KSTR
Major Contributor
A quick simulation.
Assumed is a THX-compliant Sub+Main arrangement:
- Sub is acoustic lowpass 4th order Linkwitz Riley ("LR4) at 80Hz (or 85Hz, forgot exact number).
- Sub provides a highpass output for the mains, Butterworth 2nd order at 80Hz. This must also compensate the acoustic highpass function of the sub, say 20Hz 4th order (sealed sub with subsonic filter, or a ported box sans filter).
- Mains are acoustic highpass 2nd order Buttherworth at 80Hz, and this combines with the electrical highpass for the needed total filter, LR4 again
This sums perfectly:
I actually modelled a 3-way with 300Hz 2nd order XO woofer to mid, this adds some low frequency group delay which in this case happens to be exactly compensated by moving the sub back by a millisecond (30cm).
Now let's increase the time-of-flight difference to 3.5ms having the woofer even further back a bit above one meter. Even with a polarity switch we cannot compensate the dip:
Adding in a delay (phase shifter) on the sub, we can arrive at this:
Not excactly perfect but better than without.
But it's not going to sound good, the low bass will be severly lagging.
Looking at group delay we can see why:
At 80 Hz the sub is now way late compared to the main. The step response looks equally bad:
Not that the original step response were exactly what one would call "quick", yet it is still significantly less ringy:
Now, mind you, we started with a perfectly aligned system... in any actual use case, notably with generic subs and mains, the chances of what you get is pretty much random. There was a good reason for setting up the THX specs as it does help to reduce the unknowns so the customers are not competely left in the dark.
BTW, overall magnitude and phase compensation with DSP to "fix" the total response at the listening position does not work well when the system is heavily "detuned" already. It's gonna sound better, but not really as good as it could be. You have to have the basics right, and thats the dilemma that I mentioned. Same goes for adjustable delay for the mains, optional filters, etc in AVRs, this can fix only so much (though often better than with analog means alone).
Assumed is a THX-compliant Sub+Main arrangement:
- Sub is acoustic lowpass 4th order Linkwitz Riley ("LR4) at 80Hz (or 85Hz, forgot exact number).
- Sub provides a highpass output for the mains, Butterworth 2nd order at 80Hz. This must also compensate the acoustic highpass function of the sub, say 20Hz 4th order (sealed sub with subsonic filter, or a ported box sans filter).
- Mains are acoustic highpass 2nd order Buttherworth at 80Hz, and this combines with the electrical highpass for the needed total filter, LR4 again
This sums perfectly:
I actually modelled a 3-way with 300Hz 2nd order XO woofer to mid, this adds some low frequency group delay which in this case happens to be exactly compensated by moving the sub back by a millisecond (30cm).
Now let's increase the time-of-flight difference to 3.5ms having the woofer even further back a bit above one meter. Even with a polarity switch we cannot compensate the dip:
Adding in a delay (phase shifter) on the sub, we can arrive at this:
Not excactly perfect but better than without.
But it's not going to sound good, the low bass will be severly lagging.
Looking at group delay we can see why:
At 80 Hz the sub is now way late compared to the main. The step response looks equally bad:
Not that the original step response were exactly what one would call "quick", yet it is still significantly less ringy:
Now, mind you, we started with a perfectly aligned system... in any actual use case, notably with generic subs and mains, the chances of what you get is pretty much random. There was a good reason for setting up the THX specs as it does help to reduce the unknowns so the customers are not competely left in the dark.
BTW, overall magnitude and phase compensation with DSP to "fix" the total response at the listening position does not work well when the system is heavily "detuned" already. It's gonna sound better, but not really as good as it could be. You have to have the basics right, and thats the dilemma that I mentioned. Same goes for adjustable delay for the mains, optional filters, etc in AVRs, this can fix only so much (though often better than with analog means alone).