Regarding a 24 hz Fs driver being sort of a waste to load in the A7 horn box.
To illustrate this point analogy is made here to a number of older JBL woofers being provided with quite low Fs, even if some of these had limited ability to reproduce low bass, e.g. 30 hz, with good level (SPL), often because of their cone travel limitation and/or very low Qts. Well its not really different for Altec.
A couple of JBL examples come to mind: 2205 Fs 30hz, K/E140 Fs 30-32hz, K151/E155 Fs 30hz, etc.
As i recall, around 30hz the 2205 will take about 30 W only when in a 5 cu.ft. vented box. And it will be approx. the same type of outcome for Altec's 416-8C at 24hz. No miracles available today.
In more recent times i can't remember seeing a 15" SR driver having a 25hz Fs (excluding subwoofers). The way to design, spec and determine proper woofer application has changed, they're not trying to seduce a broad range of customers with a driver anymore (the "all purpose" kind of thing).
The following more recent JBL examples, 2225, 2226 and 2227 are all 15" SR drivers and all three come with a 40 hz resonance, instead of an older common number like 30hz. The message is the new Fs number is quite sufficient for their expected duty. For the last bass octave its anticipated coverage will be handled by subs if need be. Moreover, there's a bunch of specialized drivers/cabs to take care of particular situations (e.g. under balcony speakers or Keele's Oblique angle speakers).
From a horn box one would hope, or even expect, good transient response but this depends on the woofer too.
Dickason mentions that higher values of Qts have inferior transient response, that higher Q drivers work in vented boxes but their transient capability is less than spectacular, and further reports on high Q drivers "degraded transient performance". (Vance Dickason, The Loudspeaker Design Cookbook, 5th ed, P. 44, 46)
As far as i know there isn't an exact and/or official number that dictates what is a low Qts woofer. Based on a number of observations and sampling, like what drivers JBL and Altec specified for horn use, or which they effectively used in these cabs (i looked at 14 from Altec: 416, 421 and 515 series). Low Qts appears to be mostly around 0.25 and downwards, with a few slightly higher exceptions, e.g. JBL 2225 Qts 0.28; Keele's W horn using JBL K151 Qts 0.27; Altec's 515-8G Qts 0.27).
(BTW Keele's W horn bass is also at a distance from 30hz: single unit, half-space, on-axis F3 60hz, power response F3 55hz, a little better with a pair, but still in similar waters as the A7 cab. The W horn was intended to be used in multiples, from 2 to 8 units, in order to get the bass lower. Therefore it may require a fair amount of real estate or money depending on one's LF horn ambitions.)
There's a balancing act that needs to take place between transient response and bass reach/output. Very low driver Qts implies better transients but skinnier bass. On the other hand with a relatively high woofer Qts bass is more robust, however the transient response suffers. Can't have it both ways it seems. Therefore in choosing a LF driver the user must decide what he/she prioritize in view of the intended usage. For my part, in absence of a specific application, i'd say go for a "middle of the road" compromise, to benefit from some of both worlds (bass and transients), and that would mean a Qts around 0.25 for the A7 woofer. Its pretty much what effectively happened from Altec, except for those who use the 416-8B. (There's no Qts data for the 421A driver).
Another important aspect to consider is over a period of time Altec had 15-16" woofers with power capacity ratings of 50, 75 and 100 W, some even lower. About 1978 they increased power capacity to 150W for a group of woofers: e.g. the H series 2 units and the "8LF" models. However, in doing so a little twist slipped-in the power ratings in the form of an apparent LF restriction. It is mentioned at the bottom of a specs page and has somewhat pale printing: "Power band lower frequency limit is 80hz for rated power". That clearly applies to the 150W units. Don't bother with the partial specs seen on the pic this is for 12" models, other columns on the doc deal with the 15" models. The point here being the power rating note.
A short way to explain the matter is to say this is not a thermal issue (Pe) its rather a cone displacement limitation (either Xmax 10% driver distortion level or Xlim limit before driver damage) because of the 80hz reference. In practice such driver won't be able to follow all the way down to VLF at full 150W power. Note this is not unusual, in prior days, as well as in more recent times . A situation like that may happen wether it involves a Altec, JBL or E-V. Nice this is disclosed though.
For a musician the 80hz+ for full power may or may not have consequenses. A Lead Guitar player for example might not be impaired by the limit, whereas for the Bass Guitar player this could spell trouble due to potentially excessive cone travel. The latter may also apply to high level music reproduction having a fair amount of lower frequencies.
Faced with the above case its good practice to apply some power derating on the woofer since it will prevent high distortion or damage. Considering these already have a high sensitivity in the 102-3 db range they'd still make pretty loud noise even with a power derating. I know its rarely enough...
Please don't repost or copy elsewhere.
Richard
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