C2 question on auto-distance calibration for subwoofer

[jewel]

Hi,

I'm trying to use the auto-distance calibration on C2 to set the subwoofer distance. The measured distance is 3.3m, but the auto-calib sets it to 6.7m. Should I use the actual distance or the auto-calib distance?

Also, the auto-calib distances are different if I change the phase on the subwoofer.
0 deg = 7.7m
45 deg = 8.0m
90 deg = 6.7m
135 deg = 6.7m
180 deg = 6.7m

Can someone explain what this means? Thanks.

[MarkStega]

In general you want to use the auto calibration distance as it reflects the "real world" signal path and takes into account how long it takes the sub to actually generate a sound at the microphone position from where the sub is located. Subs take longer to start generating a sound than a speaker for higher ranges so it appears to be further away.

I would suspect (guess) that the phase change deltas are related to the timing of the start of the wave. Sounds (roughly) travels at 1,000 feet per second or 1 foot per uSec. Varying the start time of the sub signal relative to the other speaker signals (phase change) will vary the apparent distance.

[Peter Nielsen]

You should use the ACTUAL distance. It is very common that the subwoofer distance can't be autocalibrated. This does not indicate a problem. You just need to set the distance manually...

Peter

[jewel]

In terms of the phase shift of the subwoofer, shouldn't the change in distance be incremental as the phase shift changes?
For example:
0deg = 8.0m
45deg = 7.7m
90deg = 7.3m
135deg = 7.0m
180 = 6.7m
But instead, my measurement shows a sudden change between 45 to 90 deg, and then a constant distance. I would say this phenomenon shows the inaccuracy of the distance auto-calib. Unless, of course, the room resonance is somehow affecting the sound wave as I vary the phase.

Any comments?

[Chris D]

Just to point out, the official book answer is found in the user's manual, page 31 for the C2 under Autocalibration:

The delay time which the C 2 calculates and displays for the subwoofer may appear quite different than its physical distance, based on 1ms as the time, at sea level, that it takes sound to travel 1 foot or .3 meters. This phenomenon is explained on page 62 in the “Technically Speaking” section near the back of this manual.

And then in "Technically Speaking" that it references on page 61:

The C 2 calculates and displays the actual required delay time as equivalent distance.If you are accustomed to thinking about equivalent distance in terms of 1ms of delay = 1 foot (or .3 meter) of distance, you shouldn’t be concerned when the C 2 displays subwoofer equivalent distance that is quite different than its physical distance from the calibration mic location. This isn’t a shortcoming or fault, but rather it is because the C 2 autocalibration algorithm is extremely sophisticated and measures the real “acoustical distance,” not simply the physical distance from the sub to the calibration mic. Acoustical distance is the actual amount of time required for a sound burst to travel from the speaker to the listening position, which the C 2 then converts to an equivalent distance in the display.

In the high frequency range with very short wavelengths, physical distance and acoustical distance are nearly equal, so equivalent distance does closely follow 1 ms = 1 foot. But at lower frequencies with longer wavelengths, the acoustical distance grows significantly longer than physical distance. Further contributing to this are delays in the low-pass crossover circuit, the voice coil mass of the subwoofer driver(s), and the fact that longer wave forms require some time to travel from wall to wall in a room before they start to resonate.

The C 2 autocalibration is the only correct way to accurately adjust for the actual acoustical distance, because it is the only way to insure that a burst of sound occurring simultaneously in all channels will also be heard simultaneously at your listening position. It is another reason why the C 2 is capable of recreating a uniquely seamless and vivid envelope of sound in your home theater.

My own technique is to use auto-cal for level, then distance, then again auto-cal level. (it hardly ever changes) Then I slightly adjust some of the numbers, such as if my left and right mains are off by 1 foot or something. As discussed in the manual, all of my settings make sense, except for my subwoofer distance, which is quite off, but it sounds right. So I stay with the Auto-cal setting.

We've discussed this in another thread here in the Club. A search should probably turn it up, if anyone is looking for more info.

[Chris D]

jewel, did I say hi and give you a banana yet? :banana: Hi! :later:

[jewel]

Chris, first I want to thank you for welcoming me to the forum.

However, regarding the C2 technical description of the auto-cal distance, it make sense on paper until I started experimenting with the phase of the subwoofer. The subwoofer phase setting should be changing the delay time of the bass generation so that it can be tuned with the front speakers. Therefore, as I change the phase, the auto-cal distance should change accordingly either in increasing or decreasing increments.

But in reality, the auto-cal distance makes a sudden shift at 45-90deg (from 8.0m to 6.7m...pretty big jump) and then stays at 6.7m. If the auto-cal distance changes incrementally with subwoofer adjustment, then I would have set the phase to 0 deg and trust the C2 auto-cal. (Please refer to first post on phase vs auto-cal distance)

So now if I were to use the C2 auto-cal distance, what setting should I use for the subwoofer phase?

[Kingdaddy]

So now if I were to use the C2 auto-cal distance, what setting should I use for the subwoofer phase?

You should not use Phase adjustments on the sub output at all unless your using multiple subs driven from different outputs (like one of the programmable outs) or have some room mode problems your trying to tame, the Phase adjustment is a tool only if you need it and a single sub system does not need phase adjustment in most cases. I use the phase adjustment because I have a pair of subs on a programmable output, (well, used to until the programmable outputs died) and one sub on the LFE out, the Phase adjustment in this case it to align the pair of subs with the one sub at the LFE output. None of this is easy to do by ear, I'm not even sure you could actually, but if you have a RTA you can see what is happening and use the Phase adjustment with great success.

[Chris D]

Kingdaddy, I've actually been wondering how you should REALLY properly dial in a subwoofer's phase. If it's just a 180 degree flip switch, it should be pretty easy using test tones to determine sound location at each setting to determine if it's in phase or not. But what about subs that have infinitely variable phase knobs?

[Kingdaddy]

Kingdaddy, I've actually been wondering how you should REALLY properly dial in a subwoofer's phase. If it's just a 180 degree flip switch, it should be pretty easy using test tones to determine sound location at each setting to determine if it's in phase or not. But what about subs that have infinitely variable phase knobs?

True to a degree, when you have multiple subs the lead and lag can be just a few degrees so a 180 switch may not be as noticeable. I've used the phase settings on the programmable outputs quite a bit with ETF running in real time and watching the FR plot and can here the difference much better when i see the response as well, however since All of my fully programmable outputs are dead now I cant go back and verify anything. As I said, I see no reason to fool with this unless you have two or more subs your trying to get together, I have three so I need to do this since they were all in different locations relative to my seating.

[Chris D]

Yeah, and I have two. I've been wondering how really to set the phase for both of them, as "jewel" asks above as well.