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New Banjo "Leg Rest Amplifier" (?) For Open Backs

Posted by yellowdog on Friday, October 22, 2010

   For the past month I have been using a little modified mailing box with my open back banjo in our  weekly Irish sessions in a local pub.   I open the lid, place the box on my thigh, rest the banjo in the box, (the banjo's rim doesn't touch the bottom of the box but rests on the doubled-over reinforced box ends), and then I start playing.  What  I and everyone else hear is a beautiful and very loud banjo sound with rich lows, bright highs and just the right amount of sustain.  I LOVE this thing!

    This simple little  reinforced cardboard box with its metal attachments is a very efficient sound surface wave amplifier, tone enhancer and audible sound emitter.  It is even a wonderful sounding banjo mute!  When used as a mute the lid of the box is held closed with a rubber band and the box is placed inside the open back banjo on the coordinator rods. It's held in place with the box's rubber mesh bottom touching the banjo head and with the left end of the box held against the banjo rim by rolled up pair of socks on the right.  It travels well inside the banjo with the banjo in its case in this mute position.  

   I have everything that I need to make and sell these "open back banjo box amplifiers" and will offer them for sale soon.  If you don't want to wait or if you prefer you can make your own from the information here.  Either way I'm sure that you will enjoy this "leg rest box amplifier". (It deserves a better name so please let me know your suggestions.)

   You can see a drawing of this box amplifier in the photos section.  You might want to take a look at the drawing and then come back here to continue reading.  I'll also be including more about it in a blog on my website.

      The box is a white-coated crush proof cardboard mailing box with interior dimensions of 5" x 3" x 2".  The flap on the lid has been removed so the lid is 3" high when open.  (It sticks up 3" behind the banjo when the banjo is played.)  Four 13/16" diameter holes are punched (or can be carefully cut) in the front face of the box and two of these have simple metal attachments that both amplify and brighten high and low frequencies of the sound. The metal parts are two one-inch squares of thin steel (can be cut from a steel can), one common steel staple and a 3/8" long steel pan head machine screw with nut, (no washer), available in almost any hardware store.

   These metal attachments were selected to amplify and enhance different frequency ranges of sound surface waves traveling around the edges of the holes and were selected from many alternatives in many trial-and-error tests.  

     The two thin steel squares are good low frequency surface wave amplifiers to surface waves on the inside and outside surfaces of the box. Sound surface waves amplified by the squares meet on the bolt and are amplified again.  The amplified low frequency waves cause the edges  of the holes, and especially the hole with the squares, to ripple, which creates low frequency (mostly) audible sound.

   The single common steel staple is located 1/8-inch above the hole second from the right side of the box.  Because the staple penetrates the box it connects the surface waves on the inside and outside surfaces of the box, especially the very high frequencies because of its small size.  The waves of similar frequencies amplify when they meet.  The staple also forms an amplifying loop for the waves consisting of mostly steel wire plus a small distance of paper between the staple points inside the box.  Any loop is a surface wave amplifier because the waves meet coming from different directions, and a staple is an excellent amplifier to harmonic frequencies which are essential to good sound quality.

   The two holes without attachments receive their amplified surface waves over the paper surfaces from the holes with the metal attachments. 

   What you don't see in the drawing are the 1/4" deep scissor and knife cuts made through the cardboard webbing around each hole.  These cuts free the inside and outside paper surfaces of the cardboard around all holes from the constraint of the webbing so that they will vibrate easily under the influence of the amplified surface waves moving around their  edges.  This flexibility allows the edges of the holes to ripple at high frequencies in the same manner as the high frequency sound surface waves that are moving around the edges.  These moving hole edges create the high quality audible sound that is loud and also beautiful because harmonics, have been greatly amplified and then accurately copied as audible sound.

   The body of the cardboard box also does its part as a surface wave amplifier, (because its flat pattern is a combination of amplifying rectangles), and its large surfaces act as a low frequency sound emitter acting much like a radio or stereo speaker cabinet.  I am old enough to remember the first little transistor radios that came out of Japan after WWII.  They sounded terrible so we kids would place them in or on a cracker box and that made them sound much better.  Paper and cardboard make wonderful high quality sound emitters and should not be sold short because of a bias for gold plated and engraved brass or whatever.  Which reminds me,... yes, we did experiment with sheet brass squares but preferred the sound of the sheet steel.

   In use the banjo is taken out of the case, the pair of socks holding the box in the banjo is removed from the banjo, the rubber band holding the box lid closed is removed from the box and put around the socks and the socks are returned to the case.  The banjo player sits down, places the box on the thigh, (the bottom of the box is covered with a thin rubber non-slip mesh to keep it on the thigh), raises the lid to the vertical position and brings the banjo around the front of the box to rest the banjo pot inside the box.  The banjo doesn't touch the bottom of the box but rests, often with the back edge of the wood rim only on the reinforced (folded over)  ends of the box.  (Two contact points between the box and the banjo sounds great.)  The banjo is played normally.  

     The height of the banjo on the leg with the box is so close to the height without it that everything feels the same and I forget that it is there.  Which has been the only problem!  I keep forgetting that it is there, take a break and come back to find the box on the floor!  I may have to tie a string between the box and the banjo if my memory doesn't improve, but I certainly don't want to do that because sound surface waves will travel down the string and the sound of the banjo will change in some unknown way.

     One final comment.  Expect a lot of questions.  My favorite was, "What is that?  A resonator?".   I said, "Yes!"  I should have said, "Yes, but it's a lot better than a regular banjo resonator because it doesn't create sound quality-killing echos in the sound chamber but improves sound quality and volume below the banjo".  (And then in my early morning wild imagination the next thing he says is, "Why is it on the floor?")

     I'll have these for sale on my website soon.  The website will also have photos on the Photos page and more details about how it works on the Blog Page.  The link to my web site is on the left.




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