by Bert Whyte
IT IS GENERALLY accepted that the hi-fi industry really began to build momentum in the late forties. In the hi-fi milieu of those days, you weren't considered one of the cognoscenti unless you owned an Altec loudspeaker. If you had limited means, you played your Prokofiev through the Model 603, a 15 in. speaker with an aluminum dome for high frequency dispersion. More usually, the "in" people owned the famous Model 604B, the granddaddy of all co-axial loudspeakers. Those who could afford the cost ... and the space ...indulged themselves with 15 in. theater woofers crossing over at 500 or 800 Hz into huge multi-cellular exponential horns.
It must be noted that these speakers were not "derived" from theater or broadcast monitor speakers, but were the actual units used in these applications. Of course, in large theaters the woofers and horns were used in multiples. Fortunately for the owners of these speakers, who used them for listening in the home, all of the models were of fairly high efficiency and could be driven to high output levels with the 10 to 30 watt amplifiers of that era. Needless to say, in those days you had to construct your own baffles for these speakers, with bass reflex design favored for the smaller speakers and back-loaded horns for the woofer/tweeter combinations.
Somewhere around 1948 Altec sensed a trend and built a handsome mahogany-and-brass-grille furniture enclosure to house the 603 or 604B speakers. It might be said that this marked the "official" entry of Altec into the consumer hi-fi market. A quarter of a century has passed since then and Altec is still going strong ... a dominant factor in the theater and public address markets and a highly respected "senior citizen" in the hi-fi establishment. Some months ago I had the pleasure of visiting the Altec plant in Anaheim, California, where I was given a fascinating "behind the scenes" look at the process of manufacturing loudspeakers.
Altec has a slight problem in respect to its visitors, as there is another fairly interesting attraction across the street . . . called Disneyland! My co-hosts at Altec were Don Davis, Vice President of Marketing, Industrial Products, and old friend John Eargle, Director of Sales, Commercial Sound Products. Don Davis is no stranger to the pages of AUDIO, having contributed a number of articles on various subjects, including one on the Altec "Acousti-Voicing process of room equalization, a project he conceived and has championed for the past several years. John Eargle is familiar to readers of this column from my descriptions of his unique process of quadraphonic synthesis and for his expertise in so many diverse areas of audio engineering. John was a colleague of mine at RCA Victor where he was director of quality control for recorded products. Then came a stint as chief engineer of Mercury Records. In his present position John really wears a number of "hats" and, as you might expect, is deeply involved with Altec products for recording studios and the professional audio market.
The first thing that struck me about the Altec plant is that it is far larger than I imagined it would be. This was impressed upon me quite physically, as I dragged my excess avoirdupois through many different production sections, up and down corridors, and even from building to building. Altec is a very successful operation, as evidenced by the extensive facilities, but I understand that sales in public address and commercial sound are burgeoning so fast that plant capacity is sorely taxed. In one huge area there are squadrons of women working on electronic subassemblies ... amplifiers, crossover networks, Acousti-Voice equalizers. Another area is devoted to speaker assembly, with cones, voice coils, spiders, baskets, magnets, and retainer plates being deftly fabricated into various models of loudspeakers, with much mechanical and electrical testing accompanying each successive step of assembly.
------Flatwire--Milled on the premises, the flat
wire passes through many processes before final attachment to cones or diaphragms.
Quite apart from the purely assembly type operations, many of the basic parts are made right in the Altec plant. For example, all voice coils used in Altec speakers are made of edge wound flat copper ribbon. This ribbon really starts out as copper wire which is fed into a special milling machine, which compresses the wire into flat wire, which in turn is annealed and varnished and then wound by a special machine onto various diameter formers to make bulk cylinders. After further heat treatment the cylinders are ready to be used. Ribbon is broken off into segments of the proper size, and excess turns are pulled off until the proper resistance is reached. The ribbon is then attached to either paper, plastic or aluminum formers and again heat treated. At this stage they are then ready to be attached either to cones or high frequency diaphragms.
One of the most fascinating processes I witnessed was the fabrication of the multi-cellular exponential horns. Depending on the size of the horn, pre-cut steel pieces are mounted on special jigs which will give the finished section the correct rate of flare. Once on the jig, the seam of each section of the horn is soldered by hand. The artisan picks up a huge soldering iron which has been heated to a fiery red by a blast torch, liberally gunks it with flux, and then with a king-size bar of solder in his hand, runs iron and solder down the seam in one smooth flowing sweep. What a technique! When I think of my own puny soldering attempts, I am in awe of such skill. Each seam is so treated in turn, and then each horn section is joined together and reinforced to prevent rattles. The entire multi cellular assembly is then coated with a material called Aquaplas, which insures good damping and freedom from resonances.
Another interesting process was one in which the aluminum diaphragms used in high frequency drivers are placed in a machine, which with a sort of embossing action forms a tangential compliance on the diaphragm, somewhat analogous to the "soft surround" on the cones of several types of loudspeakers, especially of the acoustic suspension variety.
----------Multi-Cellular Horns--Like pipe organs, they
are built from metal pieces, soldered by hand, and reinforced to prevent
rattles.
Altec has extensive machine shop facilities, including a large room I saw crammed with lathes, drill presses, etc. Much of this is devoted to the machining of the very intricate slits in the phasing plugs of the various high frequency drivers. These phasing plugs convert high pressure, low velocity soundwaves at the throat of the exponential horns to low pressure, high velocity waves at the mouth of the horns.
All the speaker frames or baskets in Altec speakers are die cast, as are certain types of cast aluminum sectoral horns. These parts are supplied by a vendor foundry with the machining facilities at Altec used for imparting a fine finish. The cone loudspeakers are ready for final assembly, with the cones with voice coils attached, being centered on the die cast frames, and spiders and magnet structures added.
The final step is magnetization of the Alnico or ceramic slug, the sensitivity and response checks are performed.
Altec makes a great deal of specialized equipment for the recording studios, including huge mixing consoles, portable mixers, monitor speakers, etc. In the area of public address, in addition to the speakers, Altec makes amplifiers and input consoles. Sort of overlapping into the PA and recording areas are the Acousti-Voice equalizers and real-time analyzers used in this process.
With John Eargle's experience in the recording field, I am sure we will be seeing some innovative ideas in the area of mixing consoles and associated equipment. He has already lent his talent to upgrading and new designs in monitor speakers. At the present time, the growing instrumental complexity and sophistication of rock music is making new and severe demands on the monitoring facilities of recording studios. The rock producers want to be able to hear the extreme 30 Hz frequencies of Fender bass and electronic music synthesizers like the Moog and Arp units. The synthesizers also require extended high frequency response. And the producers want all this at the usual ear-splitting monitoring levels prevalent in today's studios. These new requirements have set off a sort of race among several speaker manufacturers to see who can come up with a design that will capture this new market. Up to now, most studio monitor speakers (including Altec's) were characterized by a rather elevated "peaky" mid-range, a somewhat restricted high frequency response, and poor low frequency response with a rather rapid roll-off below 60 Hz. The new speaker parameters precluded any practical extension of current design practices. John came up with a solution that combined two seemingly incompatible speakers. To produce the requisite bass response and yet keep the size of the monitor speaker within usable studio limits, he turned to an acoustic suspension system, utilizing a 15 in. speaker with flexible "surround" and long-throw voice coil.
----------HF Diaphragms--Pressure-formed aluminum diaphragms
are glued to voice coil assemblies and the outer frame.
This was to operate up to 500 Hz and then a multicellular horn with a new configuration of throat and a special high frequency driver would carry on from there up to at least 15 kHz. As we all know, acoustic suspension speakers are of very low efficiency, while the exponential horn is a high efficiency device. John's answer to this was to use bi-amplification. Altec built a special 60 watt continuous output amplifier to handle the acoustic suspension section up to 500 Hz, then built an electronic crossover, and finally a 30 watt continuous output amplifier for the horn. This unit, designated the Model 9846, has proven very successful for its intended application, with a solid "gut-thumping" bottom end, well-defined mid-range and crisp top. More to the point of the studios . . . this full-spectrum output can be employed at tremendous levels without breakup. I visited John's apartment, where he had two of these speakers installed in his living room, which he had Acousti-Voiced. The sound of these units was very impressive indeed and did full justice to the spectacular 15 ips tapes he played on his big Ampex. On the famous Mercury recording of Sir William Walton's Crown Imperial march, there are some bass drum shots and some really rousing brass which are reproduced through John's speaker system with stunning impact and clarity. For "the man who has everything," you can always Christmas-gift him with the Model 9848, John Eargle's "monster monitor," which has two 15 in. woofers in an acoustic suspension system of heroic proportions, with a giant exponential horn to match. Gadzooks! All in all, I thoroughly enjoyed my visit to Altec. I think I learned a great deal, and I know I came away with a heightened appreciation of the problems and complexities of the high precision technology necessary to manufacture such high quality loudspeakers. .
--------High Frequency Drivers--Large 3 in. and small 1 3/4 in. diameter
drivers are exploded to show the intricate slits in the phasing plugs.
(Audio magazine, Dec. 1972; Bert Whyte)
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