A reference source may be needed for comparison with an unknown one in non-anechoic environment or for estimating the incident pressure in free field. However, the performance of such a source must be assessed in an anechoic room, which is not perfect below its cutoff frequency (usually 70 to 100 Hz). This presentation deals with the design of a reference source targeting lower audio frequencies (40-200 Hz), involving two loudspeakers combined in ?push-pull?. This source features an internal microphone allowing to estimate its volume velocity, which can be related to the radiated pressure. The second part of the presentation deals with the acoustic characterization of this source, which raise many difficulties as no facility can be considered as truly anechoic in the lower part of the targeted frequency band. These difficulties will be presented and discussed, showing the trade-off which had to be found and the resulting measurements.