Resonator banjos include a wooden box in the back of the body. The idea is to project more of the sound from the strings forward, toward the listener, increasing the banjo's volume. They are commonly used in bluegrass ensembles, where the banjo has to be loud enough to compete with the other instruments.
Often referred to as "bluegrass banjos," these instruments are commonly used in bluegrass music, Besides the resonator, they tend to have a number of other recurring features. They commonly have five metal strings, tuned to open chord pitch: GDGBD. They have broad fingerboards for easily playing a combination of chords and melody fills. Unlike the ukulele or mandolin-style banjos, the low fifth string has its tuning peg halfway up the neck.
The thing that distinguishes a resonator banjo from an "open-back" is the presence of a resonator. There are different types of resonators available for anyone wishing to build his own banjo or convert an open-back to a resonator. The simplest ones consist of a tapered wooden disc, which is bolted to the rear of the body and held in place with a set of thumb screws, allowing easy adjustment.
Some high-end banjos include a mechanical resonator, much like the metallic ones seen on Dobro guitars. The assembly consists of a pair of metal discs that resemble the cones of a stereo loudspeaker. When the banjo strings are struck, the plates vibrate sympathetically at a similar frequency. This increases the volume and imparts a buzzing, raspy tone to the notes, which can be very complementary to other bluegrass sounds such as slide guitar, harmonica or fiddle.
The other basic style of banjo, the open-back, essentially consists of the same components as a resonator, minus the back of the body. As a result it's fairly straightforward for someone with basic carpentry skills to retrofit a resonator to another banjo. Manufacturers are aware of this, and many sell blank banjo resonators in a variety of sizes, specifically for fitting to existing instruments. Fitting hardware is generally included. All that needs to be done is to cut a notch for the neck to sit in, and then to finish the resonator in an appropriate color.
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