This doesn't really belong here, but I still want to reply to UB. This should probably be moved somewhere else. I think we are talking about different things here. It's not about picking one thing. Obviously a perfectly flat FR, perfectly linear phase would be best in terms of measurements. Yes, this is what I was talking about. I think I got confused a bit here. All I really wanted to say is this: Here you can see the phase response and the minimum phase response. I used an old HD800 free air measurement I had for this example, but it really doesn't matter which headphone, which coupler, etc. This is just an example and the measurement as such isn't the most accurate.* As you can see the phase response tracks the minimum phase response (calculated from the FR) perfectly - the excess phase is 0. The slight deviation is probably from stuff such as ADC, DAC and other general measurement setup issues. The time-delay might also not be set 100% perfectly as there's a tiny bit of HF droop at >30kHz, but you get what I mean. You will get the same result with a single-driver, single diaphragm speaker in a sealed box, infinite baffle, OB, etc. But you will not get the same result with most speakers and their crossovers and drivers that aren't time aligned. How much this matters is a different issue. Here's a measurement of an old two way speaker I have. I think the crossover is around 1.3kHz and both drivers are connected in the same polarity. I think this one might actually be a 3rd order design, though. The step response looks similar in shape to the one @Marvey showed us. Again, this isn't really about the specific measurements, it's about the overall picture. I had to quite aggressively window the measurements because when I measured these I didn't make any attempts to get close to anechoic conditions, despite measuring them outdoors. This is why the FR starts to drop off below 400Hz. The range below the crossover frequency is delayed by multiple tenths of a ms. *Hahaha, I bet some of you will also notice something weird about the measurement. But this doesn't really matter here. It was just a random measurement I picked.