Tom Blackshear -> RE: new build for 68' Conde style flamenco (Nov. 28 2021 21:03:09)
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Hello. I received a note from an acoustics engineer in San Juan Puerto Rico and thought you guys might find it interesting, perhaps more knowledgeable than I can understand. Hello Tom, I work with Carlos Rodriguez, San Juan Mix Suite, Puerto Rico. My specialty is acoustics, mostly small room acoustics and that's what we do together. My products are the TubeTraps, which I invented and developed. I'm the owner of ASC. I'm an acoustical engineer with masters in engineering(acoustics) and physics. OK, years ago I got the opportunity to develop a lab manual for a physics book called The Acoustics of the Guitar. It was through a grant from National Science Foundation. I spent the summer studying guitar sound. I would like to share with you basically what I discovered. I've discussed these concepts with other builders and they don't seem to relate to them. Sound comes off both sides of the guitar surface. The sound off the front side projects into the open space and people believe that is what they are listening to. But this is only 50% of the sound made by the guitar, the other half of the sound is projected into the guitar body cavity. People think that the sound hole is where this inside sound comes out. Sound hole is Helmholtz resonator to reinforce lowest note of the guitar. It's the fundamental drone tone for the guitar, stimulated by the initial shock of every string pluck, no matter what the string note is. In my research I found the thump tuned "sound windows" on the top of the guitar are located in a unique position relative to the curved interior sides of the body of the guitar. A portion of a curved side reflects and refocuses the sound back to where the tuned sound window is located. This reflected sound arriving at the back side of the sound window is in phase and in time with the movement of the backside of the sound window. Essentially a standing wave is setup inside the guitar body which reinforces the sound being emitted off the top of the sound window. Here's how I found this out. I was using a glass tube with a rubber hose attached to it, plugged into my ear to explore the sound inside the guitars. They were Martin guitars. I put a vibrator on the bridge and did a tone sweep. I heard many different louder and quieter notes. I left tuner on a louder good sounding note and explored the interior of the body with my glass tube. It didn't take long to discover standing wave patterns inside the guitar I used crayons to draw the standing wave patterns I found inside the body, on the top of the guitar. (crayons can be erased easily) I could easily see what was going on, same thing note after note. Different sound windows, and different standing wave patterns. The reflected sound off the curved sides is refocused back to where the sound came from, the bottom side of the thump tuned sound window. This refocused reflected standing wave reinforces the movement of the sound window which gives the top side of the guitar more sound power. The top of the thump tuned sound window makes louder and longer lasting tones because it is utilizing the sound emitted off the back side of the window. It's a method of acoustic feedback that reinforces the sound being emitted off the front side of the guitar. The shape of the guitar is because of where the sound windows are located in the top and where the refocusing reflection off the interior sides are located. I give this to you because Carlos said you share your "secrets" of guitar building, I shared this with Carlos and I decided to share it with you. My son has one of your guitars from Carlos. I'd like to know about your thoughts and/or experiences on this discovery. Arthur Noxon
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