There is a trade-off between the amplifying effect of the resonators and the length of sustain of a ringing bar. The energy in a ringing bar comes from the initial mallet strike, and that energy can either be used to make the bar ring louder initially, or not as loudly but for a longer period of time. This is not an issue with marimbas and xylophones, where the natural sustain time of the wooden bars is short, but vibraphone bars can ring for many seconds after being struck, and this effect is highly desirable in many circumstances. Therefore, the resonators in a vibraphone are usually tuned slightly off-pitch to create a balance between loudness and sustain.