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I am a junior in high school and am participating in the science fair. My project is "how does the guitar get its sound?" can anyone help me?

I am a junior in high school and am participating in the science fair. My project is "how does the guitar get its sound?" can anyone help me?

Hmmm. Tough question. Basically, the strings (when vibrating) bend the bridge back and forth thus deforming the guitar top. The guitar top acts as an "amplifier" and the body of the guitar acts as a reflector and concentrator. Think about how drums work. Same idea. If you doubt this, strum a solid body electric guitar. I trust you will get better answers to this...especially if someone here thinks I got that wrong.

Mike

Kristi, Welcome to OLF. HERE is a good description of how a guitar works. This is about as good a description as I have ever seen, and it will let you get as much detail as you want. I hope it helps.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Kristi,

Try this:

http://www.phys.unsw.edu.au/music/guita ... intro.html

I found that site really interesting.

-Dave

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David Morris

Todd Kristi started two threads with the same title. Your disappearing post never disappeared at all - you made it in the other thread where it still lives.

I am about to merge the two threads so that everyone's replies are in one place.

Thanks

I have always thought it is Tone Faerie but there are many that present different ideas...

http://www.kettering.edu/~drussell/guitars/hummingbird.html

A couple in pdf format,

http://online.physics.uiuc.edu/courses/phys498pom/NSF_REU_Reports/2002_reu/Adam_Kempton_REU_Report.pdf

http://www.speech.kth.se/prod/publications/files/qpsr/1991/1991_32_4_011-018.pdf

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"It's a Tone Faerie thing"
"Da goal is to sharpen ur wit as well as ye Sgian Dubh"
"Sippin Loch Dhu @Black lake" ,Kirby O...

This is a far bigger project than you might think, and I fear you're going to find a certain amount of disagreement amoungst us luthiers as well. This is partly a symptom of the fact that we don't understand it all as well as we'd like, but it's also true that the people who know a lot about the physics of it simply haven't been communicating well with the people making the instruments.

I'm going to start by saying that I don't believe that the bridge rocking motion is a major producer of sound. There are several reasons for this.
1) I've measured the forces that a plucked string imposes on the top of the saddle. The two main ones are a transverse force and a tension change force. The transverse force is simply the result of the fact that the string is moving up and down, and something has to oppose that. The tension change signal arises because the string gets a little tighter when it's displaced from being straight, and tugs the top of the saddle toward the neck more strongly than usual. It turns out that for most strings the tension change signal is much smaller than the transverse one.
2) Guitar tops are built to resist twisting of the bridge toward the neck. They have to be if the guitar is to last for any length of time with strings on it at tension. It's much easier to move the bridge up and down than it is to twist it toward the neck, particularly at low (below about 350 Hz) frequencies.
3)The tension change signal is an ocatve higher than the transverse one, because the string is displaced off to the side (and thus tugs on the saddle top) twice for every cycle of vibration. If the tension change/bridge rocking signal accounted for most of the power, then flat top guitars would sound an octave higher than archtop guitars. Since the strings are not tied to the bridge on an archtop, it doesn't 'feel' anywhere near as much of the tension change signal.
4) Finally, I've done the experiment. If you have access to a signal generator you might try it. One way to drive a string into vibration is to make an electric motor of it. Get a small magnet and rig up a way to hold it so that a string runs between the poles. Use a signal generator to pass an alternating current through the string at the frequency the string is tuned to (if you're doing this with the low A string, that would be 110 Hz). This is a bit tricky, since you have to hit the frequency exactly, but it can be done.

When you do this you might expect the string to vibrate such that it gets closer and farther from the poles: if the magnet is the letter "U", you'd think the the string would be moving left and right. In fact, it moves up and down; toward and away from the bottom of the "U". You and your teacher can figure out why. The neat thing about this is that you can make the string go in any direction you want: up and down relative to the soundboard of the guitar, or across the top, simply by rotating the magnet.

What I found when I did this is that there was much more sound coming out of the guitar when the string was moving up and down that when it was moving crosswise. If the tension change/bridge rocking mechanism produced the sound then you'd expect the level to be the same with the string going in either direction, since the tension change will be the same. The fact that much more sound was produced in the up and down vibration direction shows that the transverse force is the major sound producer.

BTW, there's another way to get the string to vibrate in a specific direction, but it's a bit less reliable. Get some fine magnet wire, say about #40 or #42. Loop a short length behind a string on a guitar, and, holding both ends, pull it until the fine wire breaks. This will 'pluck' the string in the direction of the pull, if the wire breaks right behind the string. It's more reliable with smaller strings, because of the sharper bend. Agan, you can try pulling the strings in different directions and listening to the sound they make. You can also record the sounds on a computer and analyse them later with a Fast Fourier Transform (FFT) program. There are a number of these available, some for free, and they are really handy for this stuff.

Good luck, and watch out: I got started on all of this doing a science fair project on sound, and I've been at it ever since. This is addictive stuff!

As always Allen, I really appreciate the way you take the time to relate to others what you have discovered.I guess that explains one of the reasons for putting a pressure sensitive device in the bottom of the saddle slot for electronic amplification.Chris.

I would like to thank everyone for their help with my science project. I got a lot of good websites to look at and good information from the posts. Yes, this project is turning out to be more than I imagined.

Thank you once again, Kristi