The modern concert marimba is defined by the metal tubes hanging beneath its wooden bars. These are called resonators. Each resonator is cut to a specific length that corresponds to the fundamental frequency of the bar above it. This causes the air inside the tube to vibrate in sympathy with the bar, which significantly amplifies the sound and increases its sustain. If you look closely at a marimba, the resonators for the low notes are very long, while the resonators for the high notes are very short. Interestingly, the very longest resonators on a bass marimba are often 'U' shaped or have 'tunable' caps at the bottom. This is because a straight tube for the lowest notes would be too long to fit on the instrument frame without hitting the floor. The precision of these resonators is what gives the marimba its characteristic warm and mellow 'voice' compared to the xylophone.