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Height of bridges

 
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Webbedfingers
Junior Guzheng


Joined: 09 Sep 2010
Posts: 55

PostPosted: Wed Feb 09, 2011 11:58 am    Post subject: Height of bridges Reply with quote

Annk posted an interesting comment about improving the sound of a 51" travel Gu Zheng with higher bridges.

Admittedly I had never considered this, so I went out and bought three new bridges in a shop. Two are extra high ones - higher than my standard no.21 string bridge by about 1cm. The 3rd is just a short one which was the wrong colour so I haven't used it.

I took out the old bridges and put on these extra high ones and retuned them looking forward to some really deep wonderful bass.

Boy was I wrong. All I got was a flat string going "bang" and echoing hollow against the zither. I tried tightening the string another sharp or so, and then tried repositioning the bridge further and closer.

Hmmm. Nothing seems to work. I'm not getting better bass responses from the last two strings - I'm just getting worse responses!


What am I doing wrong? Is it just not possible to push a full size Shanghai Dunhuang (rosewood) this way with higher bridges?

Or are the bridges more demanding when taller, and will need more string tension at a different position altogether?

I've taken those two higher bridges out for the meantime. Will wait and see what to do with them.

Thanks for any help.
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jbpho
Plywood


Joined: 16 Aug 2017
Posts: 19
Location: Seattle

PostPosted: Thu Oct 19, 2017 1:57 pm    Post subject: effects of bridge height Reply with quote

I know it's been years since this first post but the effect of bridge height is a very interesting topic to me, so maybe it interests others as well.

This is a theory, but I think it's not just bridge height, but bridge height, tension, and string angle that control how and which vibrations get passed from a string, to the bridge, and then to the soundboard of the instrument. There's also the quality of the contact between the soundboard and bridge, but let's assume it's always great.

I believe the vibration needs to travel across the bridge in order to sound good. Strike a string on a guzheng like you normally would, but have your left fingers on the left side of the string. You can feel it vibrating, right? Now put your finger on the bridge tip to dampen the sound and strike it again. You get something closer to a "plunk", correct? Move your dampening finger to the right and left until you hear what I'm talking about. You've stopped the vibrations from crossing the movable bridge and therefore changed the sound.

The higher the bridge and the closer the bridge is to the right side, the sharper the angle the string makes with the bridge. Get the angle too sharp and the bridge dampens itself. Instead of a great resonance, you get a flat "plunk". You can see what I mean without moving your bridges.

Look at the straight bridge at the head of your instrument. See where the strings cross the fixed bridge and disappear into the tuning box? Put your finger on the string to the right of the fixed bridge and then pluck the string where you would normally. You don't feel any vibration, do you? The angle is so extreme the vibration can't cross the fixed bridge. It's dampened by the bridge itself.

There's a range that is acceptable for all of these details, but perhaps Webbedfingers' bridges were too tall and the angle was just too high? Or else the design of the bridge was different enough that the string was being dampened and couldn't pass the vibration correctly (strange notch shape, notch too small). The bridge could also just be bad - poorly made, bad materials, bad string rest material, but we can't know that from here.

Anyways, that's my theory. We see violins and cellos and the like with very shallow angles over their bridge. Perhaps guzhengs also function the best when their string angles are shallow.
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