A new experimental method for investigating the effects of obstructions on impulse resulted from flammable gas cloud deflagration is presented in this paper. Two types of obstructions-multi-rectangular hole (RH) hemisphere shells and multi-circular hole (CH) hemisphere shells-were studied here covering opening ratios ranging from 20% to 100%. The experimental results reveal that the increase in deflagration impulse depends on the geometry, size, opening ratio and layer number of obstructions. The deflagration impulse induced by the RH obstructions was higher than that induced by the CH obstruction. (c) 2007 Elsevier Ltd. All rights reserved.