Acoustic measurements at very low frequency are difficult to perform, and the interpretation of the results may often be tricky. In this paper, four subwoofer measurement techniques are compared in terms of frequency response and directivity. The methods used are the following ones: semi-anechoic room, anechoic room, isobaric room, pseudo free-field. Two subwoofers are tested: a closed box system and an active/passive system. For the semi-anechoic technique, double layer pressure measurements on a half-sphere surrounding the source are performed. Then, using a BEM integral formulation, outgoing and ingoing pressure fields are separated to recover free field conditions (i.e. removal of reflections on walls below room cut-off). Discrepancies between results are discussed and explained when possible.
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