Originally Posted by Shiki
I have a question regarding subwoofer delay adjustment.
I'm running a pair of small sealed bookshelves with dual 4'' woofers (Gallo Strada) as my fronts with a single F12 crossed over at 100Hz. The plan is to add another F12 as soon as they're available to help with localization issues.
Anyway, REW says that the phase for the fronts is -172 degrees and the phase for the F12 is 19 degrees. Is the following calculation correct?
Wavelength = 344/100 = 3.44m
Phase shift = -172 + 19 = -153 degrees
Cycles = -153/180 = -0.85
Delay adjustment = -0.85 x 3.44 = -2.924m
So I need to subtract 2.924m from the physical distance between the subwoofer and the listening position?
That 19 degrees of F12 does not seem to be right unless you have the sub much closer to you than the front speakers.
One thing we need to be careful is the sign of phase. LPF such as subs always have negative phase because the mathematical expression is actually sin(freq*t-phase_lag). The time that we get zero phase crossing is delayed to t=(phase_lag/freq). On the other hand, if the expression has been sin(freq*t+phase_lag), the zero phase crossing is t=-(phase_lag/freq) and that means the zero phase crossing at the output is before time 0, earlier than the input signal zero phase crossing at time 0. I'd like to preserve that sign of phase. So remember, phase lag is negative phase and phase lead is positive phase. These numbers are referenced to the phase at time 0.
[EDIT] note the freq above should really be 2*pi*freq. Just to keep discussion simple, I ignored 2*pi.
Nevertheless, let us take the numbers as they are and go from there. We need to convert everything to either phase lead or phase lag. I like to use phase lag because it is conceptually easier to understand. 172 degrees is a phase lead (I change the sign here). Its equivalent phase lag is -360+172=-188. The sub has a phase lag of -19. So the difference is -188-(-19)=-169 degrees. You need to put additional phase lag to the sub equivalent to -169 degrees in order to get perfect phase alignment. So that is -169/360*3.44=-1.61m. Full wavelength is 360 degrees. Now when you use speaker distance menu in AVR to achieve this delay, you need to put in the opposite of what you want to do. In this case, you need to put the sub "closer" by 1.61m distance, so AVR will apply the signal to sub channel later and hence you can get the delay lag that you want.
The best way to figure this out is to use our subtuner2.html. I will bring that back as soon as possible. There are also typos in phase3.html and I have corrected those.