Do you want to average for a specific amount of time? If so, compute the number of blocks you need to acquire using
T_avg = (# blocks to avg)*(samples per block)/(sample rate)
restart averaging after you have hit the required averaging time.
Be sure to allow sufficient time for the octave filters to settle before you start averaging.
Doug
Enthusiast for LabVIEW, DAQmx, and Sound and Vibration