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Advanced Banjo Design

Thursday, October 4, 2018

Advanced Banjo Design 

By Tom Nechville 2018

note: A condensed version of this article is followed by a needlessly complex version. 

This article  goes beyond basic banjo setup and seeks to explain the difference between standard banjos and Nechville’s helically mounted banjos.  If we want to judge sound differences between the two types, we must determine what effect the mechanical differences impart upon the sound. Being very subjective, each players’ opinion as to what constitutes “best” may vary. While it is difficult to define the positive tonal aspects of the best banjos that truly tug at our emotional heartstrings, it is obvious to most players when a negative sound such as sonic dissonance or a mechanical buzz is present.  Likewise bad intonation caused by imperfect fret or bridge position is measurable attribute that we can minimize with good building and set-up practices.

 

String vibration is a natural phenomenon that delivers a fundamental note plus  sub vibrations of the string called harmonics.  It is the job of the bridge,  head and tone chamber, or  “banjo pot” to transduce the string's vibration into the sound that we hear. Complex variations in the pressure applied upon the stretched membrane through the bridge,defined by the string's harmonic signature, compresses and stretches the air on both sides of the head, causing sound waves, which then get further processed by the instrument’s body. 

 

It is the intent of this article to consider just what are the sources of unwanted frequencies or dissonance that can interfere with the purest sound that a banjo can make. 

 

The tradition of simply using our ears and a long process of trial and error has given us the banjo we use today.  What if we wanted to design a banjo of pure sound, without distractions from unwanted resonances? A quick look back into banjo building history may shed some light on this.  

 

The mechanical hardware in a common bluegrass banjo consists of a 24 hooks, nuts, metal flange, metal tension hoop, 2 metal rods acting as bolts and stabilizers for the neck and body connection, plus the hardware needed to anchor strings to the body. All these metal connections were not particularly designed to effect the acoustical tone but they can and do impart a metallic nuance to the regular banjo's sound. 

 

In our ideal virtual design of the banjo pot, we run into a complication when we bolt the neck to the tone ring and pot. Scientifically as well as intuitively, we want to find a design that separates and frees the acoustical components from the Mechanical/structural ones. 

 

An advancement in banjo design in the 1920's allowed the tone chamber to nest into a one or two piece flange that allowed for fewer metal hardware attachments directly to the cylinder of the tone chamber. Fewer mechanical attachments to the tone chamber must be a factor in generating a better tone, since banjos based on the Mastertone” of the 1930's continue to dominate even in the bluegrass of today. The Gibson design, however, still depends on the neck being bolted directly to the tone chamber with a dual coordinator rod system. 

 

It was not until the mid 1980's that Tom Nechville brought us Helically mounted banjos, or Heli-Mounts. In the world of Nechville, the neck bolts to the integrated hardware of the Heli-Mount frame.  This allows for insertion of the tone chamber from the back of the frame and is compressed in place without any metallic stress or unwanted resonances. The Heli-Mount yields automatically even head tension while extending the rationale of the Mastertone design to its ultimate fulfillment. 

 

More information on Nechville and banjo design is available at 

 Www.nechville.com. 

 

If we want to judge banjos by sound quality, we must decide which aspects to compare. Being very subjective, each players’ opinion as to what constitutes “best” may vary. While it is difficult to define the positive tonal aspects of the best banjos that truly tug at our emotional heartstrings, it is obvious to most players when a negative sound such as sonic dissonance or a mechanical buzz is present.  Likewise in-accurate string tuning or imperfect fret or bridge position is another measurable attribute that we can minimize with basic set-up practices. This article  goes beyond basic setup and explores deeply the main difference between standard banjos and Nechville’s helically mounted banjos.  

 

Besides our own sensitive ears, we have electronic meters we can access on our smart phones that measure and display the harmonic content of notes played into it. In a normal vibrating string, for example take A = 440 cycles per second. Accepted frequencies include all octaves (880, 1760, etc) plus the frequencies you get when you divide the vibrating string into 3rds, 5ths, 7ths and their corresponding octaves. To sum up, The harmonic content of a vibrating “A” string creates the full bodied chordal content of an A7 chord all within the sound of one vibrating string.*  Dissonant harmonics are any frequencies that show up that fall outside the accepted frequencies for that note. 

 

 String vibration is a natural phenomenon that we can not change. It is in the transduction of the string's vibration into sound energy that determines what our ears hear. The instrument is the transducer. Complex variations in the pressure applied upon the stretched membrane through the bridge, caused by the string's harmonic signature, compresses and stretches the air on both sides of the head, causing sound waves. If we begin the thought process with the simplest theoretical banjo,  you have a vibrating string  anchored at both ends, a tensioned diaphragm serving as a soundboard and a bridge between these components to transmit the string vibrations into soundboard vibrations. 

 

It is the intent of this article to consider just what are the sources of unwanted frequencies or dissonance that can interfere with the purest sound that a banjo can make. 

 

The tradition of simply using our ears and a long process of trial and error has given us the banjo we use today. What would happen if we used our current knowledge to virtually design a banjo? I say we would look for a sound purely of the banjo without distractions from unwanted resonances, but with the pleasant attributes that can be imparted by the instrument’s tone chamber.  

 

We need to start by defining the variables that we want to measure while virtually constructing the ideal banjo. 

 

As described above, the string's vibration itself is determined by nature, however the tonal characteristics and amplitude of the resulting sound depends on the mass and type of material of the string. Let's make the string steel and use as a constant the properties of a traditional light gauge banjo string. If we also standardize the mass of the bridge and assume no dissipation of energy (the subject of a separate bridge article), we further simplify our analysis of the instrument itself. 

 

In evaluating the many structural variables involved in making the most efficient banjo we likewise can gain a better grasp of what is important to our design by agreeing on several more constants. Certainly the amount and even-ness of potential energy present in the tension of the banjo head profoundly affects the volume and audible presence of the harmonic content of the string, however if we assume perfectly even, standardized head tension (only really possible in helically mounted banjos), we again simplify our analysis. 

 

This leaves us with the means through which it becomes possible to accurately play chromatic notes upon the fretboard. The neck represents the majority of the banjo builder's influence over the accurate delivery of the notes in our standard musical scale. The composition and quality of the neck does impact the quality of the sound, but for sake of simplicity, we shall assume standard stiffness  and perfectly placed frets, ideal action and resulting perfect intonation.  

 

These are big assumptions when you realize that most banjo problems originate here. Nechville banjos have an exclusive neck connection and 2-way truss rod that insures ideal action as no other brands do. 

 

Now that we have standardized the neck, head, strings and bridge, what's left? You guessed it.  It is the construction of the tone chamber or body of the banjo. 

 

While the neck is a critical structural part of the banjo that determines accurate pitch and note clarity, the body contributes much of the instrument's tonal characteristics. This is where the builder's influence becomes one of art.  So far we have made constants out of many banjo parameters that you can alter to yield predictable results, but essence of the instrument's tone depends primarily on how the pot, body, or tone chamber is constructed. 

 

Much attention is given to the hoop of metal upon which the head is stretched, called the tone ring.  In some banjos there may just be a wood rim that doubles as a tone ring. Here again we have an opportunity to create a constant. While different tone rings will impart noticeably different aesthetics to the tone, let's assume as a constant the properties imparted by a usual cast bronze tone ring and 3- ply maple rim combination. 

 

This leaves one more interesting consideration. What about the means by which we apply tension energy to the head? In traditional banjos the tensioning hardware completely surrounds and attaches to the tone ring and rim. Nechville Helical mounting differs in that the hardware frame serves to stabilize the structure of neck and body while creating a means for maintaining even head tension without nuts and bolts. 

 

The structural integrity of a banjo contributes to the sustaining duration of notes and helps prevent dissipation or loss of sound. Traditional banjos have evolved into structures consisting of thick heavy rims and heavy tone rings bolted directly to the neck. Such a solid construction contributes to bright tone, yet by default integrates the structural and mechanical components together with the tone ring and rim. In designing our ideal virtual banjo, the tone chamber made by tone ring and rim should be considered an “acoustic” component vs. “structural” since its composition and shape do influence the tone we hear. 

 

The mechanical hardware in a common bluegrass banjo consists of a 24 hooks, nuts, metal flange, metal tension hoop, 2 metal rods acting as bolts and stabilizers for the neck and body connection, plus the hardware needed to anchor strings to the body. All these metal connections were not particularly designed to effect the acoustical tone but they can and do impart a metallic nuance to the regular banjo's sound. 

 

The remaining variable now becomes one of how the hardware works. In the world of virtual design of the banjo pot, we run into a complication when we bolt the neck to the tone ring and pot. Mathematically as well as intuitively, we want to find a design that separates and frees the acoustical components from the Mechanical/structural ones. 

 

An advancement in banjo design in the 1920's allowed the tone chamber to nest into a one or two piece flange that allowed for fewer metal hardware attachments directly to the cylinder of the tone chamber. Fewer mechanical attachments to the tone chamber must be a factor in generating a better tone, since banjos based on the Mastertone” of the 1930's continue to dominate even in the bluegrass of today. The Gibson design, however, still depends on the neck being bolted directly to the tone chamber with a dual coordinator rod system. 

 

It was not until the mid 1980's that Tom Nechville brought us Helically mounted banjos, or Heli-Mounts. In the world of Nechville, the neck bolts to the integrated hardware of the Heli-Mount frame.  This allows for insertion of the tone chamber from the back of the frame and is compressed in place without any metallic stress or unwanted resonances. The Heli-Mount yields automatically even head tension while extending the rationale of the Mastertone design to its ultimate fulfillment. 

 

Tom Nechville continues to advance the Banjo Revolution from his Bloomington Minnesota location. More information is available at Nechville.com.

 

*The 1st harmonic vibrates at 880, the fundamental's octave. The second harmonic is produced by 1/3 of the string's vibration which is 1320 cps, combining a 5th interval with the fundamental. The 3rd harmonic, 1760 is the next octave of the fundamental made by 1/4 of the string's vibration. 4th harmonic is 1/5 of the string's vibration and adds the 3rd interval of the fundamental to the harmonic content of that one vibrating string, completing a major triad (1st, 3rd and 5th notes of the fundamental's major scale). The 5th harmonic obviously is double the frequency of the 2nd and adds the next higher octave of the 5th interval. The 6th harmonic divides the string into 7 parts and the resulting note corresponds to the flatted 7th interval, often called the dominant 7th

 

 

 

 

 

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www.nechville.com
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