The characteristic pore dimension in the stack is an important parameter in the design of thermoacoustic refrigerators. A quantitative experimental investigation into the effect of the pore dimensions on the performance of thermoacoustic devices is reported. Parallel-plate stacks with a plate spacing varying between 0.15 and 0.7 mm are manufactured and measured. The performance measurements show that a plate spacing in the stack of 0.25 mm (2.5k) is optimum for the cooling power. A spacing of 0.4 mm (4k) leads to the lowest temperature. The optimum spacing for the performance is about 0.3 mm (3k). It is concluded that a plate spacing in the stack of about three times the penetration depth should be optimal (3k) for thermoacoustic refrigeration.