The Luthier's Handbook Virtual content
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Height of string anchor point on fixed bridge guitars: The question arose about changing the height of the string harness on classical guitars to modify the "string break angle" on these instruments. The string break angle is the angle the strings make as they go over the bridge. You can absolutely control the string break angle by raising or lowering the tie block on classical guitars, or the pin contact point on steel string acoustic guitars. However, the string break angle is really not critical on fixed bridge instruments (such as the bridges on steel-string acoustic, classical, and flamenco guitars). On these instruments, the bridge and soundboard produce sound* by the bridge being rocked back and forth, with the bridge/saddle acting as a lever. The critical factor in this lever action is the height of the top of the saddle above the soundboard. Whether the strings are anchored at the height of the saddle, somewhat below the saddle, or directly down at the soundboard (each of which changes the string break angle) doesn't matter. What matters is the height of the contact point above the soundboard. So, as you change the height of the saddle (and adjust the neck angle accordingly, of course), you greatly change the torque moment applied to the soundboard.
*As the bridge rocks back and forth, the soundboard causes compression and rarefaction inside the air chamber, and that is what generates the "sound" that we hear.
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String break angles, Chapter 5, Bridges: Some questions have arisen about down pressure problems (on movable-bridge instruments) that result from improperly set necks whose neck pitches are above 6°, or from improperly carved soundboards that result in string break angles over 18°. Click here for more about String-break-angle loads.
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