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The LT3 is larger than the LT2, and adds 4 tweeters for a total of 6, and two additional tweeter midrange units for a complement of four. In addition, the internal low frequency driver back wave structure is different than the LT2, with the additional low frequency bracing and damping within the enclosure. The LT3 uses a modified version of the patented symmetrical , balanced , first order internal crossover used in the ALIX-ANNE loudspeaker, and uses a completely new low internal volume, high cross-sectional area, backwave air pressure friction vent, which in practice, functions much like the highly damped aperiodic enclosures from the 1960ties. Since the enclosure represents a relatively low internal air molecule volume, high air molecule pressure environment, in most loudspeakers the cone has a preferred direction of travel. It is much easier for the cone to move outward, in response to a positive going electron signal input, rather than inward, in response to a negative going electron signal input, since the outward going excursion is into the high air molecule volume, low mechanical air molecule impedance of the listening room, while the inward excursion is into the low air molecule volume, high mechanical air molecule impedance of the enclosure volume. Porting, or venting, on the other hand, reduces this work differential, but at the expense of a short backwave path, with destructive acoustical interference between the front and back waves of the cone. Transmission line designs are an intelligent compromise, in which the back wave is conducted down a long pathway, and the time required for transmission down that backwave pathway represents some fraction of the lowest frequency of interest. In a quarter wave transmission line, as an example, if the lowest frequency of interest were 20 Hertz, that wave in the air, would be about fifty six feet long, at standard temperature and pressure. A quarter of that wave would be about fourteen feet long, which would then be the pathlength of a quarter wave line. Folding such a line within a modest enclosure such as the LT 3 would be quite a conjuror's trick indeed.
The LT3 takes advantage of the velocity transformation which occurs because of the difference in surface area between the total driven surface area of the cones, and the total aperture area of the port. By conducting the backwave down a relatively long transmission pathway, but not fourteen feet long, with a relatively high internal surface area, a frictional pathway is established where the acoustical energy represented by the backwave may be turned into heat through a frictional interaction with the interior walls of the transmission pathway. The LT3 is a fully first order device, with exceptional fidelity in the time domain.
THE AMERICAN POWER AND LIGHT, INC.,. LINEAR TRANSMISSION LINE TRANSDUCER, THE LT3 MECHANICAL, ELECTRICAL, ACOUSTICAL, SPECIFICATIONS THE LT3 IS TWENTY FOUR AND ONE HALF INCHES HIGH, FOURTEEN AND ONE QUARTER INCHES WIDE, AND EXACTLY THREE INCHES THICK, AND WEIGHS TWENTY EIGHT POUNDS. THE LT3 IS CONSTRUCTED ENTIRELY OF TWELVE AND EIGHTEEN MILLIMETER MULTIPLY PREMIUM BALTIC BIRCH. THE LT3 HAS A COMPLEMENT OF SIX LOW FREQUENCY MIDRANGE DRIVERS, AND FOUR HIGH FREQUENCY MIDRANGE DRIVERS, AND USES A PATENTED FIRST ORDER INTERNAL ELECTRICAL CROSSOVER. THE LT3 LOOKS TO THE DRIVING AMPLIFIER AS A DISTRIBUTED LINEAR RESISTANCE AT AN AVERAGE OF 3.2 OHMS, AND HAS A CONVERSION SENSITIVITY OF 94 DECIBELS AT ONE METER ON AXIS WITH AN INPUT OF TWO WATTS. (NOTE: THIS MEASUREMENT IS NORMALLY MADE WITH EIGHT OHM DEVICES, INTO WHICH A ONE WATT SIGNAL WOULD BE 2.83 VOLTS.) THE LT3 HAS AN IN ROOM FREQUENCY RESPONSE OF 35 HERTZ TO 22 THOUSAND HERTZ, PLUS OR MINUS 2 DECIBELS.
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