Abstract

What do a guitar, a piano, a harp and a violin have in common? Turns out a couple of things, including a soundboard. All stringed instruments use a soundboard to amplify (greatly increase) the volume of the sound coming from vibrations of the strings. The soundboard is positioned so that it gathers the sound vibrations coming from the strings and then retransmits them at an even greater volume when it begins to vibrate. Soundboards are common in the world of musical instruments, but they can even be found in nature. Did you know that you can hear the chirps of little black crickets at night thanks to small soundboards built right into their bodies? Yes, to increase the volume of their chirps, the crickets lift their wings to a 45-degree angle, creating a soundboard!

In this music science fair project, you will investigate how positioning affects the ability of a soundboard to transmit sound. In certain positions there is more damping of the soundboard, which results in a lower amplitude of sound. You will need a personal computer, either laptop or desktop, with a built-in microphone; or a tape recorder with a visual display of incoming signals. If you are using a personal computer, the Bibliography provides a link to free software that will allow you to look at and compare your sound recordings. You can choose any instrument with a soundboard to do your testing, but general instructions are given here for an acoustic guitar:

1. Sit in a chair a few feet away from the microphone with your guitar.
2. Hold the guitar snugly so that the back of the guitar body is making contact with your chest and stomach, and strum a chord several times. Pay attention to how hard you are strumming, and try to strum with the same force each time. Record or observe the amplitude of the signal.
3. Now hold the guitar in the "folk position" (also called "casual position") with the bottom of the body resting on your right thigh, and your right forearm and upper right chest contacting the top part of the guitar's body. There should be about 6 inches of space between the soundboard and your torso so that the soundboard can vibrate. Strum the same chord several times again, with approximately the same force as the first time, and record or observe the amplitude of the signal.
4. Now hold the guitar in the "classical position" with the bottom of the body resting on your left thigh, your left foot elevated with a foot stool or books, and your right forearm resting on the top of the body. Tilt the body of the guitar back until it contacts your chest. Again, there should be about 6 inches of space between the soundboard and your torso, so that the soundboard can vibrate. Strum the same chord several times with approximately the same force as the first time, and record or observe the amplitude of the signal.

In which position do you have the least contact with the guitar's soundboard? Which position produced the greatest amplitude of sound?

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