On tuning of pianos I'm advocating systems which happen to be historic that use numbers of exact pure 3:2 fifths. This puts many notes of the scale exactly on the harmonics of lower strings, with numbers of near pure thirds or sweeter thirds than we're conventionally used to, and enables the instrument to give a stillness to the sound, and the music thereby, in contrast to the conventional tuning, as well as tonal colour differences between keys, which is what classical composers wrote for . . . as well as actually improving the sound of the instrument, making it less metallic, sweeter and more harmonious.
I think that it is problematic to compare musical instrument tuning to phase anomalies in audio systems. An out of tune string has the fundamental and all harmonics intact. When we tune one string to another, everything moves more or less linearly. In audio where we have varying types of phase shift, we NEVER have this uniformity. A fundamental could be reproduced by the woofer, the first harmonics by the midrange and the higher harmonics by the tweeter.
I have played with phase in analog (with 6dB crossovers, you just change the high or low pass frequency) and digital crossovers (MiniDSP, Behringer) as well as physically aligning. I can say that all of the corrective approaches sound differently. The problem is obviously not only absolute phase. Knowing that phase is audible, the question is what can we do about it? Physically time align our speakers with a tool like Holm impulse? Create 5/6/7 way speakers with drivers only working in their sweet spot - allowing for minimal slope? Investigate point source schemes? Do we need to kill room reflections?
At the end of the day, how do we verify that what we did has “musical value”? Do we even have words to communicate these differences. Are we objective enough to isolate “Pride of Ownership” from actual performance?
I guess to get started, one would need some kind of reference. What could that be? Headphones? Which headphones are free from ear cavity caused phase shift? Do we need someone with Quad ESL63s? Do we need an approach like Romy the Cat with a spectrum analyser and phase tester? Do we need something like DIRAC? Do we just do a little bit and be content knowing that we did something - until someone else shoots holes into what we thought was the answer?
There are complications - especially for instrument tuners. We can play a trumpet tone 8MY INSTRUMENT) in perfect tune with a tuning device, but it sounds off pitch depending on how the player nuances it. I have noticed speakers that do a similar thing in certain ranges. The pitch measures correctly, but sounds too sharp or flat or even worse, the sense of pitch is not “strong”. Bass lines are not “tuneful” rather wooly.
I think that Holm Impulse is a great tool to help get a system at least physically aligned. It can at least help us visualize things that we at first do not understand. I consider this to be a start. We can compare phase vs amplitude. We can “see” problem areas deserving attention. It will not turn the deaf into golden eared audiophiles, but the results are documented in a way that we can share them. Arta is also a great tool.
I did some playing around with FIR and tweaking the phase response of a set of speakers yesterday. REW, RePhase, some Powersoft amps and a pair of Faital 10HX230 in ported boxes.
At one point, I had a flat phase (and frequency) response from 100Hz-20kHz, but that required a lot of FIR taps and a long delay time - certainly not useful for live music.
For playback, though, it was the most "correct" sound I think I've ever heard. There's a passive crossover in the speakers, so I made a custom curve that would undo the phase shifts from the crossover, and linearise the overall response.
I'd like to demo this at Owston, probably by EQing the speakers flat and then using the FIR processing to just fix the phase response. That way, it can be switched on & off easily.