Question About Group Delay

[Steve81]

So on my way home from work yesterday, I got to thinking about group delay. I assume these are the things that pop into most other people's minds as they're driving around town as well. Now, I've heard ugly rumors to suggest that GD is correlated with FR, and I've also seen it suggested that correspondingly GD will change when you put a sub in room versus ground plane given that the FR will change. Keeping that thought in mind, it occurs to me that a subwoofer's FR isn't necessarily a fixed quantity even in a ground plane setting, as it can vary quite significantly by drive level, at least according to power compression sweeps I've seen performed. The FR profile of the SVS SB12 at a 90dB nominal drive level for example is quite different than at the 105dB nominal level. Does that mean GD is variable as well? If so, at what level is the GD derived?

 

[KEW]

You know more about this than I do, but here is an interesting discussion of Group Delay. I'm not sure it answers your questions, but it has some graphs and that is always a good thing!

http://www.stereointegrity.com/docs/SIGroupDelay.pdf

 

[Steve81]

You know more about this than I do

Nah; this stuff goes well above my head by and large.

 

[Steve81]

Found an answer to at least one part of the question in that article:

There is one caveat: the Hilbert transform is valid ONLY for what is called a minimum phase system. What is a minimum phase system? The actual way to calculate one is also beyond the scope of this paper; however, for our purposes, a subwoofer driver operating in a box, in it's linear mode is a minimum phase system. Operate beyond the linear limits of the driver (where distortion, power compression, suspension compression, and other nonlinear issues come into play) and the system is no linger minimum phase, and hence cannot use the Hilbert transform.

The GD would presumably be calculated at a lower drive level where compression isn't an issue.

What is less apparent to me is the mentioned/implied relationship between FR and acoustic phase, and what (if any) changes there would be to acoustic phase as the FR changes due to compression.

 

[Speedskater]

One thing about that linked paper, it was written by Dan Wiggens. Dan is on my good guy list, he has been designing and building speakers since way back in the BASS e-mail group days. (late 1990's)

Looking deeper, I found more Dan Wiggens papers and many DUMAX and Klippel test results for various subwoofers.

Stereo Integrity | Technical 2

They don't answer this treads question, but are still interesting reading.