Originally Posted by Gooddoc
I think I understand. Group delay is a measure of "packet" amplitude that looks at signal time delays that vary with frequency, result in phase changes, and are caused by the DUT. Variable phase delay is a measure of phase change that also varies with frequency but with a constant time delay.
Both cause a phase change but by different causes.
Thanks for the explanation, I never understood the group delay vs phase delay issue since they're both interrelated.
Yes, I think you've got it. It is tricky. I think the critical thing is the "time" or "timing" that Chris K often brings up. Time and phase are two very different things and are not interchangeable.
You can compare the phase of two sine waves ..... if they are at the same frequency. For instance, you can compare the phase of an 80 Hz sine wave produced by the main and one produced by the sub. That is the essence of sub-main alignment.
But, when we talk about two different frequencies, the word "phase" is never applicable. Over time two different frequencies will never be "in phase" because they have different wavelengths. The peaks or zero crossings might line up now and then, but that's not the same as being "in phase." They have to have the same wavelength and hence the same frequency to have a phase relationship.
Group delay involves the "time" relationship of two waves of different frequencies. Whether it's a main or a sub, the group delay gets significant at lower frequencies. It does not matter whether the speaker has electronics (analog or digital) associated with it, there is group delay. It involves the enclosure. Lower frequencies are delayed in time relative to higher frequencies by as much as a wavelength of the lower frequency.
Group delay is always measured in milliseconds because it is a time issue.
I used to use a speaker cabinet design tool called WinISD that could plot group delay. I'll see if it is still available and dig up a plot if it is.EDIT
WinISD is still out there. Here is the group delay for a 15" Dayton RSS390-HF driver in a 3.6 cu ft box with and without a port (the lower curve is for the sealed box). Even with a port the group delay isn't all that bad. My previous sub was far worse, over 25ms at 20Hz (isobaric 15" drivers in an 11 cu ft box with base extension tuning).http://www.hkclark.com/GroupDelay.JPG