Article: Dirac Live 2 Digital Room Correction Software Walkthrough

[monteverdi]

I wonder why the corrected step response shows much more ringing than the uncorrected. As far as I can see it is at around 20kHz. Is it an artifact of Umik with step transitions (I see similar ringing just measuring my Wideband drivers using Umik) or is it introduced by the DSP?

I understand digital speaker correction which is what basically Dirac is doing at higher frequencies but I can not understand how the interaction of bass frequencies with room modes can be corrected by just altering the woofer output (past a single point in space). So I am looking forward to learn more about active bass management!

[monteverdi]

6 hours ago, mitchco said:

 

I did not use a UMIK-1 for this particular article, even though I do have one and have not come across any ringing artefacts with it yet. But I am going to test it out extensively in an upcoming article. As the speakers are also gone, I can't retest easily to confirm what the issue was. I could have goofed on my loopback test and resampled somewhere in the chain like 48 kHz signal through a 44.1 kHz filter. When I get a chance, I will confirm using another speaker/test, but given the amount of time I have spent on trying to identify (pre)ringing artefacts previously, it certainly inaudible to these ears. The main point I was trying to get across was the time alignment.

 

Understanding room correction is complicated A separate article is required to get though it all and I am contemplating writing one. In a nutshell, good room correction software will take an acoustic measurement and extract the minimum phase response and since speakers are minimum phase devices, correcting the frequency response will also correct the phase response. However, that is only half the picture as there is the room to deal with, which is what Dirac calls mixed phase or others call it excess phase, which includes room resonances and reflections.

 

And in the time domain, we know what the ideal speaker target is by looking at the timing (i.e. step) response of an "ideal" minimum phase speaker. So... the room correction software corrects the time domain to follow the ideal minimum phase target. You can see that if you have my book or some of the articles here at AS show that as well. But the trick is that this is wavelength dependant, so at low frequencies, the correction time window is long (like 600ms for example) and as frequency increases the correction time window becomes shorter so above the rooms transition frequency we are looking at more of the direct sound plus baffle of the speaker. Often called frequency dependant windowing. In other words, at low frequencies, the room correction software has a large time window that corrects both the direct sound and reflected sound (i.e. room resonance) towards a target response. At high frequencies, it is mostly the direct sound because if we tried to also correct for mid-range and high frequency comb filtering room reflections, the high frequency response of the loudspeaker sounds like a dentists drill. This is how one can hear "over correction" in the high frequencies.

 

Hope that helps somewhat. Like I say, requires a separate article to fully explain.

Thanks for your response, Toole and also in a similar version Geddes are proposing to use multiple subwoofers spaced over the room to allow wider zones of bass correction (with specific phase corrections). I wonder if that is something Dirac can do and if that leads to a significant improvement compared to standard 2 woofers/subwoofers? Of course that adds a lot of complexity to the setup both computational and room design wise. REW room simulator shows some benefits but I wonder how real these simulations are?

[monteverdi]

That avoiding early reflection from furnitures  for measurements  but not for listening is an interesting observation. Our brain can use these early reflection to have an acoustical map of the space around us but on the other hand direct sound from the loudspeaker and secondary sound generated by (edge) diffraction or early reflections will be merged if sufficiently short (5-10msec, I was reading slightly different numbers). So I am wondering also how much the direction of these reflection plays a role in its discrimination of direct and reflected sound. I was reading that coffee tables in front of the listener that have an audible influence and my limited experience it may be true (I have my table on the side).