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Setting subwoofer delay

I' NOT a room-correction expert...



Phase differences will ONLY be an issue in the narrow band around 80Hz where the regular woofer and sub are working together (where the soundwaves might be partially or completely out-of-phase relative to each other).

Assuming there is no "different DSP processing" to the high-pass or low-pass signal paths, and assuming neither one is inverted relative to the other, the ONLY phase-timing issues will be from difference in distance between the sub and the main speaker to your listening position.

The wavelength at 80Hz is 14 feet. So a half-wavelength is 7 feet and if the sub or main speaker is 7 feet closer than the other, you'll have a 180 degree phase-shift (at 80Hz) and the soundwaves will cancel. That's the worse case. (At 1.5 wavelengths you'll get 540 degrees and cancelation.) It wouldn't be unusual to have the main & sub 7-feet apart but a 7 foot difference would be unusual at home.

And since sound travels about 1 foot per ms, 7 feet is about 7 ms of time difference. (If you delay the nearest speaker by 7ms they will be back in- phase at 80Hz).
I have always got better results when using linear phase crossover vs minimum phase crossover

This post is about my main system:
Post in thread 'Lavoce SSF153.00 15" based dual opposed subwoofer project' https://audiosciencereview.com/foru...-opposed-subwoofer-project.51178/post-1852855

And this one is about my home office system:
Post in thread 'Another 8" dual opposed subwoofer project' https://audiosciencereview.com/foru...-opposed-subwoofer-project.55078/post-2033114
 
If you are using a UMIK-1 it is not accurate enough for timing measurements like this due to USB bus issues. You will never know what is correct and what is wrong. A UMIK-2 or an analog Mic using loopback is the best way to get timing measurements. See link to developer of REW showing the issue https://www.avnirvana.com/threads/acoustic-phase-measurements.13300/#post-99941

If you look closely at the horizontal scale, you will see the timing variance to be about +/- 50us (microseconds). That's 0.05 milliseconds.

If Paul is using a 48kHz sampling rate, then each sample is 0.02083 milliseconds. So the variance of the UMIK-1 is +/- 2.5 samples.

Next question to ask is what are the thresholds of audibility for bass group delay? The answer is: nobody seems to know. Some people think you need "several cycles" of bass before you even hear it. If you believe them, then the period of one 50Hz cycle is 1/50 sec (20ms) and "several cycles" may be 40-60ms.

In theory then, a UMIK-1 should be able to correct to below the threshold of audibility if you are using the UMIK-1 to time align a subwoofer. Where it might make more of a difference is with higher frequencies.

That said, there are a lot of other reasons to hate USB microphones:


The guy in the video makes a great point about how interfaces are so versatile. When I bought my interface, I never imagined all the different things I would use it for. When I am not using it for measuring, it is my headphone DAC/amp. It has digital inputs, so I can connect external digital sources like a CD player, a Wiim, my DAP, and so on. It has an ADC, so if I wanted to I could connect an analog source. I can use the interface to route signal from one input to another, for e.g. REW has no built-in convolver. I can send REW signal to a software digital input on the interface, and loopback that input to an output. My convolver then takes signal from that output and sends it back to the interface's DAC's.

In fact those interfaces are so versatile that I have 3 of them. Only one of them (the RME) is expensive. The other 2 (Presonus Audiobox USB and Focusrite 2i2) are downright cheap. But they don't do half the things that the RME can do.
 
Next question to ask is what are the thresholds of audibility for bass group delay? The answer is: nobody seems to know. Some people think you need "several cycles" of bass before you even hear it. If you believe them, then the period of one 50Hz cycle is 1/50 sec (20ms) and "several cycles" may be 40-60ms.

In theory then, a UMIK-1 should be able to correct to below the threshold of audibility if you are using the UMIK-1 to time align a subwoofer. Where it might make more of a difference is with higher frequencies.
20 or 40 or 60 ms is a lot. Hard to reliably ABX but having made some "50 ms mistakes" I think that is audible. I agree with your points in general but if 20 to 60 ms is "good enough" there is really no need to measure at all and you would be better off with a tape measure.
 
20 or 40 or 60 ms is a lot. Hard to reliably ABX but having made some "50 ms mistakes" I think that is audible. I agree with your points in general but if 20 to 60 ms is "good enough" there is really no need to measure at all and you would be better off with a tape measure.

Believe me, I would love to be wrong. I time align my subwoofers down to <0.5ms and I take a lot of care doing it. I think I hear a difference, it sounds "tight" and "punchy" to me, but that could easily be expectation bias given how much time I spend on it.

But the reality is that I am very likely correcting it to be way below audible. Toole in Chapter 4.8.2 talks about phase distortion in subwoofers. He notes that minimum phase high pass filters are used at multiple stages of the recording process, starting at the microphone, through various mixers, and finally through speakers and subwoofers. He does not know how much group delay has been added even before we attempt playback but it must be "enormous". So he asks: is there any point preserving the phase/time relationship at the last part of the signal chain, when so much harm has been done upstream? And more importantly, does it even make sense doing it if the room is going to distort the phase even more?

Toole cites several studies in his book, all of which report a subtle difference, but he notes that none of them were controlled double blind studies.

The only study I was able to find was by Liski et al using test signals and loudspeakers in a room (not mentioned in Toole's book).

1723661416188.png


Take note that the group delay thresholds start to shoot up like a rocket below 100Hz. But it does suggest that at 40Hz, a threshold of >10ms is potentially audible. Study can be found here: https://acris.aalto.fi/ws/portalfil...udspeaker_Group_Delay_Characteristics_AAM.pdf

This discussion started off by noting that the UMIK-1 has a variation of 0.05ms. I think it is going to be well below the threshold of audibility (if that is your aim). I think for many of us, "below audible" is not good enough ... we also want pretty graphs. No shame in that, that is my goal too. So that's why I persist even though I know it is probably a fool's errand.

There are many reasons to not use a USB microphone, but I don't think this is one of them.
 
I would like to add a question to this discussion.

If room gain contributes significantly to the bottom octave or so of subwoofer response, shouldn't we considered it (room gain) when setting the subwoofer time adjustment?

I'm not debating, just asking a question.
 
So finally today I got my UMIK-2:

20240911_101643.jpg

20240911_101650.jpg


Visually, the difference vs UMIK-1 is the colour and the capsule that looks totally different

It is no longer fully Plug & Play (at least in Windows), meaning that you have to install a driver for it to work properly. Also there is a firmware update capability which I performed since my unit was not fully up-to-date. And you will have to download the calibration file too, just like with UMIK-1

I have performed an acoustic timing reference test in REW with this unit vs my UMIK-1
I have made 5 measurements with each microphone exactly at the same position (MLP) to see how the measured delay value will look like using the front left channel as the reference and the subwoofer far behind my back as the other channel

Results:
- UMIK-2 has measured the same delay value with a two decimal precision for all 5 measurements, differences started to appear only from the third decimal (and we are talking milliseconds here)
- UMIK-1 has measured the same delay value up to the integer value only, meaning that there were differences starting with the first decimal already. This is definitely audible, especially in case of full-range channels

Additionally, I have performed a full-range measurement using the front channels of my home office system without any DSP:

1726057059141.png


Obviously I cannot tell which one is 'better' or 'more correct' but there is quite significant differences throughout the whole frequency domain reaching up to 3dB!
This means that there will be audible differences when you calibrate your system with an UMIK-1 vs UMIK-2

I am yet to try this microphone with Dirac Live 3 - will post my experience later on

So far so good, the UMIK-2 looks to be a keeper for me!
The measurement file is too big to attach, I will upload it to Google Drive and share here later today (I need my other PC to do that)
EDIT: see the measurement file here https://drive.google.com/file/d/1zP-0OmXTlPJGLRQJLkdRq7y7ZSAEKVVF/view?usp=sharing
 
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