Relationship between respiratory sounds and flow has always been of interest for researchers and physicians. However, the flow-sound relationship at very low flow rate has been questionable because breath sounds must exceed a minimum flow in order to be audible and different from the background noise. This study aimed to find the minimum critical flow rates for respiratory sounds to be audible and different from background noise. Tracheal and lung sound signals of healthy subjects in two groups of adults (12 subjects) and children (9 subjects) were studied. The values of minimum critical flow were determined comparing the spectrogram of the respiratory sounds at very low flow with that of during breath hold. The values of minimum critical flow for tracheal sounds were found to be 3.7.. 1.7 ml/s/kg and 2.7 .. 1.7 ml/s/kg for adults and children, respectively. The minimum critical flows for lung sounds were found as 6.2 .. 1.8 ml/s/kg and 4.0 .. 2.9 ml/s/kg for adults and children, respectively.