# Confounded by acoustic timing reference



## klu (Jun 20, 2018)

Hi,
I'm new to this forum and just signed up to aid and guide my journey into acoustic room treatment. REW is obviously new to me as well, and I'm still familiarising myself with the controls so to speak, but I'm already very impressed with its breadth and capabilities. I've spent the last week reading the help file and this forum, while waiting for my UMIK-1 to arrive. With everything in place now the newbie questions will come thick and fast, so here's the first one:

My setup is fairly straight-forward, a Sony AVR with 2/2.1 speakers attached, connected to my laptop via HDMI, a UMIK-1 mic and REW running on macOS.

After setting up and using the acoustic timing reference with my left speaker as reference I noticed that measurements of that same speaker show a delay of about 1.3-1.4ms. This comes as a surprise to me – after all I've read I would have expected the reference speaker to be 0ms from itself! The right speaker has a about the same delay, at least that makes sense because it is about the same distance from the microphone.

What's doing my head in is that there is a delay at all, since the timing ref bleep and the measurement sweep take exactly the same path through the left speaker. This is not specific to the left speaker either, I get exactly the same when using the right speaker as time ref. The AVR has all fancy processing turned off for this, speaker distances and levels have been entered manually from tape measure and SPL readings.

I feel I can't proceed to play with subwoofer timing until I get my head around this. Has anybody got an explanation?


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## JohnM (Apr 11, 2006)

You only measured over a limited frequency range, so you are essentially measuring the difference between the arrival time of the LF region you measured and the HF region occupied by the timing ref signal. Use full range measurements for comparing timings, you should then see a result closer to zero for the speaker used as the reference. It may still be slightly away from zero for much the same reason - the overall timing of the measurement can be slightly different than the timing of the tweeter-dominated timing ref signal, but the difference should be consistent so fine for relative measurements.

The timing ref isn't really useful for relative timing of a subwoofer, there are other approaches for that.


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## klu (Jun 20, 2018)

Excellent point, thank you. A full-range (20Hz-20kHz) measurement I've just done brought delay between the left speaker and itself down to about 0.4ms.

I'm reading my way through the driver phase alignment thread now... :smile:


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