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
Reply With Quote