The
Advent
Loudspeaker…
and The Smaller
Advent Loudspeaker
First of All, Who and What Is
Advent?*
Advent Corporation is a new company formed to develop new kinds of home entertainment products. Most of us involved have had long prior experience in making and marketing high-performance audio products for the home. We are in business now to produce new home entertainment products, both audio and visual, that go well beyond the generally limits generally accepted at any given moment, and explore new or significantly different approaches to design. One of our main product objectives is a color television system that uses a projected image instead of a conventional picture tube, producing a picture of 41/2 by 6 feet.
Our President and General Manager, Henry E. Kloss, was previously President and a founder of KLH Research and Development Corporation, and, prior to that, General Manager and a founder of Acoustic Research, Incorporated. He has been directly responsible, over the past fifteen years, for a major proportion of the important and lasting audio products for the home, including roughly half of the total number of loudspeaker now in use across the country in component stereo systems and high-per- formance three piece music systems, radios and phonographs. His credits would be embarrassingly long to present complete, but they include:
• The longest lived best selling component speaker system on the market, unchanged in design and undiminished in popularity after twelve years.
•The revolutionary miniature full range speaker that made it possible to provide sound of high quality in radios and phonographs of modest size.
•The first high performance FM radio.
•The first high performance portable stereo phonograph.
•The development of the three piece compact stereo system as a primary medium for home audio.
All of us at Advent share with him some convictions developed over several years of working together, about the way in which products can and should be developed. The Advent Loudspeaker is one result of those convictions, and we will have some more to say about them as we describe the speaker.
What is the Advent Loudspeaker?
Our first Product, the Advent Loudspeaker, is based on a premise that is not easy to accept. We believe that it is possible to produce a speaker system, for a moderate price, that will be nothing Iess than the right, completely satisfying choice for most people with a demanding interest in music and sound. The Advent speaker is designed to combine the following objectives:
•To fit the highest category of loud speaker quality, with overall performance at least the equal - in every audible and useful respect - of the most expensive speakers available.
•To do that for about half the average cost of the speakers now generally considered the best available.
•To be small enough, unobtrusive enough, and uncritical enough in placement to fit gracefully and usefully into a home.
•To produce enough output at low distortion to permit listening to music at satisfyingly loud levels in even the largest living room.
•To be driven comfortably by the majority of good amplifiers and receivers now available, with a power margin sufficient for the most demanding musical material.
•To sound convincing not only on the best recordings, but on the great majority of recordings of all kinds.
In developing the Advent Loudspeaker, our initial interest was in the category just below the “ultimate” in performance and a long way below it in price - the category in which most serious listeners, believing that further improvements are not worth the added cost, decide to buy. We knew that we could produce a speaker that would be both significantly better and significantly less expensive than the speakers considered the best value in that category. Our aim was to do that and establish a new point of diminishing returns that would be closer to the highest level of speaker performance.
But as we began to apply some new thinking about old concepts and to take advantage of new materials and manufacturing techniques, it became really clear that we could provide a really tremendous gain in perform- ance-per-dollar. So much, in fact, that the difference between something very good and the highest level of performance was essentially nonexistent in actual cost. It made no sense, then, to stop short of a speaker in the best category.
We didn’t stop short of that point. As difficult as it may be to believe of a speaker system of this cost and apparent simplicity, we know of no way to make any useful improvement in its performance.
*About our name: When you form a new business, and are eager to get on with things before the lawyers get all the papers signed and the incorporation proceedings over with, you (our) lawyers call you “an advent company”. We decided the term suited our continued desire to get on with things, so we appropriated it for our permanent company name.
How is all that possible?
The performance of the Advent Loudspeaker is due in great part to some important specifics of design and construction. But the key to all of them, and to the entirely new level of value the system represents, is that it is the first loudspeaker system to exploit the full potential of two way loudspeaker design.
For just about as long as home audio has been of real importance, it has been assumed that the three-way speaker system, which assigns at least one specialized speaker to each of the three segments (low, middle, high) of the frequency range, is a necessity for optimum sound. The two-way system has been accepted as a “good” medi- um-price expedient, but the idea of extra speakers and further specialization for optimum quality fits in both with the idea of separate, specialized audio components and with the widespread feeling that “more is better”. But the superiority of the three-way system is one of those “absolutes” that remain true only so long as no one really challenges them. After a thorough review of today’s materials and manufacturing technology, we knew that the old assumption no longer need be true.
The trouble with the concept of the three-way system - a crucial one - is that it’s based on the limitations of an earlier generation of loudspeakers. When high fidelity first gained wide interest for home listeners, the only suitable speakers available for use in a home audio system were those used in theater and public address systems. The individual speakers in theater systems were specialized and restricted in range for the purpose of maximum powerhandling to fill large public spaces, and they converted to full-range use in home systems only in three-way designs.
As new cone materials and new techniques were developed specifically for home loudspeakers, the need for three-way systems has disappeared. It’s simply no longer true that if a woofer is to be able to deliver excellent bass response, it will not have enough mid-range output, or that a tweeter can not combine excellent high frequency and mid-range response. Nor is it true that the three-way system is somehow capable of wider range or subtler characteristics.
Today’s three-way systems fall into three cate-
gories:
•Those that sound genuinely excellent.
•Those designed that way because of the severe limitations or their individual (and outmoded) drivers.
•Those that simply throw in an extra speaker or two because It looks better as a selling point.
Of these three, only the second has to be a threeway system.
The Two-Way System
We began the design of the Advent Loudspeaker with the knowledge that the two-way system had basic advantages, both theoretical and practical, over any other way of designing a full range speaker system for the home. They include:
•Less interference between drivers than any other practical system, Yielding more uniform characteristics over the listening area. A single speaker would be better here as an ideal, but lacks either the range or power-han- dling required in the ‘“best”’ category, or both.
•A better transition between drivers than the three-way system, because there is no need to sharply cut off the operating range of any driver. Whether such a cutoff is made for good reasons or simply to satisfy the arbitrary parameters of three-way design, its ill effects are audible.
•One simple crossover network instead of the two more complex networks needed in a three-way system.
•A cost significantly lower for excellent sound than that of an equivalent system of three-way - or any other - design.
Those advantages are important enough in themselves, but there is an overall advantage that seems even more important to us. It is that simplest feasible approach to an objective produces the best kind of engineering - a concept not too far from the notion that the shortest distance between two points is either a straight line or the one that is straightest under the circumstances “Over-engineer- ing is a common problem in audio equipment, and one for which the customer often pays heavily in many ways. Good design to us is represented by the simplest approach that permits reaching an objective without compromise, not by an over-elaboration that makes the buyer pay for our painting the lily.
If the two-way system has effective limitations, they apply to the tremendous power-handling required for use in a theater auditorium or in some laboratory applications. An extra driver provides (all other things being equal) both greater power-handling for these purposes, and the extra radiating area helpful both for radiating power and dispersing sound over a large public area. But those are the only significant limitations of two-way design.
They have no importance in the design of the best possible speaker system for use in a home.
The Specifics of the System
The Advent Loudspeaker is a “system” in the fullest sense of the term. All of its components, including the cabinet, have been designed from the outset for no application other than this one, and all of them are interdependent. That interdependence is worth stressing here. The relationship of cabinet size and speaker size, for instance, is critical. If the size of the low frequency driver were increased (all other things remaining equal), the result would be less bass: a larger driver in the same cabinet would raise the effective stiffness of the air in the enclosure and would establish a higher bass roll-off frequency.
Some other factors are equally important and we will try to make them clear as we go along.
The Low-Frequency System
The operating range of the low-frequency driver extends from the lowest frequency of musical importance to the upper crossover point of 1,000 Hz. To achieve this objective, we chose to build the driver on a standard 12inch frame that provides at low cost the width and frame depth needed for maximum cone excursion at lowest frequencies and highest power. The piston diameter of the cone combines maximum usable response at 30-35 Hz with a maximum of dispersion in the upper part of the operating range.
The driver’s capabilities at lowest frequencies is made possible in great part by a new outer suspension design of thermally-formed fiber-reinforced poly-urethane. The new suspension, a highly durable and reliable centering design, permits long linear cone motion, yet remains an effective airtight seal that maintains the full operating effectiveness of the system’s sealed cabinet. In the acoustic-suspension design used for low frequencies in the system, the mechanical rigidity of conventional suspensions is replaced by a combination of a free-moving suspension and the stiffness of the air trapped inside a sealed enclosure. For optimum performance both at first and over years or continuous operation, the free moving suspension of the speaker should have no tendency to develop air leaks, however small. The new suspension of the Advent Loudspeaker is more effective than any in our experience.
The felted cone of the low frequency speaker is formed by a low-vacuum process developed to assure high uniformity from speaker to speaker and the proper resistance to the transmission of excess energy through the cone, (The “live” energy-transmitting cone still used in many speaker systems is a prime source of the kind of distortion generally called cone “break-up”). The composition of the cone is as important as any other factor in achieving the smooth operation of the driver in the upper part of its range.
The heavy magnetic structure of the low-frequen- cy driver is designed to provide “critical damping” for the speaker in its specific cabinet volume - that is the amount of damping that could not be increased without rolling off bass response unnecessarily). (Like everything else in speaker design, the improvements secured from increasing the weight and flux density of a magnet go only so far; increasing beyond that point just rolls off bass response). Also part or the magnetic system is a double-wound, fourlayer voice coil that is significantly longer than the magnetic gap in which it operates. This is to allow a uniform portion of the coil to stay in the gap at all times, even when cone and voice coil make their longest excursions, and is a vital factor in keeping bass harmonic distortion low at even the highest listening levels.
The efficiency of the system has been carefully chosen to provide for reproduction of the lowest usable frequencies with amplifiers and receivers of good quality and medium power. While efficiency is lower than that of some comparably priced speakers of more limited range, and will require a slightly higher setting of a volume control for the same acoustic level, it is no more likely to tax the actual power capabilities of the amplifier or receiver used in a home. This doesn’t hold for auditoriums or, in many cases, for large (and sometimes noisy) audio showrooms, but it is emphatically so for home listening at even the highest usual loudness levels.
The Advent Low-Frequency Driver
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