How Banjo Works
The acoustic properties of the banjo have been subjected to very little scientific study. The few studies that exist have used the five-string banjo. Dickey used a structural dynamics model to simulate the effects of features such as loss factor, head tension, bridge mass, and string excitation location on qualities such as loudness, brightness, and sound decay.
He showed that his model predictions agreed well with accepted banjo setup practices. Rae and Rossing published some of the first performance data obtained from sound and vibration measurements from real banjos. Stephey and Moore studied banjo bridge impedance and head motion using electronic speckle pattern interferometry.
Classification of Banjos
Banjos come in three different classifications. There is a four-string banjo, which usually has a resonator attached and is popular in ragtime and Dixieland jazz music. It is usually played with a flat pick in a strumming or flat picking style.
There is an often resonator-less five-string banjo used in old-time music. It is often played in what is called claw hammer style where the strings are picked and brushed with the fingers, often without the aid of picks. The third style of banjo is also a fivestring banjo, but it includes a resonator. It is generally played with finger picks and is popular in bluegrass music.
Much of the uniqueness of banjos stems from the fact that many of their important parts are not made from wood. Rather, they are made from various metals, which have acoustic impedances quite different from those of wood.At several key locations on the instrument, vibration transfer must occur across wood–metal or metal–wood interfaces where the mismatch of acoustic impedance is expected to be large. In addition, the major sound-radiating surface is not wood. Rather, it is a thin, tightly stretched membrane made of Mylar, which is more responsive than typical sound-radiating surfaces on other string instruments. On banjos used for playing bluegrass music, the back surface of the instrument is a curved resonator made of laminated wood secured to the upper part of the banjo by way of four metal bolts.
With the resonator in place, the instrument has an enclosed air cavity, as found on many other string instruments. Another unique feature of the banjo is that most of its parts are not glued together and are amenable to modification (tuning) after the instrument has been constructed. For example, the Mylar head is attached to the instrument by a hoop and 24 hook-bolt devices that can be used to change the tightness of the head over a rather wide range. The bridge is not attached so it can be changed with ease. There are many bridge designs constructed of many different kinds of material, and they offer increased flexibility for changing the banjo’s sound with a minimum of effort.
Many banjos have adjustable tailpieces that can alter the angles that the strings make over the bridge, thus allowing adjustment of the magnitude of the string tension force vector that is directed downward toward the head. It is also possible to use adjustable-height bolts (called Raejusters) to change the spacing between the resonator and the upper part of the banjo so that adjustments of the frequencies of the cavity resonances can be made in a matter of seconds. Most of these adjustments have not yet been studied with good scientific methods, so the observations are anecdotal, without good theoretical models to predict what is measured. This chapter stems from the author’s post-retirement hobby.
While a few of the studies were done in the laboratory of Dr. Tom Rossing, most were done in the author’s own basement. The results presented are part of a series of trials done on experimental banjos constructed by the author from a large number of quality parts. Many of the parts were custom-made to the author’s specifications by well-known banjo parts makers. The findings were compared to those from six high-quality commercial banjos in the author’s personal collection.