This paper presents a study on the influence of the diaphragm on the uncertainty of measurements and the repeatability of pressure steps using a TC-20 shock tube. The tests were carried out with several diaphragms, in two different metrological shock tubes. A step-type transfer function was used as the input of the system, along with the output signal from a pressure transducer. The results show that, over a broad range of frequency bands, the characteristics of the diaphragm material and the way in which it bursts influence shock formation, causing variations in the gain of the output signal from the pressure transducer.