| Soundboard Bracing | |
|---|---|
| Top plate | Bottom plate |
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The Early '60's Hummingbird guitar is a flat-top folk guitar with a standard Dreadnought shape. Click here for the specs. The images at right show the rib bracing.
The photo at left is copyrighted by Gibson
In the images and animations below the red square represents the experimentally measured motion of the air in the sound hole. The modal analysis for the sound hole was performed using a microphone instead of an accelerometer to measure the air motion. While we were not initially confident this would work, we were pleasantly surprised to find that the phase and amplitude of the air motion compared to the body matches what we expected for the various modes of the guitar, and also matches what has been reported in the literature.
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Click on the image to download an animated GIF movie In this mode the guitar bends like a free-free beam. This mode probably contributes relatively little to sound radiation and the overall behavior of the guitar. An exception, perhaps, may be slight changes in the string tension as the guitar flexes. |
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Click on the image to download an animated GIF movie In this mode the guitar "breathes" - that is, as the top and bottom plates expand outwards (with opposite phase) air is drawn into the guitar cavity, and as the plates move inwards this air is expelled. This mode results from close coupling between the first vibrational modes of the top and bottom plates with the Helmholtz mode of the cavity. |
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Click on the image to download an animated GIF movie This mode is the first of two modes with closely spaced frequencies (188 Hz and 203 Hz) in which the top plate and the air in the sound hole move together in phase. The only difference between the two modes is the motion of the bottom plate. In the lower frequency mode (shown at left) the bottom moves with slightly opposite phase compared to the top plate, only with much less amplitude. |
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Click on the image to download an animated GIF movie This mode is the second of two modes with closely spaced frequencies (188 Hz and 203 Hz) in which the top plate and the air in the sound hole move together in phase. The only difference between the two modes is the motion of the bottom plate. In this higher frequency mode the bottom moves with the same phase as the top plate. This mode also shows significant bending motion of the neck. |
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Click on the image to download an animated GIF movie This mode looks very much like the second bending mode of a free-free beam. The motion of the guitar body is quite similar to mode #4. The tell-tale difference between modes 4 and 5 can be seen in the motion of the air in the sound hole. In mode #4 the air in the sound hole is moving, with significant displacement, in phase with the top plate. In moden #5 the air in the sound hole is not moving relative to the top of the plate - only the guitar body is vibrating. |
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Click on the image to download an animated GIF movie
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Click on the image to download an animated GIF movie In this mode the top and back plates are undergoing a (0,1) type motion with one nodal line (line of zero motion) across the width of the guitar. The top and bottom plates move with opposite phase (in terms of their outward normal directions) and the top moves with much smaller relative displacement than the bottom plate. The opposite phase motion means that the air inside the guitar is primarily sloshing back and forth, and because the displacement of the bottom plate is greater than the top plate, the air in the sound hole has a net motion in and out of the guitar. |
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Click on the image to download an animated GIF movie In this mode the top and back plates are undergoing a (1,0) type motion with one nodal line (line of zero motion) along the length of the guitar. Actually, the nodal line appears to lie directly along one of the diagonal ribs under the top plate. The top and bottom plates are moving with opposite phase (in terms of their outward normal directions) so that there is very little net motion of the air inside the guitar. As a result the air in the sound hole moves very little. |
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Click on the image to download an animated GIF movie In the (0,2) mode there are two nodal lines across the width of the back plate. The top plate exhibits very little motion. |
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Click on the image to download an animated GIF movie In the (1,1) mode there are two nodal lines, one across the width and one along the length of both the top and back plates. The top and back plates move with opposite phase so that the air in the guitar cavity simply sloshes back and forth. Since the sound hole lies on a node, there is no motion of the air out of the guitar cavity. |
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Click on the image to download an animated GIF movie This vibrational pattern appears to be a combination of two different modes: a (1,2) mode on the top plate and a (1,3) mode on the bottom plate. Because of differences in stiffness and mass between the top and bottom plates, these two modes appear at the same frequency. Because each mode involves several adjacent regions moving with opposite phase, these vibrational pattern will not be an effective radiatior of sound. |