Coaxials - Reality vs. Experience?

Hello everybody, please help me with a dumb question that I do not know the answer to.

closenplay stated, 

"To add clarity, concentric is a subset of coaxial. In other words, all concentric drivers are coaxial but not all coaxial drivers are concentric. @erik_squires Pardon the presumption; you are probably referencing most designs intended for the hifi home user, of which most (if not all) are concentric."

I know what "coaxial" means but how are you defining "concentric"? Are you suggesting that two coaxial drivers (i.e. their voice coils) may me mounted  at an angle to each other? That they are centered but not perpendicular? Or? 

sincerely

herb

closenplay stated,

"To add clarity, concentric is a subset of coaxial. In other words, all concentric drivers are coaxial but not all coaxial drivers are concentric. @erik_squires Pardon the presumption; you are probably referencing most designs intended for the hifi home user, of which most (if not all) are concentric."

I know what "coaxial" means but how are you defining "concentric"? Are you suggesting that two coaxial drivers (i.e. their voice coils) may me mounted at an angle to each other? That they are centered but not perpendicular? Or?

@herbreichert - Not to speak for @closenplay but my interpretation was that he’s focusing on the z-axis (forwards / backwards) alignment. An egregious example of z-axis non-alignment would be in some car stereo drivers, where they (sometimes) clumsily suspended & mount a tweeter in front of a woofer. That’s what I think he was calling out.

But the reality is that none of the coaxials really seem to have perfect z-axis alignment (relative to the 2 drivers’ acoustic centers). Certainly not Tannoys, which mount the tweeters well behind the back of the woofer, coupled through a relatively long waveguide. Tannoy’s brief attempts to time-align the drivers via electrical means were generally regarded a failure. Their pepperpot waveguide drivers at least have phase alignment at the crossover point; not sure about the tulip drivers.

Tannoy chose to call theirs "dual concentric" early on, probably to help distinguish their approach from far less refined coaxial arrangements of the time.

@herbreichert wrote:

I know what "coaxial" means but how are you defining "concentric"? Are you suggesting that two coaxial drivers (i.e. their voice coils) may me mounted at an angle to each other? That they are centered but not perpendicular? Or?

sincerely

herb

Pragmatically speaking it would seem mostly to be a terminological twist; Tannoy calls it ’Dual Concentric,’ whereas others typically refer to it as ’Coaxial.’ Whatever differences there are between different iterations/designs of these point source transducers, not that they're irrelevant, should have no practical impact on whether they’re called one or the other.

Coaxials with acoustic centers which are not coincicent does kind of beg for an active, DSP crossover doesn't it? :)

more dumb, not rhetorical, questions:

what is the definition of "acoustic center" ?

how do we know when they are aligned? A tone burst? 

what is the definition of a point source? 

I am completely confused. 

Is there a book on this stuff?

herb