Why gain factor when reconstructing the signal?

[AlanLiudx]

Hi seanwood @seanwood , I'm a junior in BSS and thank you for your useful and effective open-source GCC_NMF. I happened to came across an unknown parameter when applying istft and reconstructing waveform:

def getTargetSignalEstimates(targetSpectrogramEstimates, windowSize, hopSize, windowFunction,numSamples):
    numTargets, numChannels, numFreq, numTime = targetSpectrogramEstimates.shape
    stftGainFactor = hopSize / float(windowSize) * 2
......
    return array(targetSignalEstimates) * stftGainFactor

Of course the outcome is good, but I really don't know why multiplying 2*hop_size/n_fft. Could you please give an explanation? Thank you.