Summing at crossover - LR24db/BU24db

[Justin Zazzi]

Did you try to align left/right or only one side drivers only?

All the sides.

[Cathul]

Which frequency did you use for left/right midbass or tweeter?
Or how did you align left/right with the alignment tool?

I did the alignment midbass/tweeter each side and left/right with impulse responses and only the sub with the alignment tool as i did get weird numbers for midbass tweeter alignment (f.e. 3ms for a tweeter that is only 20cm nearer as the same side midbass, even with 1/1 octave smoothing).

[Justin Zazzi]

I used about 140hz to align all speakers, but I had four coaxial and no components or tweeters separate. This was a slightly unusual car. The alignment feature asked for 15ms of delay from all speakers VS the sub which seems like a lot, but I tried it and it worked.

[Justin Zazzi]

I'm guessing your "odd" time alignment suggestion might be because using the tape measure method is very different from aligning phase at a narrow band of frequencies. These two methods can give wildly different results, especially if your frequency response is not textbook-perfect.

I also haven't put much time into using this feature or the REW phase features so it might also be an anomaly with the measurement or the post processing settings or any number of things. Extracting reliable phase information from a chaotic "room" like a car interior is really challenging.

[Cathul]

I've read on avnirvana that you can add a frequency dependant window to your measurements.
I'll try that later and see if i get different results.

In addition i have to work with acoustical reference as i don't have a soundcard with two outputs/inputs and/or a XLR microphone.
In addition the mixer in the Mosconi D2 100.4 DSP doesn't allow for independant routing like in the Helix.
So when using the left tweeter as reference channel i cannot use the left midbass as output. The mixer simply doesn't let me do so.

[Cathul]

Ok, did some quick tests...

Here are the impulse responses of right side drivers with no delay referenced to the left front tweeter.



You can clearly see that i should need approx. 0.95ms delay to bring the start of the IRs on top of each other.
When doing the alignment tool for both drivers it proposes 3.5ms delay on the midbass driver.

For left side the IR is approx. 1.01ms apart and alignment tool proposes 0.85s delay to get them in phase at 2300Hz.
While left side could be reasonable the right side is way off, even with a frequency dependent window of 5 cycles and a smoothing of 1/3 octave.
IR measurements were repeated several times to check if something is off with the measurements, but all measurements were within a few thousands of a millisecond.

[mauian]

Here is a neat guide on using the acoustic timing reference feature in REW:

https://www.minidsp.com/applications...ing-time-delay

The guide is from mini dsp and for the home environment but can be done in the car as well replacing the nanoAVR with your dsp.


Sent from my iPhone using Tapatalk

[Cathul]

Yes, I know that page. Still prefer to do the math myself.
Did another round of test. Remeasured distances again and found that I was off on my right tweeter by about 4 inches for whatever reason. This also explains why I had to flip polarity, as the missing distance put the tweeter almost 180degrees out of phase.

Did time alignment for tweeters/midbass by impulse measurement and sub with the phase alignment tool in REW.
Measurements don’t look fundamentally different than before.
Will post some screenshots tomorrow.

[blockrocker]

Cathul, I just have to say how impressed I am to watch you work through these issues. It has inspired me to work to understand and solve my own stereo concerns, as well. Keep up the great work and thanks for sharing your experiences.

[mauian]

Yes, awesome stuff Cathul.

Just in case people are interested, and this doesn't contradict anything Cathul is doing, I found this interesting blog post regarding the use of impulse responses (IR) vs phase measurements to align speakers. Apparently, IR should not be used to do phase alignment of subwoofers (which Cathul is NOT doing). It's just not accurate enough. It can help with making sure certain frequencies are arriving at the same time, but it doesn't actually guarantee that they are in phase.

https://bobmccarthy.wordpress.com/20...ulse-response/


An interesting paragraph from this post (emphasis is mine):

"Simply put, you can’t time align a subwoofer to the mains. Why? because your subwoofers are stretched over time – the highest frequencies in your subwoofer can easily be 10-20 ms ahead of the lowest frequencies. Whatever delay time you choose leaves you with a pair of unsettling realities: (a) you are only aligning the timing for a limited ( I repeat LIMITED) frequency range, and (b) you are only aligning the timing for a limited ( I repeat LIMITED) geographical range of the room. So the first thing we need to come to grips is with is the fact that our solution is by no means a global one. There are two decisions to make: what frequency range do we want to optimize for this limited partnership and at what location."

And this section really opened my eyes as to what is going on with an impulse (emphasis is mine):

"what is a perfect impulse? A waveform with flat amplitude AND phase. That can’t be the pink noise described earlier, because pink noise has random phase. So what is it? A single cycle of every frequency, all beginning at the same time. Ready set, GO, and all frequencies make a single round trip and stop. They all start together, the highest freq finishes first, and the lowest finishes last"

That last part really got me. How do you align an impulse where different frequencies are finishing all at different times?

Additionally, "the peak (of the impulse response) will be found where there are more data points: the top end – 4 to 16 kHz."
- this struck me as the peak of the impulse response will be at the top end of each driver, which means the peak for a midbass is at a frequency range that is VERY different than where the peak for the midrange is (same for midrange and tweeter, etc.)

This made me realize a few things:
- that aligning IR peaks can be an inaccurate way of phase alignment
- using the start of the IR would also not be very accurate for aligning phase for drivers with different frequency ranges
- you CAN use IR for alignment of drivers with the same frequency ranges with some accuracy (L and R tweeter for example) as their impulse responses will mirror each other with where the most energy is, etc.

This is not to say that using IR for time alignment is bad, it's a good starting point. But it was interesting for me to learn that just aligning the peaks of two impulse responses or the starting points of two impulse responses only means you are time aligning a specific frequency (range) of one of the drivers to a specific frequency range of the other driver and that this still can (or likely) will result in phase issues that you need to look at either by paying close attention to how the drivers are summing, using a tool like SMAART, or this phase align tool in REW.

Justin, and others, does this understanding sound about right?