Double-check my work? Timing mains and subs to each other

[cvanc]

Please, I need fresh eyes on my effort to time mains and subs together. I've attached an mdat with all delays set to zero. My (shaky) interpretation says I need to delay the mains about 11ms and not invert the bass. What does it look like to you? Thank you.

 

[sm52]

I would recommend delaying the main ones by 24ms and not inverting the bass.

 

[Keith_W]

Go to "Overlays" then click on "Impulse". Play with the zoom settings a bit, and you will be looking at something like this:

Red/Green = main speakers. Blue = sub, Yellow = same sub but 180 inverted.

The goal is to have all the impulses start at t=0. Hold down Ctrl and the right mouse button, and drag a box from t=0 to the start of the subwoofer impulse. That will tell you the delay.

But there is another issue:

I inverted the yellow measurement, which you labelled "Bass 0 delay -180". If ALL YOU DID was invert the polarity, then the yellow/blue curves should perfectly align. Clearly you also introduced additional delay when you inverted the polarity.

Anyway, the perfect tool for the job is REW's Alignment Tool. In the "All SPL" tab, right click on the graph and select it. Please see the relevant entry here in the REW manual (it's slightly more than halfway down).

 

[cvanc]

Rats, I've no idea how I managed to do that but yes the file is wrong. Here is a fresh set of sweeps.

 

[Keith_W]

Before you go any further, I should note that you are making a mistake.

Your L/R looks like it's high-passed at 80Hz, and you only have ONE subwoofer. This means that you will only have one bass source in your room below 80Hz. This is generally a bad idea. Welti's paper says you need minimum 2, preferably 4. The more bass sources you have, the flatter the response, and the more even the coverage over a listening area. It might be better to NOT high-pass your main speakers, and simply apply a LPF to the sub at 80Hz.

Having said that, your bass looks pretty flat in this SINGLE measurement from ONE listening position. That might be OK if it's for one listener only. Read this very carefully: we can only time align the subwoofer to a single measurement from a single position. But we generally take spatially averaged measurements for bass correction.

A more ideal solution would be to allow your main speakers to play full range, then phase align the subwoofer to the combined response (i.e. L+R). You will need to centre the microphone for that measurement (see my book for a microphone centring procedure, you want Book 2). Then take a spatially averaged measurement and use that as basis for DSP.

Why don't you give that alignment tool a go. It's easy to use and has a brilliant and intuitive design.

You didn't ask this question, but I thought I should bring it to your attention. See the main impulse at 7ms, then all the spikes at 15ms, 22ms, 29ms, etc? Those are very loud flutter echoes. I am generally hesitant to recommend room treatment, but this is probably what you need. My book will tell you how to diagnose the cause of these echoes (see the section on the ETC and the Wavelet section in the Spectrogram). The appearance of your measurement suggests that you have a pretty bare room (small volume, flutter echoes, high-ish T30). You can worry about this later. Tackle problems one at a time.

 

[cvanc]

Many thanks for your input.

The bass is composed of ten subwoofers spread all over the room, calibrated as a subsystem using Multi Sub Optimizer. And yes it's an 80Hz 48dB/octave crossover.

My mains are electrostatic so I don't want to feed them deep bass, but it's my understanding that if you use MSO you can not incorporate your full range speakers into your 'fleet' of bass transducers. Others probably know better than I.

And yeah I've got a bare brick wall in the room that's unlikely to change due to aesthetics. Presumably the source of the slap echo. This is not a purpose-built room.