Group delay
Mathematically group delay is the negative derivative of the phase response. Phase response was measured using the ETF-5 program which calculates it from the measured impulse response. Since frequency response and phase response correlate with each other, also FR and GD correlate with each other. Meaning flat or gently sloping FR results in low GD. That's why the final in-room frequency response should be as flat as possible. There are no studies defining the audibility of GD at low frequencies, but the threshold is suggested to be in the range of 1-1.5 cycles. Therefore I have included the 1 cycle curve on the graphs.
Ideally GD should be as low as possible, or at least gradually/gently rising.
