The Powered Altec A7 style cab

This horn loaded Powered Altec 1209B is part of their Sound Reinforcement arsenal. This type of enclosure isn't intended for ultra low bass, its mainly designed for high output, more so when considering the choice of LF driver.

Maybe one thought the powered system idea might have been something relatively young... Well, in this case it goes back to 1973, therefore 50 years old. The A version of the 1209 in the LH Altec library hasn't much info shared, particularly for acoustical data, kind of boring.

However, i came across the B version info of this SR cab where there's more meat available, making it an interesting read.The biamplifier with electronic crossover used is a nice feature for the era, but my interest for now is some acoustical aspects of the woofer(s) and box.

One might think this is just another A7 with a Altec 416XX or 515XX woofer as normally happens. Not the case though, since it uses a Musical Instrument LF driver (Altec 421A), while the other more standard driver options were available, i checked. The Alu dustcap seen on this woofer adds to confirm use of a MI unit. Be aware that Altec also released a A7 type cab using the 421-8LF woofer, a SR and/or MI unit depending on document, and its Tech Sheet says Fs 35 hz VS 40 hz for the 421A. (For LF duty in Altec's Stanley Screamers cabs many 421-8LF were used). So here you have FOUR Altec tested A7 LF drivers, instead of the usual two (416XX & 515XX). I doubt if there's more official and valid such drivers from Altec for this specific application.

(Food for thought: Notwithstanding the preceding, in trying to provide members some other POTENTIAL LF options, i'll risk adding a few drivers from Altec, sort of last resort alternatives for those who may need this, considering some good older woofers are getting more difficult to find, or their pricing may reflect greed. The MI Altec 418XX i have in mind may be viewed as "borderline" here, not best nor worst for the job, but there's hope. It would take too long to discuss 418XX analysis, but partially: forget the very old 418B and 418-8H since their 55 hz cone resonance is pretty high to expect good A7 bass VS 421A 40 hz, 515-8G 37 hz, 421-8LF 35 hz, etc), the gap is large;

HOWEVER, somewhat more recent MI Altec driver updates (1975-1978) have shown new 418 versions (418-8C, 418-8LF, 418-8H series 2), with improvements such as rated Fs 47 hz or less depending on document, sensitivity 102-3 db, etc. Each case needs to be looked at for suitability, but one could be an acceptable substitute. (Note the 421-8H was also updated to series 2). No tech sheets found for the "newer" ones, but i have some specs and TS, though not a full deck of cards).

The 421A box wasn't the sole Altec using a MI for LF, and some JBL e.g. 4560 versions were also loaded with an MI woofer, E130 or E140 for example. BTW i found that Altec made SR cabinet model 415E, very similar looking to JBL's 4560. I've got from Altec components suggestions to load the 415E, if someone here dares to build it (have no plans, but have a few ext. dimensions).

Why go for a Musical instrument woofer instead of a conventional one in the 1209B? First, probably in order to stretch the cab's sensitivity as high as possible using the rising response a MI driver gives. Second, because of the lower power that amplifiers had at the relevant time (exception e.g. Phase Linear amps). Altec's Biamplifier output: 60W LF, 30W HF. The key is the higher the speaker sensitivity the lower the input power required to reach a target SPL.

In the 421A tech sheet the woofer's response is presented using a 5 cu.ft. SEALED box, hence the weaker bass range. Its better in the 1209B (A7) enclosure, though not great. Altec mentions 421A low cone resonance as well as optimum cone resonance. The 40 hz Fs rating is adequate for an MI in this box. Plus as i understand it its a matter of having optimized the cone's resonance to result in higher efficiency (possibly lower cone mass). Eargle has an interesting note regarding a similar low-power available context, taking place in the old days too. Remember Eargle worked at Altec before joining JBL. (Quote: John Eargle, Loudspeaker Handbook, 2nd ed, p.246).

Among the 421A sensitivities given the 98 db one makes the most sense in view of the bandwidth applied. That woofer is similar to say JBL K140 (bass guitar). The reference to bass guitar appears in the other version of the 421A tech sheet i also have (not shown here).

DON'T FORGET: during the 70's Altec was using a 4' distance (not one meter) for their db measurements. The correction factor for one meter is + 1.7 db. Then the 421A sensitivity is 99.7 db @ 1 M, pretty much 100 db.

For the same reason the loaded 1209B cab sensitivity of 103 db at 4' is now 104.7 db, so 105 db @ 1 M. In this case, it helped to show a pretty high sensitivity when the pink noise bandwidth used was 100 hz to 10 khz, thereby benefiting from the high level midrange output in the data. As expected, the 60 hz bass level on the response graph is much lower, around -10 db, due to MI woofer louder mid response and the LF horn loading. Incidentally an Altec document indicates the LF horn (front wave) is in action from 120+ hz, while below that frequency the system is bass-reflex controlled (back wave). The LF horn contributes approx. +5 db of level in the mid-band sensitivity, none in the low bass range.

I wouldn't put too much faith in the 421A/1209B mentioned responses from 35 hz (have seen some A7 rated from 30 hz). These numbers appear overly optimistic. In real life here its closer to 55-60 hz with driver enclosed. This is more realistic, since below such number response falls rapidly. Although its surrounded by bells and whistles, the A7 type enclosure is approx. a 55 hz box (F3) using a bass driver e.g. 515-8G. In the Sound Reinforcement world such F3 is acceptable and frequent, its the price to pay for having very high mid-band sensitivity.

Moreover, near completion of this post i found two, plus the one posted here, more recent Altec A7 docs indicating a RATED low-end response of 50 hz, instead of the previously frequent & optimistic 30 and 35 hz. Altec was exhibiting some lucidity in this regard.

I'm including a pic showing an example of a typical case Altec A7 bass (F3 54 hz) using the 515-8G bass driver (not MI). The "Usable -10 db" indicated is not really "usable" in my book... This system is likely the most recent Altec A7 version made (from Altec Lansing Technologies) and the LF remain away from VLF territory, like it did in the past, if one wants level (SPL). VLF is not the objective in such box, here efficiency is King! Among the group of three woofers in Altec's G series (515-8G, etc) the other two having low/very low driver Qts aren't necessarily good choices to get A7 deeper bass from the bass-reflex vent. The 515-8G, 421-8LF or 421-8H series 2 for example, appear to be better options for this.

At last the Engineers at Altec Lansing Technologies seem to have done without the traditional opaque cloth covering the A7 reflex vent (resistive vent), its about time, replacing it with a more transparent cloth. Now one can see the yellow fiberglass lining the box, should help air flow. A step in the right direction for improved vent action i assume.

The Altec Lansing Technologies A7 version was a limited edition, niche product, therefore they didn't intend to make a whole lot of these on a continuous basis, and the market for such was probably limited too. They might have taken orders, made a production run of these, then the A7 revival story could have ended there. Still an interesting system in my view.

Please don't repost or copy this article elsewhere as i have a fair amount of time in this.

Richard

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Members may have noted that in my previous post i practically didn't say anything about these often used drivers in the A7. This wasn't an oversight, i thik these woofers require more than a line of text for A7 duty. Some people may not have expected the info here and may not like what they read, but it is what it is.

As experienced in my searches, Altec's data was not always consistent, nor error free, with their specs and/or TS parameters. (hence my "depending on document" i mentioned twice in the first post). The author of the Unofficial Altec web site referred to this error issue, but did say they "been checked and corrected, where necessary". So i assume these TS numbers are correct, and i'm using a few of them. Note his table is missing info for more drivers.

416-8B

Fs 25 hz, Qts 0.32, Vas 26.47 cu.ft., Efficiency 2.7% (Eargle's table: 2.5% = 96 db sen.)

416-8C

Fs 24 hz, Qts 0.25, Vas 22.74 cu.ft., Efficiency 3.3% (Eargle's table: 3.15% = 97 db sen.)

Quick comparison of B VS C: Fs essentially the same, for C version Qts was dropped markedly and now in the low Qts range or so, Vas of C somewhat lower leading to a little stiffer suspension system, efficiency on C is increased a little but enough to raise sensitivity by 1 db. BTW the kind of C version improvements here are similar/typical to those of a Sound Reinforcement driver. Not really the case of B version.

Very high sensitivity is often an indirect indicator that VLF reach has been compromized to allow mid-band efficiency to fly high.

The 416-8B has a rather high 0.32 Qts for horn use in a A7 for example. The B looks to me as more appropriate for Hi-Fi in a vented box like the Altec Model 19 where it was used.

The 416-8C significant Qts reduction makes it more suitable for LF SR horn use, while other relevant driver parameters remained mostly comparable (somewhat less for Vas). I've read again D.B. Keele's paper LF Horn Design using TS parameters, and is pretty clear about low Qts woofers being more suitable to load a horn box.

An SR cab's job isn't to chase 20-30 hz bass, instead it should deliver very high output, with subwoofers added when required. I see the 24 hz Fs on the C version as sort of a waste in a LF horn box such as the A7. That 24 hz is pretty much out of reach and out of usefulness in this kind of cab. The bass-reflex vent of the back wave is exactly to compensate for low-frequencies otherwise bass would be lacking.

My impression is, apart from the mandatory switch to ferrite, Altec didn't want or need to develop a whole new woofer to save on costs, keeping essentially the type of cone, etc and simply "cooked a few things" in order to make that C version better suited for another application (i.e. SR type horn-loaded like the A7).

For comparison GPA's 416-8B "clone" has some comparable TS numbers (not Vas), for other relevant data here (e.g. Qts .323), then two of the same are better than one. Though i cannot reconcile that driver's 98 db sen with its own 1.76% efficiency. Eargle's sen./eff. table: 98 db = 4% eff and 1.76% eff = about 94 db sen. So GPA's sen/eff don't seem to add-up, by almost 4 db, while the piston band sensitivity is effectively derived from efficiency...

The picture attached is from an Altec catalog and i agree with them the B version is best in a vented box, even if they did use it in the A7 horn.

Please don't repost or copy elsewhere.

Richard

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

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