Stereo downmix needs -6dB on center channel to sound right. Help!

Nuendo
1

Hi there!

I work mainly in 5.1 and have recently swapped my center speaker due to a bad contact on the volume pot that would make the volume vary randomly. That of course prompted a re-calibration of the room.

Since then I’ve noticed that center channel needs around 3dB extra reduction to sound right when downmixing to stereo. I say sound right because when I compare an excerpt with only dialog in the center channel, bounced both in 5.1 and 2.0 using the common -3dB C downmix preset, the statistics match perfectly, except for the expected 3dB differences in peak levels and max RMS.

But to match that when listening I have to turn the center channel down to around -6dB in the monitoring section downmix preset.

Of course the first thing I did was double check my speaker levels. In REW, using both full spectrum and speaker cal signals and measuring it with a calibrated meter in Slow/C curves, all speakers are within 0.5 dB from each other.
In sine sweeps in REW, all speaker curves coincide reasonably well, and all are within ±5dB at 1/24 dB/oct smoothing, disregarding an intended tilt.

So I believe these measurements show me that calibration is OK and there is nothing wrong with my signal path, right?

Now why do I have to monitor the center channel at -6dB when downmixing for it to sound right?

The only thing that comes to mind is that perhaps because my room is entirely covered in 60cm deep fluffy insulation, and L/R curves never diverge more than 2dB (in psychoacoustic smoothing) at any given point, and diverge less than 0.3dB up to 170Hz and 1.5dB at 200Hz (1/24 smoothing), that my acoustic summing is closer to +6dB due to a higher degree of correlation between the speakers.

Anyone could chime in and please help me see where I’m messing up and what should I do about this?

Of course my room and speakers are far from perfect, but the differences I’ve found in level and FR between them don’t seem enough to explain this difference in percieved volume. Besides, after calibrating them and all seeming normal, I don’t know what else I should do.

Perhaps I simply adjust my monitoring downmix preset and move on?

What are your thoughts?

Thanks"!

P.S.: I forgot to say that when I measure both L+R at the same time with pink noise in REW I do get a roughly 6dB increase in the sound level meter.

P.s.: When I mute my subwoofers I don’t feel as much of a change in volume. I was thinking that this corroborates the acoustic summing theory, since all bass is coming from the same subwoofers and when the signal doubles electrically it also perfectly doubles acoustically.

2

I mean… If you’ve delivered your mixes one way so far, both in stereo and 5.1, and your clients haven’t complained or nobody noticed a big difference, then I would think you should look at the room setup rather than downmixing.

I have my pan law set to constant power and I leave any other automatic/default in place, and in my room it sounds fine. It’s not a well treated studio or stage but rather a decently treated home studio, which probably makes a difference. But I’d think that maybe you should try to A/B in a different environment to see how it feels, or just step out of the room and leave the door open and check it that way.

3

Yes, this is my reasoning when I say that both 5.1 and stereo downmix measure exactly as expected in Statistics, and why I’m thinking about changing the monitoring downmix only. I believe the issue is between my speakers and my ears.

I should have thought of this earlier, but I monitored the 2.0 downmix using headphones (I found a quick way to mute main speakers and change the downmix preset at the same time - why on earth can’t we choose the phones channel downmix separately?), and it seems to me the issue is indeed LR summing to around +6 acoustically. On headphones I feel things sit pretty much at the same spot as in 5.1 when downmixed to -3dB Center channel.

I think for now I’ll just use headphones for checking the stereo mix on the usual downmix specs and then change the downmix to -6dB Center when doing it using my monitors.

I dislike using headphones and avoid them whenever I can.

Thanks a lot Mattias!

P.s.: If anyone can confirm or contradict my theory of my room summing LR to around +6dB acoustically and any ways to mitigate this I’d love to hear it!