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Make A Great Tone Brightener and Amplifier For Almost Nothing

Posted by yellowdog on Tuesday, September 2, 2014

About a year ago I decided that my banjo which I use for Irish Traditional Music (A modified 17-fret Gold Tone CC "Irish Tenor") was not bright enough and came up with an idea how to brighten the tone.  I was pleasantly surprised to find that the gadget not only brightened the tone but amplified everything to an amazing degree.  As if that wasn't enough, it was also very easy to make, install and costs almost nothing.  You can see pictures of it in the photos section of my homepage.  I have a couple of drawings but am having trouble getting those posted on my homepage.  (I will keep trying.)  I don't have any recordings to prove how well it works so you'll just have to take my word for it.

This new "Brightener/Amp" consists of only three parts:  (1) A 2 inch by 1/2 inch rectangular strip of .010 inch thick brass sheet (steel from a steel can should also work) with a 5/32 inch diameter hole is punched in the center and two small holes drilled along the center line 1/4 inch from each end.  The diameter of these small holes is slightly larger than the diameter of the two steel eyelet screws mentioned next; (2)  Two each small steel eyelet screws with their loops opened to form two hook shapes using a pair of needle nose pliers.  

After the holes are punched and drilled in the brass rectangle the rectangle is formed with the hand into an even curve so that it will act as a leaf spring when the large center hole is placed atop a rim bolt head inside the banjo.  The 5/32 inch diameter hole should be smaller than the head of the rim bolt which, preferably, is round or rounded.  The edge of the hole needs to touch the edge of the rim bolt head only in one location to work, and probably also works best when it only touches at one location.

The opened eye screws are now hooks and are screwed into the wood rim so that the hook ends enter the small hole in the rectangle and the hook shafts are around the ends of the rectangle.  Because the brass rectangle is bent into a curve, the brass rectangle with its center hole resting on the rim bolt head acts like a spring when the two ends of the rectangle are fed under the ends of the hooks.  The spring force of the rectangle will cause the ends of the rectangle to rise until the ends of the rectangle are touching the hooks.  The inside of the small drilled holes will also touch the hooks near the hook ends at one or more points.  You can turn the hooks as needed to get everything to fit.

When everything is together play the banjo and you will hear a surprisingly loud, bright and high quality sound.  If that sound ever degrades simply remove the brass rectangle and reset the curve of the brass to increase the spring force, reassemble and the sound will return.

The theory of why this works so well is interesting.  High frequencies on the head are transferred to all parts of the banjo but are best "managed" by the banjo's metal parts.  These high frequencies are in and on the surfaces of the metal rim where they are transferred to the hooks and then to the shoes and the rim bolts, including the rim bolt heads inside the sound chamber.  The problem is that the rim bolt heads are terrible at transferring these vibrations into the air in the banjo's sound chamber.  This gadget solves that problem by allowing the waves on the surfaces of one rim bolt (called surface waves) to flow onto the brass rectangle wherever the rim bolt head touches the edge of the hole in the center of the brass rectangle.  Meanwhile, the surface waves from the wood rim enter both ends of the brass rectangle at the two locations near each end where the edge of the hooks touch the drilled hole edge and the edges of the two ends of the rectangle.  These waves have the tonal "character" (timbre) of both the wood of the rim and the steel of the hooks, but they have the same frequencies of the waves entering the center of the brass rectangle from the rim bolt.   Everywhere these waves meet on the brass surfaces (top and bottom) of the brass coming from different directions they will add (amplify) because they are the same frequency or of similar frequencies.  The rectangular shape of the brass is also an amplifier to waves on its surfaces when these waves reflect onto themselves from edges.

The result of all this wave motion is that the brass rectangle vibrates considerably under the influence of the amplified high freauency and low frequency waves on its surfaces and moves a lot of air in the sound chamber to create a lot of new sound.  Because so many high frequencies are amplified so are harmonics which causes sound quality to be noticeably better also.

To my knowledge there is nothing like this being used today.  I don't intend to patent it or make them to sell, so if you want to make them to sell to others or just one for your own use that is fine with me.

One additional thought.  I wouldn't be concerned with damaging the wood rim with the eye screws.  The screw threads are so small that the holes are barely noticeable and are easily filled.  You can even start the screws without drilling.  Just pushing a sharp small pick into the wood rim is all that is required for the tiny threads to catch.

Frank Geiger (Yellowdog)



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