Ring-down as a method to interrogate acoustic resonators was established in 1954. In the context of the QCM, it was described by Hirao et al. and Rodahl et al. The active component of a QCM is a thin quartz crystal disk sandwiched between a pair of electrodes. The application of an AC voltage over the electrodes causes the crystal to oscillate at its acoustic resonance frequency. When the AC voltage is turned off, the oscillation decays exponentially ("rings down"). This decay is recorded and the resonance frequency (f) and the energy dissipation factor (D) are extracted. D is defined as the loss of energy per oscillation period divided by the total energy stored in the system. D is equal to the resonance bandwidth divided by the resonance frequency. Other QCM instruments determine the bandwidth from the conductance spectra. Being a QCM, the QCM-D works in real-time, does not need labeling, and is surface-sensitive. Current QCM-D equipment enables measuring of more than 200 data points per second.