on the subject of the three main natural frequencies of a guitar: the main air, the top, and the back. The recognized experts (one of which I am NOT) tend to trim these frequencies to specific and absolute values during the construction of an instrument. My opinion is the mathematical relationship between these is more important than the absolute frequencies. Recognized luthiers tend to trim these frequencies to conicide exactly with musical notes. I disagree with this approach for two reasons: first, this will tend to make the guitar voice these notes at much higher amplitudes (or kill them in the case of the wolf note); and second, the least common multiple of these frequencies will introduce a beat frequency that cycles within the time domain of a normal decay. By adjusting the frequencies to prime numbers, a beat frequency still occurs, but at an interval much longer than a decay period. Depending on the combination of natural frequencies, this interval can be never be any less than about thirty seconds and on up to several minutes. I have never received a comment, either positive or negative, on this theory, but I have been applying it successfully for about two years now.
OK, back to mass. Let's say I have a guitar with a bad warble. I analyze the sound signature and find both the top and main air, or Helmholtz frequency are both even numbers. I find the nearest prime number from the top frequency is 3Hz lower, and the nearest prime to the main air is 1Hz lower. When altering mass, adding mass decreases the natural frequency and subtracting mass increases the natural fequency. The bad news is adding a lot of mass reduces volume and responsiveness. The good news is the natural frequency of a top changes with insanely small changes in mass. More good news is that as you reduce the top frequency, the main air frequency comes down a little, too.
Just a large change in humidity will change the mass enough to alter the vibration signature of a top! I make most of these trims with about 2 to 5 grams of mass. Sometimes it's even possible to do this by changing the material of the bridge pins. For this reason, I prefer to make alterations by adding mass with small wood clamps attached to braces. When I need to remove mass, I determine the frequency change per unit of mass, then remove material from the braces and weigh it. All the work is done through the sound hole. So in our test guitar, I need to add some mass to the top to reduce its natural frequency by 3 Hz. I will tape a coin to the center of the bridge, then take a tap test and analyze it. I will find the correction mass in two or three passes, then make a wooden clamp of the correction mass and attach it to the X brace intersection. I sometimes glue the correction mass to the bridge plate (if I know the change is not weather and must be permanent). If the main air needs further trimming (and it usually doesn't because the guitar was fairly well trimmed at its construction), this is done by altering the opening of the sound hole - opening reduces frequency, and restricting it increases the frequency. The back is done with the same procedure as the top.
Now you know why my head was spinning in complete circles when wiley wrote about his experience with the O-Port! The mass of the O-Port is most certainly altering the vibration signature to a significant degree, but in a random way since it is not tuned to the specific guitar, and the mass is most certainly excessive.
And then there's the wolf notes, the top to back phasing issues, and the environmental factors, and the....................AAAARRRRRGGGGHHHHH!!!!!
OK, this should ruin your evening. And maybe a spare guitar.
Dan