directional speakers

ATC is not directional. They do have very small waveguides but it’s a very small mid and they are very wide dispersion. They are very much set and forget, not varying a lot by location.

My comment was based on the specs for those mids with those waveguides. I stand by that statement.

That's not to say they don't have excellent off-axis imaging. 

Regarding the radiation pattern of ATC speakers which use that big 3" dome midrange on a shallow waveguide -

As a general ballpark rule of thumb, a horn or waveguide starts to lose directional control below the frequency where its dimension in a given direction is less than 1/2 wavelength, though it still has some directional control down to the 1/4 wavelength frequency.

It looks to me like the round waveguide is about 3" in radius (6" in diameter) and maybe 1.75" deep. The 1.75" depth is the limiting factor. Assuming these numbers are correct, theoretically the waveguide will start losing directional control around 3.8 kHz, and it won’t have much effect below about 1.9 kHz. The directional control won’t fall off a cliff because the radius of the wageguide is still large enough to impart some control, but it’s being largely short-circuited by the shallowness of the waveguide.

You can see the off-axis response out to 40 degrees in the measurements made by Troels Graveson, scroll down about 1/3 of the way:

http://www.troelsgravesen.dk/ATC-SM75-150.htm

It looks to me like useful pattern control is maintained down to about 1.6 kHz, so a little bit lower than my calculation.

In the SCM40 the upper crossover point is 3.5 kHz, so the shallow waveguide is narrowing the radiation pattern from there down to about 1.6 kHz, a range of a little over an octave.

I would expect the direct radiator dome tweeter that takes over north of the 3" midrange to have a wide radiation pattern in the crossover region, narrowing of course as we go up in frequency and the wavelengths become shorter. So even if the midrange dome’s waveuide was large enough to exert serious directional control across its range, the tweeter would still have a wide pattern.

I used to be a dealer for ATC, and remained one up until the time I started manufacturing loudspeakers. I think very highly of ATC, and in particular I think very highly of that dome midrange, but "directional" is not a term I would have used to describe ATC speakers.

Duke

dealer/manufacturer

This has off axis response graphed.  

"Despite the significant size, the 30 degree lateral output is almost as good as the on-axis trace, as is the 45 degree response. Only by 60 degrees laterally off-axis does this enclosure begin to become directive at higher frequencies."

Hi-Fi_Critic_SCM50PSL_WEB.pdf (atc.audio)

Thanks for providing better source materials than my poor memory, @audiokinesis and @jon_5912 - It seems I was very much mistaken.  My bad.

I will say that I don't think "directional" is a bad thing, so I certainly didn't mean it as a negative, but in this case my expectations of the objective behavior of those speakers was clearly wrong.  I apologize.

@erik_squires, when the day comes that it's MY bad memory in the spotlight, I hope to remember your example and be half as gracious as you.  Mad respect.

Duke