The Erhu


 

Bowing

Lots of instruments are bowed. Violins, violas, cellos, basses, and all types of huqin including the erhu use a clump of horsehair, dragged across a string. So why does bowing produce a sound at all?

First, let's look at the basics of how sound is produced by a stringed instrument. Without amplification, the string does not produce much sound at all; to see how the sound is made louder by using a sound box, visit the Amplification section. The first step to achieving a sound, however, is making the string vibrate.

The easiest way to achieve string vibration is to pluck it. In this case, dampened harmonic motion is achieved. Dampened harmonic motion is similar to simple harmonic motion, except that the amplitude of the oscillations decreases over time. Imagine that the diagram below describes the amplitude of a string's vibration after being plucked.

As a string is plucked, it vibrates side to side with a large amplitude, and then gradually returns to its stationary position when it has lost all of its energy. This quick burst of vibration transmitted to the sound box causes a momentary loud sound which decays very quickly.

With a bow, the sound waves are much more complicated in shape as the fundamental as well as the harmonics are all sounded. The vibration of the string is determined by the interaction between the bow and the string; this is all about static versus sliding friction. The horsehair itself doesn't have much grip on the string, so rosin is used to increase the friction between the two interacting surfaces.

Static friction, however, is ALWAYS greater than sliding friction, which is the property behind the concept of bowing.

When the bow drags across the string, it pulls the string slightly to the side, because static friction is greater than the horizontal component of string tension. At the point where the horizontal component of string tension exceeds static friction, the string travels back to its original position and further the other way, because the string tension creates a tendency toward harmonic motion; the string tension has to be greater than the sliding friction force during this period, as the bow has to slide across the string.

But instead of the harmonic motion dying out, at some point the the horizontal component of tension will get small enough due to the decreasing amplitude of the harmonic motion and thus the sliding friction becomes greater; the string stops on the bow, and static friction takes over.

This cycle, repeated numerous times per bow stroke, produces a complicated wave pattern. The movement of the string as the bow slides across it in this "slip-slide" manner is shown to the right.

 

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Copyright 2008 Harvest Zhang & Karen Kaminsky. All Rights Reserved.