Calibration experiments are disturbed by seismic noise and tilt, and should therefore be carried out in a basement room. However, the large operating range of modern seismometers permits a calibration with relatively large signal amplitudes, making background noise less of a problem than one might expect. Thermal drift is more serious because it interferes with the long-period response of broadband seismometers. For a calibration at long periods, seismometers must be protected from draft and allowed sufficient time to reach thermal equilibrium. Visible and digital recording in parallel is recommended. Recorders must themselves be absolutely calibrated before they can serve to calibrate seismometers. The input impedance of recorders as well as the source impedance of sensors should be measured so that a correction can be applied for the loss of signal in the source impedance.