The Erhu



 All amplification of stringed instruments is achieved by using a sound box. All sound boxes have a few things in common: they are hollow, they contain openings, and they are always in close proximity to the strings. Some plucked instruments like guitars do not have a direct connection from the sound box to the string, but for ALL bowed instruments, there is a bridge that connects the soundbox to the string directly.

It is very important to note that the word "amplification" in this sense does NOT refer to true amplification, which is when additional energy is put into a system; all the energy, whether the sound box is there or not, comes from the bow. In this case, then, amplification refers to the more efficient conversion of mechanical energy (bow against string, string vibration) to actual sound energy. By itself, a string does not create much sound; it is so slender that it can't move much air. Think of trying to pluck or bow an electric guitar without the amp plugged in; the sound is very soft. Sound boxes convert the mechanical energy of the string's vibration to sound energy by utilizing larger surfaces in order to push greater quantities of air around.

In the violin shown below, we can see the relatively large sound box in comparison to the entire instrument as well as the larger bridge. However, the openings, or f-holes, are much smaller. The special shape the case is built in creates many resonance frequencies that usually coincide with the fundamental frequencies of the strings.

Now, the soundbox on an erhu looks like this:

It is much smaller in comparison to the size of the body, and the main body (the wood part) does not vibrate like a violin. Instead, only the taut python skin, connected to the strings by a small bridge, actually vibrates. It cannot be seen here, but the back of the sound box is completely open. It is important to keep the back open because, if it were closed, you would have a "closed-closed tube" resonance system and therefore half of more of the total sound energy wouldn't ever really make it out of the instrument; you are left only with the sound coming off the front.

In an erhu, the way sound is made can be likened more to a speaker; the python membrane vibrates at the same frequency as the string, pushing air and creating loud sound waves. The actual bamboo box serves as an "open-closed tube" system that amplifies certain resonance frequencies, but it is mostly the python membrane that amplifies the sound.

The bridge serves two main purposes: it provides the clearance necessary to bow the string, and it also helps to transfer vibrations more effectively to the sound box. The vibratory energy from the string without a bridge creates minimal sound waves; it dissipates quickly and only very little energy hits the membrane. The bridge serves to conduct vibrations through a solid and focused medium directly to the resonating membrane, focusing more of the energy onto the part of the instrument that actually creates the majority of the sound.

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