Increasing interest for traceable infrasound measurements has caused the Consultative Committee for Acoustics, Ultrasound and Vibration (CCAUV) of BIPM to initiate a key comparison calibration project (CCAUV.A-K2) on pressure reciprocity calibration down to 2 Hz. Ten national metrology institutes, including the Danish Primary Laboratory of Acoustics (DPLA), take part in this project. In addition DPLA has started its own infrasound calibration project, which is described in this paper. The purposes of this project are verification of the CCAUV results and development of methods for calibration of general types of measurement microphone between 0.1 and 250 Hz. The project includes the design of an active comparison coupler, an experimental low-frequency reference microphone, and new methods for its frequency response calibration. One method applies an electrostatic actuator and requires a low-pressure measurement tank, while the other requires an additional microphone, whose design is closely related to that of the reference microphone that is to be calibrated. The overall calibration uncertainty (k=2) for ordinary measurement microphones is estimated to less than 0.05 dB down to 1 Hz and less than 0.1 dB down to 0.1 Hz, if the reference is calibrated in the latter mentioned way, i.e., by the related microphones method.