Unlike the RTA, the ear can process and register both the direct and the reflected sound. Since it’s louder than the reflected sound, the direct reading is what the mic sends to the RTA. At the same time, the mic is also getting the full-volume signal from the speaker’s direct, as-yet unreflected soundwave. By the time the reflected sound bounces around the room a few times and reaches the mic, it arrives at the mic at a lower sound pressure level. The problem is that the mic and RTA are “dumb.” The mic only “sees” the loudest sound pressure level it gets at a given frequency, and the RTA registers what the mic “sees.” Everyone knows that SPL dwindles with distance. I’ve never been able to buy into the idea that reflections unduly influence RTA readings. So is that 250ms correct or where is the problem? How fast signal we need to exclude those room modes? Am I confusing room modes and normal reflections? I have measured with both ways, QS and pink noise and the results are the same, so Murphy is of course right. Because of the wide gate, room reflections will influence measurements when testing indoors." In the TrueRTA help John L Murphy says "There is gating but it is sufficiently wide that it does not exclude room reflections on the acquired sweep. This indicates that QS would be too fast for room to response and therefor room modes should't be built. If it's linear each major third (1/3 oct) should take about 50ms or less (1500ms/31).
QS takes about 1500ms and it goes 10Hz-20kHz. Now we come to TrueRTA and it's Quick Sweep. If a good decay is say 600-800ms then that 250ms would be quite correct. Here (bottom) they actually show a formula for this. It is said that it takes about 250ms signal to get those room modes ringing.