My findings on the audibilty of excess phase:
First of all, like anything subtle (phase errors sure fall into the subtle category), it takes some training and skills to hear it. Just like's Amir's outstanding capabilities to identify subtle codec artifacts in 320k MP3's vs original WAV required a long training period, phase error detection skills need the same level of dedication, so to say.
The problem area of excess phase is mainly below 1kHz or so where our hearing is not only a frequency/level analyser but also judges actual waveform and the waveforms get distorted by excess phase (and by the normal minimum phase of LF rolloffs also, that is). This has a slight effect on the "timbre" of steady-state signals that are asymmetric (even order overtones dominating), organ notes are a good example for this.
For "bass transients" like kick-drums, plucked upright bass and slapped electric bass etc, excess phase at LF spreads out the energy in time, the lower freqcuency components arriving later. This makes the transient sound less punchy, compact and impactful and also slightly sharper/spikey (the HF content gets masked less by the LF content). This is probably the most drastic effect one will hear after while. Subwoofer crossovers at the typical 80Hz do a lot of damage here, as do other bass to low-mid crossovers in the 200...400Hz range, the higher the order the worse.
I also found a sharpening / more pin-point stereo imaging and stage depth when excess phase is removed, but this is likely a side-effect of the reduced "computing power" we need apply to seperate transient incidents because those are more compact.
I also found that LF room modes etc don't have too much masking effect, the difference still is heard even when the bass region is quite corrupted in the time domain by room effects. On the other hand, phase errors are usually more benign when listening with headphones, not something one would expect and I don't have a good explanation for this.
My overall take: when designing speakers, don't sacrifice other more important aspects just for a notably better phase response. Linear phase can easily installed afterwards by inversely pre-distorting the signal with a FIR-"phase-unwrapper". Linear phase nomally means minimum phase, of the equivalent single driver speaker, but in some cases a bit of phase unwrapping of the minimum phase bass roll-off can be beneficial to speed up the bass response, for ported boxes with rather high cutoff (50Hz'ish) and the ususal 2nd order electrical subsonic filter found in active monitors, making the roll-off 6th-order.
Well, thank you for taking time to give such a detail answer!
First, let me comment that Amir, as an ex-Microsoft executive, is very good in self promotion. The thing that he can distinguish recording of jungling keys in mp3 vs wav is really nothing to what his wife can from the kitchen tell about speaker's response. Luckilly for us she has decided to step out of his shadow and help him evaluate the speakers.
What is your overall take when doing EQ regading phase alignement/correction? I did some in-room mesaurements and according to them it turns out that not only magnitude response but also the phase response varies from point to point. As modern EQ tools (and rePhase, if one chooses to do correction mannualy) beside FR also adjust the phase response at LP, would you say that this also results in subtle but audible gains in SQ, or is it just a marketing gymmick/waste of time?
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