The frequency at which a system offers the lowest impedance, allowing the highest power transfer. In the case of physical systems, it is the frequency at which the vibration is greatest for a given input. For loudspeaker drivers, the resonant frequencies of the drivers are very important characteristics to take into account in the design process, avoiding placing them in the range of operation for higher frequency drivers, and carefully designing the enclosure to make best use of the resonant frequency of the woofer.