Resonators are thin-walled tubes, typically made of aluminum, but any suitably strong material can be used. They are open at one end and closed at the other. Each bar is paired with a resonator whose diameter is slightly wider than the width of the bar and whose length to the closure is one-quarter of the wavelength of the fundamental frequency of the bar. When the bar and resonator are properly in tune with each other, the vibrating air beneath the bar travels down the resonator and is reflected from the closure at the bottom, then returns to the top and is reflected back by the bar, over and over, creating a much stronger standing wave and increasing the amplitude of the fundamental frequency. The resonators, besides raising the upper end of the vibraphone's dynamic range, also affect the overall tone of the vibraphone, since they amplify the fundamental, but not the upper partials.