The author of the Thiele-Small parameters listing on the Unofficial Altec Web site seems to have dropped the ball for a half-dozen or so woofers i'm interested in, regarding their cone excursion Xmax parameter. I don't deny the author's work, and thank him for doing it, but for the cone travel spec in particular tagging 0.05 inch (1.27 mm) Xmax on numerous models isn't the right answer for a part of them.
It is evident he simply compiled data in a table form but didn't ask himself if some of that repeated number ever made sense, because some of these DO NOT make any sense when considering driver purpose, applications and manufacturer description. If he didn't know or lack good data why not put a question mark next to those ?
There's many 0.05 inch Xmax in the Altec drivers TS table. No doubt some are justified while some others appear to be doubtful. Examples of Bass drivers from the "LF" group having questionable excursion: 421-8/16 LF, 418-8LF. On the other hand some non-LF suffix drivers are still bass speakers, e.g. 421A, 421-8H series 1 and 2, also with questionable excursion data. That issue makes these models look bad unfairly in my view, and could keep potential audiophiles away from owning such due to a TS error.
The 416 and 515 woofer series excursion data appears legitimate considering the period of time involved where such Xmax numbers were not astronomically high for most, even for MI/SR 15" woofers. Power amps juice grew way faster than what woofers could handle, specially in terms of cone excursion capability. This has to be seen with the eyes of say the 1975-78 era, not judged by today's standards. Since this is about Sound Reinforcement, not Hi-Fi, i had a look at their ER/MI series too. Altec 15" SR woofers often had Xmax around the 3.3-4.6 mm range for bass drivers. Therefore to see a number having a rated 1.27 mm of travel is certainly weird.
The Altec 418 and 421 series drivers in the TS table are showing unusually low displacement capability, and that involves different models being rated the exact same 0.05 inch (1.27 mm) Xmax number. I may concede an odd one for lead guitar with high efficiency but limited LF spectrum for example, might not require higher excursion.
Lets take the 421- 8LF to illustrate the issue (its also "gifted" with 1.27 mm in the TS table). Well, Altec describes it as "our strongest 15-inch", Bass guitar driver, LF guitar speaker, and for application its Bass, typical applications are Bass, Electric Bass, organ, Sound Reinforcement woofer, etc. If that isn't enough this model's "LF" suffix should tell.
Moreover its use in Stanley Screamer (SS) would not have lasted long with the high levels in Touring sound, if it did have 1.27 mm of excursion, even worst in the SS subwoofer cabs. Seems the author of the TS table didn't know any of that.
The 421-8LF has Maximum Cone Excursion of 1/2" (spec sheet). The way its written this is likely the travel limit before damage (Xlim), and where driver distortion could be quite notable at high drive. But if we try to ESTIMATE a more reasonable number away from the extremes (i.e. Xmax) it would be somewhere between 1.27 and 12.7 mm (1/2" converted to metric).
An "educated guess" might be to cut the 12.7 mm in half. This leads to an Xmax of 6.35 mm, and assumes Xmax is about half of Xlim (no guarantee here though). That is still higher than for most Altec SR woofers (except for some 31 series). One might say 6.35 mm remains overly generous in the context VS other Altec drivers. Then if we cut another 30% of travel it would bring the estimated Xmax here to 4.445 mm, and that is near the top of the range for a number of 15" Altec SR woofers (about 3.3- 4.6 mm). Maybe not perfect but at least it makes more sense than 1.27 mm for the 421-8LF...
I'm not trying to rewrite the TS table for the better or worst, simply looking for numbers that can hold the road...
On my last post i forgot to show an Altec 421A pic. The one in the spec sheet posted earlier is sideways view whereas in the attached its full front view, and comes from an Altec AD involving a Bass musician.
Please don't repost or copy elsewhere.
Richard
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