It's not too high, just too high for full range horn loading. As Wayne mentioned, the size requirements for low bass are prohibitive in practical use. However, we are often talking about 80-120 hz and up being horn loaded. Below that many of these type boxes did act as a reflex box, with less output.

"What I was wondering is this: isn't the horn cutoff the limit at which the horn effectively loads the driver(impedence matches it to the room)?"

Yes.

"Below that cutoff, isn't the driver basically a direct radiator (in the case of Altec designs, a typical driver operating in a bass reflex cabinet arrangement)?"

Yes

Is the problem really that the spl level below the horn cutoff will be comparatively too low (yes) resulting in a very unbalanced sound? (and not necessarily; you may, upon listening decide it's not bad, or you may augment with a subwoofer)

"And maybe more to the point, is there any real benefit to horn loading the bass/mid in a domestic environment where the needed "throw" is so very short anyway?"

Yes. The "throw" is not what we are after, the efficiency and low distortion, etc. IS

"I suspect that the "old" Altec cabinets were a necessary augmentation to the driver technology of the time;"

the drivers were fine; they needed to be efficient for the low power amps available then, and they needed the clarity and projection of the horn. The concept is still valid and still very much used today; we just never get away from the size requirements for lower bass

"that all this is moot with modern 15" drivers since all the two-way cinema speakers that I know of use a bass reflex cabinet and do not horn load the bass/mid any longer."

More to do with space requirements and money, than issues of quality sound reproduction.

Thanks again for your thoughts.

These systems do have a stepped response with the area below 100hz down several db in level. However the quality of the bass is very good. IME an 825 with a 416 (A-7) sounds very good on it's own but an 825 with a 515 needs a subwoofer. I have 825s with 515Bs (A-5) and augment them below 100hz with a pair of JBL 4648 bassbins. The upside of the A-5 is the absolutely finest, most glorious and lifelike upper bass and lower midrange I've ever heard and I've heard alot.

The bass drivers used in these VOT systems are still IMO the finest sounding woofers ever made. The only areas in which they are inferior to modern woofs is in power handling, personally I'll take better sound over higher power handling in a home system. :-)

The move to direct-radiating bassbins in theaters was driven by several things; the need for deeper bass, size and cost, and the misinformed personal prejudices against basshorns of the guy who was in charge of developing the THX theater standards.

• Chicago Horn Speaker Club

Regards,
stpower