Testing phase at the crossover point.

[Grant Bunter]

During my time here I have learnt a lot.
And the more you learn, the less you think you know.

Even in the last couple of weeks, there has been discussion in a thread about polarity vs phase.

Polarity is relatively easy, plus or minus, as is, in an electrical connection, the result being a speaker cone will move in the opposite direction if swapped.
If one of two cabs has polarity swapped, frequency cancellation will occur.

Phase is different, it's frequency dependant. And, if you view phase as frequency vs time, it could be described as delay, given different frequencies have different wavelengths.
So some frequencies take longer to get to the listening position, hence they are delayed.

One of Bill's sayings , in essence, with our systems, is to make sure "the subs are in phase with the mains at the crossover point.
If the crossover point is say 100Hz, the goal will be to get the highest possible SPL at 100Hz.

Yes, we could use a polarity swapped lead to induce 180 degrees out of phase, or hit a phase swap button in DSP, and use RTA at 100Hz while playing a 100Hz wave through a tone generator to see which is louder or loudest.
Is that the simplest solution?

What do you do to test phase response at 100Hz without FFT?

Let's throw in a curve ball, namely delaying mains to backline. I totally get why that's a good thing as a musician up the back more often than not.
But what does that do to phase alignment at the crossover frequency?

Love to hear your thoughts on this complex subject...

[Bill Fitzmaurice]

This brings up the fact that you can't hear phase unless you have two different sources playing the same frequency and the phase is at or close to 180 degrees apart between the two sources. The easiest way to test it is to play a tone at the crossover frequency, do a polarity flip on either the subs or mains, and note if the level goes up or down. If it goes up leave it flipped. If it goes down flip it back again. If you're going to delay either the subs or mains do this after setting the delay, as delay changes phase. Don't use delay to account for a polarity issue. Use delay for time alignment, use polarity flip to fix polarity. If you can't hear any difference with a polarity flip don't worry about it. What you can't hear doesn't matter.

delaying mains to backline. I totally get why that's a good thing as a musician up the back more often than not. But what does that do to phase alignment at the crossover frequency?

It might not do anything. But it gets complicated when you have folded horn subs that also add delay via the path length. You may need to delay the mains to time align to the backline, but not the subs, or by a lesser amount, because of the path length. Whatever you need to do with time align do that first, then do the polarity check.

[Grant Bunter]

Thanks as always Bill.
Great answers.

If one decides to move on to FFT, then it is quite easy to produce 2 signal sources out of phase, as one can play a tone at the crossover point through both sub, and mains, with polarity flip if required to make them out of phase compared to each other.
Is this correct?

[Bill Fitzmaurice]

There's no need to take a phase reading with FFT. You may, but this is one case where your ears as tools are sufficient.

[Grant Bunter]

There's no need to take a phase reading with FFT. You may, but this is one case where your ears as tools are sufficient.

Thanks again!

For those who may not/don't understand why testing phase at the crossover point is important, I'm going to throw some stuff in to explain:

It's often said here in the forum, Don't mix cabinets of different designs in the same band pass.
Why? Because every design has a different phase response. It's pretty much impossible to address all the phase response issues when using more than one design.

There should be only one situation where you do have different designs at the same frequency, and that is subs with tops/mains, at the crossover point. Hence the need to test.

How often do you need to do this?
Once, but only if the system is in an install that is never moved ever again.

That's not how most of us use our systems though.
Multiple venues mean we set up differently every time. Even returning to a venue again might see us setting up differently. Subs further forward or backward, mains/tops further forward, or backward, or wider apart or closer together, means phase response is altered. Sometimes this can also be due to relative placement, mains/tops not perfectly in line as an example.
So we need to test phase every time.

Some may say "my venue doesn't allow me time to do that".
Sometimes the answer to that is, get there early.
If you aren't allowed that either, I'm unsure about the driverack series, but the DEQ2496 will allow a certain frequency to be allocated. RTA the crossover frequency alone, with music playing, hit the invert button on subs or tops/mains and see if the RTA changes up or down (or your ears tell you one setting was better than the other). It takes 10 seconds. That shouldn't be a problem.

Tips:
As Bill said above, enter delay before testing.

Also, be aware that delay causes phase shifting.

Only EQ in the chain up to the input side of your DSP. EQ alters phase response. So if you EQ on the output side of your DSP (which is post crossover) after testing, you alter phase, meaning you're back to where you started...

[Wobbly]

I am shorty moving next to a road. Could I employ my HT and a mic on delay to match the worst of the rumble at 180° at night with the individual truck passing by?

[Bill Fitzmaurice]

In theory, yes. That's the same way that noise cancelling headphones work. In practice it might not be that simple.

[Wobbly]

Thanks.might to hang onto a PA amp with delay unless I get lucky with the horn path and distance to the bedroom vs window. Could move the mic around I suppose. Good to know.

Set up sub A playing at worst rumble frequency out front and see what happens?

[Bill Fitzmaurice]

For it to work the speaker and mic would have to be very close to the listening position, because phase isn't just a matter of time, it's also a matter of distance. For that reason it's easy to make noise cancelling headphones, and relatively easy to make noise cancelling audio systems for cars. If it was easy for homes there would be a huge market for them, but it's not easy, so they don't exist. You mention 'window'. While windows are less resistant to sound passage than walls the low frequencies that make up most road noise pass through all but the most robust walls with relative ease.

The best method to deal with night noise is with a sleep sound machine. I've used one since living in Manhattan in the 1970s, and can't sleep well without one even though I'm in the country now. It's too quiet.

[Wobbly]

Not following.

Time is distance is same for all frequencies leaving the road

Small differencesi n responses of amp, speaker shift the resultant sound by enough to cause problems blending two sources Together with delay, horn path and physical distance we can align a crissover.

Not seeing why the mic needs to be at the LP with the speaker to get cancellation.

[Bill Fitzmaurice]

The distance between the mic and the listening position must be very short. Where lows are concerned with wavelengths of 10 feet and longer, plus the room reflections, the region where cancellation is possible is very small. Not an issue with headphones, not too great an issue in a car, but a pretty much insurmountable issue in a room.

[Wobbly]

Got it.

I have been doing the sleep sounds thing since something got screwed with in 2020. There was no rogue beat frequency until a switch got flipped. Some nights it is absent and I much prefer the silence.

My landlord couldn't hear it, didn't believe me. I wired up my bass speaker as a microphone upstairs in the corner and hooked into HT downstairs. Had to kll it as feedback got going. He could hear it then!