THE SPECIFICS 0F THE SYSTEM.
The New 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 relation- ship 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 1,500 Hz. To achieve this objective, we chose to build the driver on a standard 12-inch frame that provides at low cost the width and frame depth needed for maximum cone excursion at lowest frequencies and
The Advent Low-Frequency Driver
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 an outer suspension design of thermally-formed polyurethane. The 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 of continuous operation, the free-moving suspension of the speaker should have no tendency to develop air leaks, how- ever small. The 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-frequency 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 of the magnetic system is a double-wound, four-layer voice coil that is significantly longer than the magnetic gap in which it operates. This is to allow a uniform proportion 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