Acoustic Timing Reference Question

[MikeVW]

I have a question about the meaning and interpretation of the “Acoustic Timing Reference” in REW that I am hoping someone can help me with. From the documentation I gather that it is supposed to represent the time delay in the system, but I am getting delays that are all over the map and often seem unrealistic. There is a lot leading up to this question, but keeping to the point, I am running a Mac with Catalina and have a Umik Mic. And, I should note that the results are the same for both 5.18 and 5.20. When I run REW using the internal Mac speakers and mic I get a time delay of about 0.05 ms. However, when I plug the USB Mic in and load the calibration file, using the computer speakers, I get a delay of 85 ms. The results get worse when I use a set of powered bookshelf speakers driven off the headphone jack with a 6 foot cable. I am including several screenshots showing the parameter set up for the Umik Mic. What am I missing?

Many thanks in advance for any help,

Mike

P.S. When I first set up 5.18 back in November the delays were very small. Something has changed. I wonder if this has something to do with the latest version of the MacOS - maybe not.

 

[Plipie Plip]

The only thing i could think of that its the delay of the mic > USB > pc > program > pc-output > speaker has more delay than direct pc output > mic input.
The other thing i noticed they have slightly different buffers. But I can't tell if that makes any difference to the delay.

 

[John Mulcahy]

It is better to select an actual output rather than leave the setting as default, the default can change.

Delay figures with limited bandwidth measurements use an estimated IR start value, on the assumption the bandwidth has been limited because the measurement is of a band limited speaker, such as a subwoofer. Your measurement looks very noisy, so the IR start estimation may have been corrupted by noise in the measurement. It is difficult to tell from images though, better to post mdat files so the data can be looked at. For delays of speakers that include a tweeter you should measure to at least 20 kHz.

 

[MikeVW]

Thanks John, Good point about band-limited measurements. I had it running just fine in Nov, so I'll take another pass at full range with my big speakers. I find it hard to believe that I damaged the Mic, but I am pondering that possibility if this doesn't work.