Folk, country and bluegrass music often feature the distinctive sound of the banjo. Twangy, bright and almost metallic, the banjo is a stringed instrument but sounds nothing like its cousin the guitar, which can be smooth and warm. If you want a side-by-side comparison of how guitars and banjos differ, look no further than the 1972 movie thriller "Deliverance" and the famous "Dueling Banjos" scene, where two characters go head to head with their stringed instruments.
Although both instruments look quite similar -- stringed, with a bridge and a neck -- the sounds they produce are somehow completely different. Why? The short answer is that a banjo's strings react in a very complex way when plucked. It has to do with acoustics, physics and the way the banjo is crafted.
The banjo is basically a drum with a long neck and strings stretched across it, affixed at either end. A bridge supports the strings over the drum membrane. The drum membrane is very thin, usually about 12 thousandths of an inch thick. By contrast, a guitar's soundboard is typically about 1/8th of an inch thick, with the bridge affixed to the board [source: Hunn]. When you pluck a banjo string, the drum resonates and the bridge vibrates, emitting a sound. A banjo pluck is much louder than a guitar pluck because the banjo's thin head vibrates much more.
A Nobel Prize-winning physicist named David Politzer studied this phenomenon in depth. Politzer believes the twangy, loud banjo sound comes from something called frequency modulation. You can modulate the frequency of a stringed instrument by changing the tension of the string. For example, guitar players modulate the frequency of a string when they push it sideways and get that tremolo, or shaking, sound.
The tension of a plucked banjo string changes as it vibrates. Because a banjo's drum is so thin, the pluck makes the bridge vibrate as well. So the tension of the string changes twice: once from the initial pluck and then again from the resulting motion of the bridge. This change in frequency modulation is what gives the banjo its bright, twangy sound.
You might be wondering why this doesn't happen with mandolins or other stringed instruments like violins, where the bridge and the strings move in tandem. The answer is pretty simple -- it's because those instruments are made of thick wood. Their soundboards (as compared to the drum/membrane of the banjo) are too heavy to vibrate. In fact, Politzer points out that if you replace the thin drum membrane of a banjo with wood, it no longer sounds like a banjo.