Adjusting delays to correct frequency response

[NeuroBloo]

Something’s been bugging me, and I’m hoping this group’s wisdom can help me understand.

Aside from EQ filters, one of the common methods for improving a subwoofer’s frequency response is to adjust delay. I have seen success with this approach many times in REW. After tweaking the delay, the peak or (minor) dip in the frequency response flattens out, so we all pat ourselves on the back and declare mission accomplished.

But is it? Doesn’t this approach misalign timing between the sub and the mains? If so, are we smearing low frequency instruments like bass drums? Are we prioritizing frequency response at the expense of time coherence?

I ask because after using Dirac to measurably and audibly improve frequency response, I sometimes hear noticeable timing misalignment between subs and mains. If my ears are sensitive to such anomalies, should I use REW’s more hands-on adjustments instead? Or are these anomalies an unavoidable consequence of subs and small-room acoustics?

Lots of questions, but hoping this group can provide some insight. TIA!

 

[lovinthehd]

Why would adjusting delay affect a subwoofer's frequency response? Maybe as far as being in phase with mains or not, but I wouldn't particularly call that a frequency response adjustment for the sub.

 

[NeuroBloo]

Why would adjusting delay affect a subwoofer's frequency response? Maybe as far as being in phase with mains or not, but I wouldn't particularly call that a frequency response adjustment for the sub.

Exactly. And yet it’s a common recommendation to fiddle with delay until the frequency response curve flattens. But that seems like an ancillary benefit from misaligning the timing.

 

[TLS Guy]

Why would adjusting delay affect a subwoofer's frequency response? Maybe as far as being in phase with mains or not, but I wouldn't particularly call that a frequency response adjustment for the sub.

The OP is right. You can use time shift to correct FR, but he is correct it causes time smear. You can have a perfect FR from two sources but with time delay and music and speech would be unrecognizable. A fourth order crossover, puts the drivers a whole cycle out of time. This produces significant time smear and is enough to make a square wave look like a sine wave.

This is actually one of my beefs about these auto correction systems, and one of many reasons why I regard them as "fools gold". That is a major reason why my system is designed the way it is. That gives my system a very well preserved transient response. That is why I don't consider spacing subs away from mains optimal at all.

 

[3db]

I adjust the sub's distance via the AVR menu to integrate it with the mains and I don't hear any smear.

 

[William Lemmerhirt]

I adjust the sub's distance to integrate it with the mains and I don't hear any smear.

I’ve done this as well. It can improve impulse response too.

 

[TLS Guy]

I adjust the sub's distance via the AVR menu to integrate it with the mains and I don't hear any smear.

You have your subs fairly close to your mains and you don't know what the time delay of the crossover is. If you are using a full AVR crossover the phase difference is 270 degrees. So let's pick a 80 degree sine wave. Which is the usual crossover frequency. The wavelength is 13.75'. That is a time smear of 0.01 sec. at the crossover frequency. That is if the speaker and sub are next to each other. You can see also that with the wavelengths involved it is very easy to get peaks and nulls just by position, before we even consider room modes.

 

[highfigh]

Why would adjusting delay affect a subwoofer's frequency response? Maybe as far as being in phase with mains or not, but I wouldn't particularly call that a frequency response adjustment for the sub.

Phase cancellations at/near the LP/HP frequency, although not allowing the HP to dip too low helps a lot. The energy from the sub hits the back wall far more than the mains, so that affects the phase relationship.

I wouldn't attempt to do this without REW because it's very easy to see when they're aligned, or close to it.

 

[3db]

You have your subs fairly close to your mains and you don't know what the time delay of the crossover is. If you are using a full AVR crossover the phase difference is 270 degrees. So let's pick a 80 degree sine wave. Which is the usual crossover frequency. The wavelength is 13.75'. That is a time smear of 0.01 sec. at the crossover frequency. That is if the speaker and sub are next to each other. You can see also that with the wavelengths involved it is very easy to get peaks and nulls just by position, before we even consider room modes.

Im used REW to measure the frequency response everytime I changed the su. distance on the AVR. I picked the smoothest transition from 80 to 120 Hz and that was the distance I set.